Answers to Review Questions
Chapter 52 (p. 1052)
1. 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.
2. The three major components of blood plasma are metabolites and wastes, salts and ions, and proteins.
3. 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. Arteries contain extra elastic fibers in their walls, allowing them to recoil with each heartbeat. The degree of elasticity decreases as the diameter of the vessels decrease. Smaller vessels have more smooth muscle to resist bursting. Veins have less smooth muscle since the pressure is one-tenth less than that in the arteriole system. Large veins have valves to prevent backflow of blood. Capillaries possess only the single layer of endothelium to ensure gas, nutrient, and metabolite transfer from blood to cells.
4. Resistance to blood flow is inversely proportional to vessel diameter (r4). A decrease in diameter by half increases the resistance 16 times. Resistance to flow is regulated by constriction/dilation of the smooth muscle layers in the arterioles. Some organs also have precapillary sphincters to regulate flow into certain capillary beds.
5. The pressure created when the movement of body muscles squeezes vessels drives the flow of fluid. The direction of flow is one-way into the venous blood flow.
6. The sequence of chambers in a fish heart is 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 amphibian heart has two atria. The right receives deoxygenated blood from the body and the left receives oxygenated blood from the lungs. Blood from both travels to the single ventricle, although little mixing occurs since the conus arteriosus is partially divided by a septum, diverting deoxygenated blood to the lungs and oxygenated blood to the aorta. Amphibians were the first vertebrates to exhibit this new pulmonary circulation.
7. Baroreceptors respond to a change in blood pressure and relay the information to the cardiac control center of the medulla oblongata, which then modifies the activity of the autonomic neurons that regulate heart and blood vessels. ADH causes the kidneys to excrete less urine, retain more water in the bloodstream, and hence, increases blood pressure. ADH is secreted when an increased osmotic concentration of the blood is sensed. Aldosterone promotes the retention of Na+ in the kidney, conserving water and increasing blood pressure. Aldosterone is secreted (via rennin-angiotensin-angiotensin I-angiotensin II system) when the kidneys sense a decrease in blood flow.