18.1 exchanging materials: basic principles

Living things are complex machines with many parts that must work together in a coordinated fashion. All systems are integrated and affect one another in many ways. For example, when you run up a hill, your leg and arm muscles move in a coordinated way to provide power. They burn fuel (glucose) for energy and produce carbon dioxide and lactic acid as waste products, which tend to lower the pH of the blood. Your heart beats faster to provide oxygen and nutrients to the muscles, your breathe faster to supply the muscles with oxygen and to get rid of carbon dioxide, and the blood vessels in the muscles dilate to allow more blood to flow to them. As you run you generate excess heat. As a result, more blood flows to the skin to get rid of the heat and sweat glands begin to secret, thus cooling the skin. All of these automatic internal adjustments help the body maintain a constant level of oxygen, carbon dioxide, and glucose in the blood; constant pH; and constant body temperature. They can be summed up in the concept of homeostasis. Homeostasis is the maintenance of a constant internal environment as a result of monitoring and modifying the ing of various systems. To explore the various mechanisms that help organisms maintain homeostasis, we will begin at the cellular level.

Cells are highly organized units that require a constant flow of energy in order to maintain themselves. The energy they require is provided in the form of nutrient molecules that enter the cell. Oxygen is required for the efficient release of energy from the large organic molecules that serve as fuel. Inevitably, as oxidation takes place, waste products form that are useless or toxic. These must be removed from the cell. All these exchanges of food, oxygen, and waste products must take place throught the cell surface.

As a sell grows its volume increases, and the amount of bolic activity required to maintain ir rises. The quantity of materials that must be exchanged between the cell and its surroundings increases. Thus growth cannot continue indefinitely. The ultimate size of a cell is limited by one or more of the following interrelated factors:

1.                          the strength of the membrane: as the cell increases in size, the membrane will eventually not be strong enough to withstand the forces caused by the mass of material inside it. If you had a balloon and kept adding water to it, eventually the balloon would burst. Similarly, dams have failed when too much water was accumulated behind them.

2.                          the cell surface area: if materials are to enter a cell they must pass through a surface. The cell membrane is a selectively permeable barrier to the passage of material in and out of the cell. The amount of surface will determine how much material can pass. If you were to pour water through two coffee filters of different size, the one with the largest surface area would allow the water to pass through more rapidly.

3.                          the surface area-to-volume ratio: the bolic needs of a cell are determined by its volume and the ability of exchange materials between the cell and its surroundings are determined by its surface area. As a cell increases in size, its volume increases faster than its surface area. This relationship between surface area and volume is often expressed as the surface area-to-volume ratio.

The ability to transport materials into or out of a cell is determined by its surface area, whereas its bolic demands are determined by its volume. So the larger a cell becomes, the more difficult it is to satisfy its needs. Some cells overcome this handicap by having highly folded cell memebranes that substantially increase their surface areas. This is particularly true of cells that line the intestine or are involved in the transport of large numbers of nutrient molecules. These cells have many tiny, folded extensions of the cell membrane called microvilli.

In similar way, the structure of an automobile radiator increases the efficiency of heat exchange between the engine and the air. The radiator has many fins attached to tubes through which a coolant fluid is pumped. Because of the large surface area provided by the fins, heat from the engine can be efficiently radiated away.

In addition to the limitation that surface area presents to the transport of materials, large cells also have a problem with diffusion. The molecular process fo diffusion is quite rapid over short distances but becomes very slow over longer distances. Diffusion is generally insufficient to handle the needs of cells if it must take place over a distance of more than a millimeter. The center of the cell would die before it received the molecules it needed if the distance were greater. Because of this and the problems presented by the surface area-to-volume ratio, it is understandable that the basic unit of life, the cell, must remain small.

All single-celled organism are limited to a small body size because they handle the exchange of molecules through their cell membranes. Large, multicellular organisms consist of a multitude of cells, many of which are located far from the surface of the organism. Each cell within a multicellular organism must solve the same materials-exchange problems as single-celled organisms. Large organisms have several interrelated systems that are involved in the exchange and transport of materials so that each cell can meet its bolic needs. Diffusion, facilitated diffusion, and active transport are all involved in moving molecules across cell membranes. These  topics are presented in detail in chapter 4.

18.2 circulation

Large, muticellular organisms like humans consist of trillions of cells. Because many of these cells are buried within the organism far from the body surface, there must be some sort of distribution system to assist them in solving their materials-exchange problems. The primary mechanism used is the circulatory system.

    The circulatory system consists of several fundamental parts. Blood is the fluid medium that assists in the transport of materials and heat. The heart is a pump that forces the fluid blood from one part of the body to another. The heart pumps blood into arteries, which distribute blood to organs. It flows into successively smaller arteries until it reaches tiny vessels called capillaries, where materials are exchanged between the blood and tissues through the walls of the capillaries. The blood flows from the capillaries into veins that combine into larger veins that ultimately return the blood to the heart from the tissues.

The nature of blood

Blood is a fluid that consists of several kinds of cells suspended in a watery matrix called plasma. This fluid plasma also contains many kinds of dissolved molecules. The primary of the blood is to transport molecules, cells, and heat from one part of the body to another. The major kinds of molecules that are distributed by the blood are respiratory gases ( oxygen, and carbon dioxide), nutrients of various kinds, waster products, disease-fighting antibodies, and chemical messengers (hormones). Blood has special characteristics that allow it to distribute respiratory gases very efficiently. Although little oxygen is carried as free, dissolved oxygen in the plasma, red blood cells, contain hemoglobin, an iron-containing molecule, to which oxygen molecules readily bind. This allows for much more oxygen to be carried than could be possible if it were simply dissolved in the blood. Because hemoglobin is inside red blood cells, it is possible to assess certain kinds of health problems by counting the number of red blood cells. If the number is low, the person will not be able to carry oxygen efficiently and will tire easily. This condition, in which a person has reduced oxygen-carrying capacity, is called anamia. Anemia can also result when a person does not get enough iron. Because iron is a central atom in hemoglobin molecules, people with an iron deficiency are not able to manufacture sufficient hemoglobin. They can be anemic even though their number of red blood cells may be normal.

Red blood cells are also important in the transport of carbon dioxide. Carbon dioxide is produced as a result of normal aerobic respiration of food materials in the cells of the body. If it is not eliminated, it causes the blood to become more acidic (lowers its pH), eventually resulting in death. Carbon dioxide can be carried in the plasma, about 23% is carried attached to hemoglobin molecules, and 70% is carried as bicarbonate ions. An enzyme in red blood cells known as carbonic anhydrase assists in converting carbon dioxide into bicarbonate ions, which can be carried as dissolved ions in the plasma of the blood. The following reversible chemical equation shows the changes that occur.

When the blood reaches the lungs, dissolved carbon dioxide is lost from the palsma, and carbon dioxide is released from the hemoglobin molecules as well. In addition, the bicarbonate ions reenter the red blood cells and can be converted back into molecular carbon dioxide by the same enzyme-assisted process that converts carbon dioxide to bicarbonate ions. The importance of this mechanism will be discussed later when the exchange of gases at lung surface is described.

Heat is also transported by the blood. Heat is generated by bolic activities and must be lost from the body. To handle excess body heat, blood is shunted to the surface of the body, where heat can be radiated away. In addition, humans and some other animals have the ability to sweat. The evaporation of sweat from the body surface also gets rid of excess heat. If the body is losing heat to rapidly, blood flow is shunted away from the skin, and bolic heat is conserved. Vigorous exercise produces an excess of heat so that, even in cold weather, blood is shunted to the skin and skin feels hot.

The plasma also carries nutrient molecules from the gut to other locations where they are modified, bolized, or incorporated into cell structures. Amino acids and simple sugars are carried as dissolved molecules in the blood. Lipids, which are not water soluble, are combined with proteins and carried as suspended particles, called lipoproteins. Most lipids do not enter the bloodstream directly from the small intestine but are carried to the bloodstream by the lymphatic system. Other organs, like liver, manufacture or modify molecules for use elsewhere; therefore they must constantly receirve raw materials and distribute their products to the cells that need them through the transportation of the blood.

In addition, many different kinds of hormones are produced by the brain, reproductive organs, digestive organs, and glands of the body. These are secreted into the bloodstream and transported throughout the body. Tissues with appropriate receptors bind to these molecules and respond to these chemical messengers.

The immune system           

Table 18.1 lists tha variety of cells found in blood. Whereas the red, hemoglobin-containing erythrocytes serve in the transport of oxygen and carbon dioxide, the white blood cells carried in the blood are involved in defending against harmful agents. These cells help the body resist many diseases. They constitute the core of the immune system. The various WBCs participate in providing immunity in several ways. First, immune system cells are able to recognize cells and molecules that are foreign to the body. If a molecules is recognized as foreign, certain WBCs produce antibodies that attach to the foreign materials. The foreign molecules that stimulate the production of antibodies are called antigens. When harmful microorganisms (e.g., bacteria, viruses, fungi), cancer cells, or toxic molecules enter the body, other WBCs recognize, boost their abilities to respond to, move toward, and destroy the problem causers. Neutrophils, eosinophils, basophils, and monocytes are specific kinds of WBCs capable of engulfing foreign material, a process called phagocytosis. Thus they are often called phagocytes. Although most can move from the bloodstream into the surrounding tissue, monocytes undergo such a striking increase in size that they are given a different –macrophages. Macrophages can be found throughout the body and are the most active of the phagocytes.

   The other major type of white cells, lymphocytes, work with phagocytes to provide protection. The two major types are T-lymphocytes and B-lymphocytes. T-cells are involved in a cell-mediated immune response in which cells directly attack potentially dangerous objects. This highly complex response involves the release of chemical messengers that coordinate the response, an increase in the population of T-cells and B-cells, and stimulation of B-cell and macophage activities. Some T-cells are capable of killing dangerous cells by destroying their cell membranes. T-cells are primarily involed in fighting infections of viruses, fungi, protozoa, worms, and cancer cells.

B-cells are involved in antibody-mediated immunity in which B-cells produce antibody molecules that are released into the blood stream and are distributed to all parts of the body. These antibodies attach to the foreign molecules, causing them to clump together. This clumping may destroy their harmful properties, make them more susceptible to chemical attack, or make them more recognizable for phagocytes. Many kinds of disease caused by viruses and bacteria are controlled by antibodies produced by B-cells.

Another kind of cellular particle in the blood is the platelet. These are fragments of specific kinds of white blood cells and are important in blood clotting. They collect at the site of a wound where they break down, releasing molecules. This begins a series of reations that results in the formation of fibers that trap blood cells and form a plug in the opening of the wound.

The heart

Blood can perform its transportation only if it moves. The organ responsible for providing the energy to pump the blood is the heart. In order for a fluid to flow through a tube, there must be a pressure difference between the two ends of the tube. Water flows through pipes because it is under pressure. Because the pressure is higher behind a faucet than at the spout, water flows from the spout when the faucet is opened. The circulatory system can be analyzed from the same point of view. The heart is a muscular pump that provides the pressure necessary to propel the blood throughout the body. It must continue its cycle of contraction and relexation, or blood stops flowing and body cells are unable to obtain nutrients or get rid of wastes. Some cells, such as brain cells, are extremely sensitive to having their flow of blood interrupted because they require a constant supply of glucose and oxygen. Others, such as muscle cells or skin cells, are much better able to withstand temporary interruptions of blood flow.

The hearts of humans, other mammals, and birds consist of four chambers and four sets of valves that work together to ensure that blood flows in one direction only. Two of these chambers, the right and left atria (singular, atrium), are relatively thin-walledd structures that collect blood from the major veins and empty it into the larger, more muscular ventricles. Most of the flow of blood from the atria to the ventricles is caused by the lowered pressure produced within the ventricles as they relax. The contraction of the thin-walled atria assists in emptying them more completely.

The right and left ventricles are chambers that have powerful muscular walls whose contraction forces blood to flow through the arteries to all parts of the body. The valves between the atria and ventricles, known as atrioventricular valves, are important one-way valves that allow the blood to flow from the atria to the ventricles but prevent flow in the opposite direction. Similarly, there are valves in the aorta and pulmonary artery, known as semilunar valves, the aorta is the large artery that carries blood from the left ventricle to the body, and the pulmonary artery carries blood from  the right ventricle to the lungs. The semilunar valves prevent blood from flowing back into the ventricles. If the atrioventricular or semilunar valves are damaged of improperly, the efficiency of the heart as a pump is diminished, and the person may develop an enlarged heart or other symptoms. Maling heart valves are often diagnosed because they cause abnormal sounds as the blood passes through them. These sounds are referred to as heart murmurs. Similarly, if the muscular walls of the ventricles are weakened because of infection, damage from a heart attack, or lack of exercise, the pumping efficiency of the heart is reduced and the person develops symptoms that may include chest pain, shortness of breath, or fatigue. The pain is caused by an increase in the amount of lactic acid in the musle because the heart muscle is not getting sufficient blood to satisfy its needs. It si important to understand that the muscle of the heart receives blood from coronary arteries that are branches of tha aorta. It is not nourished by the blood that flows through its chambers. If heart muscle does not get sufficient oxygen for a peroid of time, the portion of the heart muscle not receiving blood dies. Shortness of breath and fatigue result because the heart is not able to pump blood efficiently to the lungs, muscles, and other parts of the body.

The right and left sides of the heart have slightly different jobs because they pump blood to different parts of the body. The right side of the heart receives blood from the general body and pumps it through the pulmonary arteries to the lungs, where exchange of oxygen and carbon dioxide takes place and the blood returns from the lungs to the left atrium. This is called pulmonary circulation. The larger, more powerful left side of the heart receives blood from the lungs, delivers ir through the aorta to all parts of the body, and returns it to the right atrium by way of veins. This is known as systemic circulation. Both circulatory pathway are shown in figure 18.4. the systemic circulation is responsible for gas, nutrient, and waste exchange in all parts of the body except the lungs.

Arteries and veins

Arteries and veins are the tubes that transport blood from one place to another within the body. Figure 18.5 compares the structure and of arteries and veins. Arteries carry blood away from the heart because it is under considerable pressure from the contraction of the ventricles. The contraction of the walls of the ventricles increases the pressure in the arteries. A typical pressure recorded in a large artery while the heart is constracting is about 120 millimeters of mercury. This is known as the systolic blood pressure. The pressure recorded while the heart is relaxing is about 80 millimeters of mercury. This is known as the diastolic blood pressure. A blood pressure reading includes both numbers and is recored as 120/80. (originally, blood pressure was measured by how high the pressure of the blood would cause a column of mercury to rise in a tube. Although the devices used today have dials or digital readouts and contain no mercury, they are still calibrated in mmHg.

The walls of arteries are relatively thick and muscular, yet elastic. Healthy arteries have the ability to expand as blood is pumped into them and return to normal as the pressure drops. This ability to expand absorbs some of the pressure and reduces the peak pressure within the arteries, thus reducing the likelihood that they will burst. If arteries become hardened and less resilient, the peak blood pressue rises and they are more likely to rupture. The elastic nature of the arteries is also responsible for assisting the flow of blood. When they return to normal from their stretched condition they give a little push to the blood that is flowing through them.

Blood is distributed from the large aorta through smaller and smaller blood vessels to millions of tiny capillaries. Some of the smaller arteries, called arterioles, may contract or relax to regulate the flow of blood to specific parts of the body. Major parts of the body that receive differing amounts of blood, depending on need, are the digestive system, muscles, and skin. When light-skinned people flush, it is because many arterioles in the skin have expanded, allowing a large volume of blood to flow to the capillaries of the skin. Because the blood is red, their skin reddens. Similarly, when people exercise, there is an increased blood flow to muscles to accommodate their increased bolic needs for oxygen and glucose and to get rid of wastes. Exercise also results in an increased flow of blood to the skin, which allows for heat loss. At the same time, the amount of blood flowing to the digestive system is reduced. Athletes do not eat a full meal before exercising because the additional flow of blood to the digestive system reduces the amount of blood available to go to muscles and lungs needed for vigorous exercise. Muscular cramps may results if insufficient blood is getting to the muscles.

Veins collect blood from the capillaries and return it to the heart. The pressure in these blood vessels is very low. Some of the largest veins may have a blood pressure of 0.0 mmHg for brief periods. The walls of veins are not as muscular as those of arteries. Because of the low pressure, veins must have valves that prevent the blood from flowing backward, away from the heart. Veins are often found at the surface of the body and are seen as blue lines. Varicose veins results when veins contain faulty valves that do not allow efficient return of blood to the heart. Therefore, blood pools in these veins, and they become swollen bluish networks.

Because pressure in veins is so low, muscular movements of the body are important in helping return blood to the heart. When muscles of the body contract, they compress nearby veins, and this pressure pushes blood along in the veins. Because the valves allow blood to flow only toward the heart, this activity acts as an additional pump to help return blood to the heart. People who sit or stand for long periods without using their muscles tend to have a considerable amount of blood pool in the veins of their legs and lower body. Thus less blood may be available to go to the brain and the person may faint.

Although the arteries are responsible for distributing blood to various parts of the body and arterioles regulate where blood goes, it is the of capillaries to assist in the exchange of materials between the blood and cells.

Capillaries

Capillaries are tiny, thin-walled tubes that receive blood from arterioles. They are so small that red blood cells must go through them in single file. They are so numerous that each cell in the body has a capillary located near it. It is estimated that there are about 1,000 square meters of surface area represented by the capillary surface in a typical human. Each capillary wall consists of a single layer of cells and therefore presents only a thin barrier to the diffusion of materials between the blood and cells. It is also possible for liquid to flow through tiny spaces between the individual cells of most capillaries. The flow of blood through these smallest blood vessels is relatively slow. This allows time for the diffusion of such materials as oxygen, glucose, and water from the blood to surrounding cells, and for the movement of such materials as carbon dioxide, lactic acid, and ammonia from the cells into the blood.

In addition to molecular exchange, considerable amounts of water and dissolved materials leak through the small holes in the capillaries. This liquid is known as lymph. Lymph is produced when the blood pressure forces water and some small dissolved molecules through the walls of the capillaries. Lymph bathes the cells but must eventually be returned to the circulatory system by lymph vessels or swelling will occur. Return is accomplished by the lymphatic system, a collection of thin-walled tubes that branch through out the body. These tubes collect lymph that is filtered from the circulatory system and ultimately empty it into major blood vessels near the heart. As the lymph makes its way back to the circulatory system, it is filtered by lymph nodes that contain large numbers of white blood cells that remove microorganisms and foreign particles. There are many lymph nodes located throughout the body. The tonsils and adnoids are large masses of lymph node tissue. The speen also contains large numbers of white blood cells and serves to filter the blood. The thymus gland is located over the breastbone and is large and active in children. Its primary is to produce T-lymphocytes that are distributed throughout the body and establish themselves in lymph nodes. The thymus shrinks in size in adulthood, but the descendants of the T-lymphcytes it produced earlier in life are still active throughout the lymphatic system. Figure 18.7 shows the structure of the lymphatic system.

Some of this leakage through the capillary walls is normal, but the flow is subject to changes in pressure inside the capillaries and in the tissues, and changes in the permeability of capillary wall. If pressure inside the capillary increases, more fluid may leak from the capillaries into the tissues and cause swelling. This swelling is called edema, and it is common in circulatory disorders. Another cause of edema is an increase in the permeability of the capillaries. This is commonly associated with injury to a part of the body: a sprained ankle or smashed thumb are examples you have probably experienced.

18.3 gas exchange

The lungs demonstrate this interplay between blood flow, caplillary exchange, and surface area.

Respiratory anatomy

The lungs are organs of the body that allow gas exchange to take place between the air and blood. Associated with the lungs is a set of tubes that conduct air from outside the body to the lungs. The single large-diameter trachea is supported by rings of cartilage that prevent its collapse. It branches into two major bronchi (singular, bronchus) that deliver air to smaller and smaller branches. Bronchi are also supported by cartilage. The smallest tubes, known as bronchioles, contain smooth muscle and are therefore capable of constricting. Finally, the bronchioles deliver air to cluster of tiny sacs, known as alveoli (singular, alveolus), where the exchange of gases takes place between the air and blood.

The nose, mouth, and throat are also important parts of the air-transport pathway because they modify the humidity and temperature of the air and clean the air as it passes. The lining of the trachea contains cells with cilia that beat in a direction to move mucus and foreign materials from the lungs. The foreign matter may then be expelled by swallowing, coughing, or other means. Figure 18.8 illustrates the various parts of the respiratory system.

Breathing system regulation

Breathing is the process of moving air in and out of the lungs. It is accomplished by the movement of a muscular organ known as diaphragm, which separates the chest cavity and the lungs from the abdominal cavity. In addition, muscles located between the ribs (intercostal muscles) are attached to the ribs in such a way that their contraction causes the chest wall to move outward and upward, which increases the size of the chest cavity. During inhalation, the diaghragm moves downward and the external intercostal muscles of the chest wall contract, causing the volume of the chest cavity to increase. This results in a lower pressure in the chest cavity compared to the outside air pressure. Consequently, air flows from the outside high-pressure area through the trcachea, bronchi, and bronchioles to the alveoli. During normal relaxed breathing, exhalation is accomplished by the chest wall and diaphragm simply returning to their normal, relaxed positions. Muscular contraction is not involved.

However, when the body’s demand for oxygen increases during exercise, the only way that the breathing system can respond is by exchanging the gases in the lungs more rapidly. This can be accomplished both by increasing the breathing rate and by increasing the volume of air exchangeed with each breath. Increase in volume exchanged per breath is accomplished in two ways. First, the muscles of inhalation can contract more forcefully, resulting in a greater change in the volume of the chest cavity. In addition, the lungs can be emptied more completely by contracting the muscles of the abdomen, which forces the abdominal contents upward against the diaphragm and compresses the lungs. A set of internal intercostal muscles also helps compress the chest. You are familiar with both mechanisms: when you exercise you breathe more deeply and more rapidly.

Several mechanisms can cause changes in the rate and depth of breathing, but the primary mechanism involves the amount of carbon dioxide present in the blood. Carbon dioxide is a waste product of aerobic cellular respiration and becomes toxic in high quantities because it combines with water to form carbonic acid:

As mentioned previously, if carbon dioxide can not be eliminated, the pH of the blood is lowered. Eventually, this may result in death.

Exercising causes an increase in the amount of carbon dioxide in the blood because muscles are oxidizing glucose more rapidly. This lowers the pH of the blood. Certain brain cells and specialized cells in aortic arch and carotid arteries are sensitive to changes in blood pH. When they sense a  lower blood pH, nerve impulses are sent more frequently to the diaphragm and intercostal muscles. These muscles contract more rapidly and more forcefully, resulting in more rapid, deeper breathing. Because more air is being exchanged per minute, carbon dioxide is lost from the lungs more repidly. When exercise stops, blood pH rises, and breathing eventually return to normal. Bear in mind, however, that moving air in and out of the lungs is of no unless oxygen is diffusion into the blood and carbon dioxide is diffusion out.

Lung

The lungs are organs that allow blood and air to come in close contact with each other. Air flows in and out of the lungs during breathing. The blood flows through capillaries in the lungs and is in close contact with the air in the cavities of the lungs. For oxygen to enter the body or carbon dioxide to exit the body the molecules must pass through a surface. The efficiency of exchange is limited by the surface area available. This problem is solved in the lungs by the large number of tiny sacs, the alveoli. Each alveolus is about 0.25 to 0.5 millimeters across. However, alveoli are so numerous that the total surface area of all these sacs is about 70 square meters—comparable to the floor space of many standard-sized classrooms! Associated with these alveoli are large numbers of capillaries. The walls of both capillaries and alveoli are very thin, and the close association of alveoli and capillaries in the lungs allows the easy diffusion of oxygen and carbon dioxide across these membranes.

Another factor that increases the efficiency of gas exchange is that both the blood and air are moving. Because blood is flowing through capillaries in the lungs, the capillaries continually receive new blood that is poor in oxygen and high in carbon dioxide. As blood passes by the alveoli, it is briefly exposed to the gases in the alveoli, where it gains oxygen and loses carbon dioxide. Thus, blood that leaves the lungs is high in oxygen and low in carbon dioxide. Although the movement of air in the lungs is not in one direction, as is the case with blood, the cycle of inhalation and exhalation allows air that is high in carbon dioxide and low in oxygen to exit the body and brings in new air that is rich in oxygen and low in carbon dioxide. This oxygen-rich blood is then sent to the left side of the heart and pumped throughout the body.

Any factor that interferes with the flow of blood or air or alters the effectiveness of gas exchange in the lungs reduces the efficiency of the organism. A poorly pumped heart sends less blood to the lungs, and the person experiences shortness of breath as a symptom. Similarly, diseases like asthma, which causes constriction of the bronchioles, reduce the flow of air into the lungs and inhibit gas exchange.

Any process that reduces the number of alveoli will also reduce the efficiency of gas exchange in the lungs. Emphysema is a progressive disease in which some of the alveoli are lost. As the disease progresses, those afflicted have less and less respiratory surface area and experience greater and greater difficulty getting adequate oxygen, even though they may be breathing more rapidly. Often emphysema is accompanied by an increase in the amount of connective tissue and the lungs do not streth as easily, further reducing the ability to exchange gases.

The breathing mechanism is designed to get oxygen into the bloodstream so that it can be distributed to the cells that are carrying on the oxidation of food molecules, such as glucose and fat. Obtaining food molecules involves a variety of organs and activities associated with the digestive system.

18.4 obtaining nutrients

All cells must have a continuous supply of nutrients that provides the energy they require and the building blocks needed to construct the macromolecules typical of living things. They specific functioons of various kinds of nutrients are discussed in chapter 19. this section will deal with the processing and distribution of different kinds of nutrients.

The digestive system consists of a muscular tube with several specialized segments. In addition, there are glands that secrete digestive juices into the tube. Four different kinds of activities are involved in getting nutrients to the cells that need them: mechanical processing, chemical processing, nutrient uptake, and chemical alteration.

Mechanical and chemical processing

The digestive system is designed as a disassembly system. Its purpose is to take large chunks of food and break them down to smaller molecules that can be taken up by the circulatory system and distributed to cells. The first step in this process is mechanical processing.

It is important to grind large particles into small pieces by chewing in order to increase their surface areas and allow for more efficient chemical reactions. It is also important to add water to the food, which further disperses the particles and provides the watery environment needed for these chemical reactions. Materials must also be mixed so that all the molecules that need to interact with one another have a good chance of doing so. The oral cavity and the stomach are the major body regions involved in cutting and grinding food to increase its surface area. This is another example of the surface area-to-volume concept presented at the beginning of this chapter. The watery mixture that is added to the food in the oral cavity is known as saliva, and the threee pairs of glands that produce saliva are known as salivary glands, saliva contains the enzyme salivary amylase, which initiates the chemical breakdown of starch. Saliva also lubricates the oral cavity and helps bind food before swallowing.

In addition to having taste buds that help identify foods, the tongue performs the important service of helping position the food between the teeth and pushing it to the back of the throat for swallowing. The oral cavity is very much like a food processor in which mixing and grinding take place. Figure 18.12 describes and summarizes the s of these structures.

Once the food has been chewed, it is swallowed and passs down the esophagus to the stomach. The process of swallowing ivolves a complex series of events. First, a ball of food, known as a bolus, is formed by the gongue and moved to the back of the mouth cavity. Here it stimulates the walls of the throat, also known as the pharyx. Nerve endings in the lining of the phyrynx are stimulated, causing a reflex contraction of walls of the esophagus, which transports the bolus to the stomach. Because both food and air  pass through the pharynx, it is important to prevent food from getting into the lungs. During swallowing the larynx is pulled upward. This causes a flap of tissue called the epiglottis to cover the opening to the trachea and prevent food from entering the trachea. In the stomach, additional liquid, called gastric juice, is added to the food. Gastric juice contains enzyems and hydrochloric acid. The major enzyme of the stomach is pepsin, which initiates the chemical breakdown of protein. The pH of gastric juice is very low, generally around pH 2. consequently, very few kinds of bacteria or protozoa emerge from the stomach alive. Those that do have special protective features that allow them to survive as they pass through the stomach. The entire mixture is churned by the contractions of the three layers of muscle in the stomach wall. The combined activities of enzymatic breakdown, chemical breakdown by hydrochloric acid, and mechanical processing by muscular movement result in a throughly mixed liquid called chyme. Chyme eventually leaves the stomach through a valve known as the pyloric sphincter and enters the small intestine.

The first part of the small intestine is known as the duodenum. In addition to producing enzymes, the duodenum secretes several kinds of hormones that regulate the release of food from the stomach and the release of secretions from the pancreas and liver. The pancreas produces a number of different digestive enzymes and also secretes large amounts of bicarbonate ions, which neutralize stomach acid so that the pH of the duodenum is about pH 8. the liver is a large organ in the upper abdomen that performs several s. One of its s is the secretion of bile. When bile leaves the liver, it is stored in the gallbladder prior to being released into the duodenum. When bile is released from the gallbladder, it assists mechanical mixing by breaking large fat globules into smaller particles. This process is called emulsification.

Emulsification is important because fats are not soluble in water, yet the reactions of digestion must take place in a water solution. Bile causes large globules of fat to be broken into much smaller units (increasing the surface area-to-volume ratio) much as soap breaks up fat particles into smaller globules that are suspended in water and washed away. The activity of bile is important for the further digestion of fats in the intestine.

Along the length of the intestine, additiona watery juices are added until the mixture reaches the large intestine. The large intestine is primarily involved in reabsorbing the water that has been added to the food tube along with saliva, gastric juice, bile, pancreatic secretions and intestinal juices. The large intestine is also home to variety of different kinds of bacteria. Most live on the undigested food that makes it through the small intestine. Some provide additional benefit by producing vitamins that can be absorbed from the large intestine. A few kinds may cause disease.

Several different kinds of enzymes have been mentioned in this discussion. Each is produced by a specific organ and has a specific . Chapter 5 introduced the topic of enzymes and how they work. Some enzymes, such as those involved in glycolysis, the krebs cycle, and protein synthesis are produced and used inside cells; others, such as the digestive enzymes, are produced by cells and secreted into the digestive tract. Digestive enzymes are simply a special class of enzymes and have the same characteristics as the enzymes you studied previously. They are protein molecules that speed up specific chemical reactions and are sensitive to changes in temperature or pH. The various diegestive enzymes, the sites of their production, and their s are listed in table 18.2.

Nutrients uptake

The process of digestion results in a variety of simple organic molecules that are available for absorption from the tube of the gut into the circulatory system. As we move simple sugars, amino acids, glycerol, and fatty acids into the circulatory system, we encounter another situation where surface area is important. The amount of material that can be taken up is limited by the surface area available. This problem is solved by increasing the surface area of the intestinal tract in several ways. First, the small intestine is a very long tube; the longer the bube, the greater the intestinal surface area. In a typical adult human it is about 3 meters long. In addition to length, the lining of the intestine consists of millions of fingerlike projections called villi, which increase the surface area. When we examine the cells that make up the villi, we find that they also have folds in their surface membranes. All of these characteristics increase the surface area available for the transport of materials from the gut into the circulatory system. Scientists estimate that the cumulative effect of all of these features produces a total intestinal surface area of about 250 square meters. That is equivalent to about half the area of a football field.

The surface area by itself would be of little if it were not for the intimate contact of the circulatory system with this lining. Each villus contains several capillaries and a branch of the lymphatic system called a lacteal. The close association between the intestinal surface and the circulatory system and the lymphatic systems allows for the efficient uptake of nutrients from the cavity of the gut into the circulatory system.

Several different kinds of processes are involved in the transport of materials from the intestine to the circulatory system. Some molecules such as water and many icons, simply diffuse through the wall of the intestine into the circulatory system. Other materials, such as amino acids and simple sugars, are assisted across the membrane by carrier molecules. Fatty acids and glycerol are absorbed into the intestinal lining cells where they are resynthesized into fats and enters lacteals in the villi. Because the lacteals are part of the lymphatic system, which eventually empties its contents into the circulatory system, fats are also transported by the blood. They just reach the blood by a different route.

Chemical alteration: the role of liver

When the blood leaves the intestine, it flows directly to the liver through the hepatic portal vein. Portal veins are blood vessels that collect blood from capillaries in one part of the body and deliver it to a second set of capillaries in another part of the body without passing through the heart. Thus the hepatic portal vein collects nutrient-rich blood from the intestine and delivers it directly to the liver. As the blood flows through the liver, enzymes in the liver cells modify many of the molecules and particles that enter them. One of the s of the liver is to filter any foreign organisms from the blood that might have entered through the intestinal cells. It also detoxifies many dangerous molecules that might have entered with the food.

Many foods contain toxic substances that could be harmful if not destroyed by the liver. Ethy alcohol is one obvious example. Many plants contain various kinds of toxic molecules that are present in small quantities and could accumulated to dangerous levels if the liver did not perform its role of detoxification.

In addition, the liver is responsible for modifying nutrient molecules. The liver collects glucose molecules and synthesizes glycogen, which can be stored in  the liver for later use. When glucose is in short supply, the liver can convert some of its stored glucogen back into glucose. Although amino acids are not stored, the liver can change the relative numbers of different amino acids circulating in the blood. It can remove the amino group from one kind of amino acid and attach it to a different carbon skeleton, generating a different amino acid. The liver is also able to take the amino acid group off amino acids so that what remains of the amino acid can be used in aerobic respiration. The toxic amino groups are then converted to urea by the liver. Urea is secreted back into the bloodstream and is carried to the kidneys for disposal in the urine.

18.5 Waste disposal

Because cells are modifying molecules during blic processes, harmful waste products are constantly being formed. Urea is a common waste; many other toxic materials must be eliminated as well. Among these are large numbers of hydrogen ions produced by bolism. This excess of hydrogen ions must be removed from the blood stream. Other molecules, such as water and salts, may be consumed in excessive amounts and must be removed. The primary organs involved in regulating the level of toxic or unnecessary molecules are the kidneys.

Kidney structure

The kidneys consist of about 2.4 million tiny units called nephrons. At one end of a nephron is cup-shaped structure called Bowman’s capsule, which surrounds a knot of capillaries known as glomerulus. In addition to Bowman’s capsule, a nephron consists of three distinctly different regions: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. The distal convoluted tubule of a nephron is connected to a collecting duct that transports fluid to the  ureters, and ultimately to the urinary bladder, where it is stored until it can be eliminated.

Kidney

As in the other systems discussed in this chapter,the excretory system involves a close connection between the circulatory system and a surface. in this case the large surface is provided by the walls of the millions of nephrons, which are surrounded by capillaries. Three major activities occur at these surfaces: filtration, reabsorption, and secretion. The glomerulus presents a large surface for the filtering of material from the blood to Bowman’s capsule. Blood that enters the glomerulus is under pressure from the muscular contraction of heart. The capillaries of the glomerulus are quite porous and provide a large surface area for the movement of water and small dissolved molecules from the blood into Bowman’s capsule. Normally, only the smaller molecules, such as glucose, amino acids, and ions, are able to pass through the glomerulus into the bowman’s capsule at the end of the nephron. The various kinds of blood cells and larger molecules like proteins do not pass out the blood into the nephron. This physical filtration process allows many kinds of molecules to leave the blood and enter the nephron. The volume of material filtered in this way through the approximately 2.4 million nephrons of our kidneys is about 7.5 liters per hour. Because your entire blood supply is about 5 to 6 liters, there must be some method of recovering much of this fluid.

     Surrounding the various portions of the nephron capillaries that passively accept or release molecules on the basis of diffusion gradients. The wall of the nephron are made of cells that actively assist in the transport of materials. Some molecules are reabsorbed from the nephron and picked up by the capillaries that surround them, whereas other molecules are actively secreted into the nephron from the capillaries. Each portion of the nephron has cells with specific secretory abilities.

     The proximal convoluted tubule is primarily responsible for reabsorbing valuable materials from the fluid moving through it. Molecules like glucose, amino acids, and sodium ions are actively transported across the membrane of the proximal convoluted tubule and returned to the blood. In addition, water moves across the membrane because it follows the absorbed molecules and diffuses to area where water molecules are less common. By the time the fluid has reached the end of the proximal convoluted tubule, about 65% of the fluid has been reabsorbed into the capillaries surrounding this region.

   The next portion of the bubule, the loop of Henle, is primarily involved in removing additional water from the nephron. Although the details of the mechanism are complicated, the principles are rather simple. The cells of the ascending loop of Henle actively transport sodium ions from the nephron into the space between nephrons where sodium ions accumulate in the fluid that surrounds the loop of Henle. The collecting ducts pass through this region as they carry urine to the ureters. Because the area these collecting ducts pass through is high in sodium ions, water within the collecting ducts diffuses from the ducts and is picked up by surrounding capillaries. However, the ability of water to pass through the wall of collecting duct is regulated by hormones. Thus it is possible to control water loss from the body by regulating the amount of water lost from the collecting ducts. For example. If you drank a liter of water or some other liquid, the excell water would not be allowed to leave the collecting duct and would exit the body as part of the urine. However, if you were dehydrated, most of the water passing through the collecting ducts would be reabsorbed, and very little urine would be produced. The primary hormone involved in regulating water loss is antidiuretic hormone. When the body has excess water, cells in the hypothalamaus of the brain respond and send a signal to the pipuitary and only a small amount of ADH is released and water is lost. When you are dehydrated these same brain cells cause more ADH to be released and water leaves the collecting duct and is returned to the blood.

The distal convoluted tubule is primarily involved in fine-tuning the amounts of various kinds of molecules that are lost in the urine. Hydrogen ions, sodium ions, chloride ions, potassium ions, and smmonium ions are regulated in this way.

    Some molecules that pass through the nephron are relatively unaffected by the various activities going on in the kidney. One of these is urea, which is filtered through the glomerulus into Bowman’s capsule. As it passes through the nephron, much of it stays in the tubule and is eliminated in the urine. Many other kinds of molecules , such as minor bolic waste products and some drugs, are also treated in this manner. Figure 18.16 summarizes the major s of the various portions of the kidney tubule system.