ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Forty-Six Review Answers 1. Nitrogen is the most common gas in dry air, occupying 78.9% by volume. Oxygen occupies 0.95%, and carbon dioxide 0.03%. 2. Fick's Law of Diffusion is where R = The rate of diffusion, in this case, the amount of oxygen or carbon dioxide diffusing per unit of time; D = the diffusion constant, whose value depends upon the material through which the diffusion is occurring; A = the area over which diffusion takes place; _p = the difference in partial pressures in the two regions, in this case, the interior of the organism and the external environment; and d = the distance across which diffusion takes place. Over evolutionary time, the rate of diffusion (R) has been optimized. 3. Gases can only diffuse at a specific rate through a thin enough area; this serves to put severe limitations on the sizes organisms can attain if they are respiring entirely through diffusion. As an organism increases in size, its surface-to-volume ratio decreases, and those areas more distantly removed from the surface will receive less or no oxygen than regions near the surface. 4. The disadvantages associated with an external gill are that the water must constantly circulate over the gills, which requires a lot of energy to move the highly branched organs through the water. Gills associated with branchial chambers are on the inside of the body, and the various structures formed to move water over the gill surface meet with less resistance. The counter-current flow of oxygenated water and deoxygenated blood makes the gills of fish the most efficient of all respiratory organs. 5. The two major kinds of respiratory systems in terrestrial animals are the insect trachea and lungs. In trachea the external openings close whenever the body CO2 levels fall below a certain level; in lungs air flows in and out through the same passage, reducing both O2 diffusion and water loss. 6. This greater efficiency is achieved by their greater surface area and more numerous alveoli. In birds air flow is one way. In terms of associated blood flow, there is a countercurrent flow of blood versus air. 7. Air flow in the human lung is as follows: air enters the nostrils, goes into the nasal cavity, and then goes through the back of the mouth to the larynx and down the trachea. Next it goes into one of the branching bronchi, into the bronchioles, and into the alveoli. This is a one-cycle system in that air completes one cycle with only a single inhalation and exhalation. The lungs are connected at the junction of the lung and the bronchus and supported by the water tension of the interpleural fluid between the two pleural membranes. 8. The carrier molecule is hemoglobin, and it is based on iron. It is contained and synthesized in erythrocytes or red blood cells. The oxygen-hemoglobin dissociation curve indicates the amount of O2 that combines with hemoglobin plotted against the partial pressure of O2. The Bohr effect occurs when a high CO2 concentration lowers blood pH, causing hemoglobin to dissociate from O2 at a greater rate than normal. Metabolically this means that blood unloads more O2 where it is especially needed. Carbon monoxide binds to hemoglobin 300 times more easily than oxygen and dissociates much less readily. Medically, a small amount can irreversibly prevent O2 from getting to cells. 9. One-fifth of the carbon dioxide is carried bound to hemoglobin, and the rest diffuses into the cytoplasm of red blood cells. Increased absorption of CO2 is achieved in red blood cells as CO2 is catalyzed by carbonic anhydrase to dissociate into bicarbonate and H+. CO2 is unloaded at the lungs because lower CO2 levels cause a reverse reaction to occur; at low CO2 levels hemoglobin has a greater affinity for O2 than CO2, causing the cells to give up the CO2 for new O2. 10. Respiration is a primitive function, carried out by specific areas in the brainstem. Chemoreceptors in the circulatory system are sensitive to CO2 levels in the blood. When they sense an increase in CO2, they will tell the brain stem to accelerate breathing. 11. A closed circulatory system contains blood within a muscular hollow pumping heart and blood vessels. An open circulatory system conventionally has a pumping organ; the blood moves freely in the body cavity without the constraints of vessels. Insects have open circulatory systems, vertebrates have closed circulatory systems. A closed circulatory system permits more efficient delivery of oxygen and nutrients to the tissues of the organism. 12. The four functions of the vertebrate circulatory system are: nutrient and waste transport, oxygen and carbon dioxide transport, temperature regulation, and hormone circulation. The essential structure of a blood vessel is an inner smooth, single-celled layer of endothelium, surrounded by a layer of elastic fibers, surrounded by a smooth muscle layer, surrounded by a connective tissue layer by which the vessel is anchored into place. Capillaries do not exhibit this level of structural organization. 13. Arteries carry blood away from the heart, while veins return blood to the heart. Flow resistance is inversely proportional to the fourth power of the tube radius. Therefore small vessels exhibit greater flow resistance. The greatest amount of resistance is at the capillaries, which is where gas and metabolite exchange between the blood and tissues occurs. 14. A precapillary sphincter is a ring of smooth muscle at the entrance to a capillary bed which can close off the flow to a specific capillary. 15. The lymphatic system gathers liquid from body tissues as it seeps out at the capillaries and returns the fluid to the cardiovascular system. It is an open system. The flow of fluid is driven by the pressure created when vessels are squeezed by the movement of body muscles. The direction of flow is one-way into the venous blood flow. 16. The three major components of blood plasma are metabolites and wastes, salts and ions, and proteins. Erythrocytes function in gas exchange, leukocytes function in protection and defense, and platelets function in clotting. 17. The sequence of chambers in this type of heart are sinus venosus, atrium, ventricle, and conus arteriosus. The sinus venosus and atrium are collecting chambers, and the ventricle and conus arteriosus are pumping chambers. The major advantage of this heart is that blood is pumped directly to the gills, thus all blood delivered to the body is fully oxygenated. The major disadvantage of this heart is that the flow of blood through the gill capillary bed reduces its force, so that the flow through the body is generally sluggish. 18. The excitatory tissue of the SA node in the wall of the right atrium is derived from the sinus venosus of ancestral vertebrates. It is the point of origin for each heartbeat, retained in higher vertebrates, and serves a pacemaker function. 19. The depolarization travels through the atria to the AV node, and then to the Bundle of His (atrioventricular bundle), whereupon the wave moves into the ventricles. When the atria contract, the ventricles relax and vice-versa. The tricuspid, pulmonary semilunar, bicuspid, and aortic semilunar valves keep blood flowing in a single direction. Blood flow is as follows: blood flows from the body into the vena cava, which empties into the right atrium and then proceeds into the right ventricle via the tricuspid valve. From here, blood moves into the pulmonary arteries via the pulmonary semilunar valve to the lungs, and is returned to the heart by the pulmonary veins. Freshly oxygenated blood is dumped by the pulmonary veins into the left atrium, whereupon it proceeds into the left ventricle via the bicuspid valve. Blood then leaves the heart through the aorta via the aortic semilunar valve and travels to the body, where the cycle then repeats itself. In one cardiac cycle, a wave of depolarization is initiated at the SA node, high up in the wall of the right atrium. 20. Baroreceptors respond to a change in blood pressure, and relay the information to the cardiac rhythm region of the brainstem, which then compensates. ADH causes the kidneys to excrete less urine, retaining more water in the bloodstream, and hence, increasing blood pressure. Aldosterone ultimately has the same effect, by concentrating sodium ions in the kidney, more water is conserved, and blood pressure increases.