Chapter Summary

• The metabolic needs of the cells of many small animals are met by direct exchange of materials with the external medium. The metabolic needs of the cells of larger animals are met by a circulatory system that transports nutrients, respiratory gases, and metabolic wastes throughout the body.

• In open circulatory systems, extracellular fluid called hemolymph leaves vessels and percolates through tissues before returning to the heart. In closed circulatory systems, blood—a portion of the extracellular fluid—is contained in a system of vessels. Closed circulatory systems have the ability to selectively direct blood, and therefore hormones and nutrients, to specific tissues. Review Figure 49.1

• The circulatory system of vertebrates consists of a heart and a closed system of vessels containing blood that is separate from the interstitial fluid. Arteries and arterioles carry blood from the heart; capillaries are the site of exchange between blood and interstitial fluid; venules and veins carry blood back to the heart.

• The vertebrate circulatory system evolved from a single circuit in fishes to partially or completely separate pulmonary and systemic circuits in amphibians, reptiles, and mammals.

• In the single-circuit system of fishes, blood flow is unidirectional and is propelled by one-way valves between the sinus venosus and the atrium, between the atrium and the ventricle, and between the ventricle and the bulbus arteriosus.

• In birds and mammals, blood circulates through two completely separate circuits. The pulmonary circuit transports blood between the heart and lungs, and the systemic circuit transports oxygen-rich blood between the heart and tissues. See Activity 49.1

• The mammalian heart has four chambers. Valves in the heart prevent the backflow of blood. Review Figure 49.2, Activity 49.2

• The cardiac cycle has two phases: systole refers to the contraction phase, and diastole refers to the relaxation phase. The sequential heart sounds (“lub-dup”) are made by the closing of the heart valves. Review Focus: Key Figure 49.4, Animation 49.1

• Blood pressure can be measured using a sphygmomanometer and a stethoscope. Review Figure 49.5

• Pacemaker cells of the sinoatrial node set the heart rate as a result of the properties of their ion channels. The autonomic nervous system controls heart rate: sympathetic activity increases heart rate, and parasympathetic activity decreases it by altering the rate of depolarization of the pacemaker cell resting membrane potentials following the termination of systole. Review Figures 49.6, 49.7

• The sinoatrial node controls the cardiac cycle by initiating a wave of depolarization in the atria, which is conducted to the ventricles through a system consisting of the atrioventricular node, bundle of His, and Purkinje fibers. Review Figure 49.8

• Sustained contraction of ventricular muscle cells is due to long-duration action potentials that are generated by voltage-gated Na⁺ and Ca²⁺ channels. Review Figure 49.9

• An electrocardiogram (ECG or EKG) records electrical events caused by the depolarizations and repolarizations of the cardiac muscles. Review Figure 49.10

• Blood consists of a plasma portion (water, salts, and proteins) and a cellular portion (erythrocytes or red blood cells, platelets, and white blood cells). All of the cellular components are produced from stem cells in the bone marrow. Review Figure 49.11

• Erythrocytes transport oxygen. Their production in the bone marrow is stimulated by erythropoietin, which is produced in response to hypoxia (low oxygen levels) in the tissues.

• Platelets, along with circulating proteins, are involved in blood clotting, which results in a meshwork of fibrin threads that help seal damaged vessels. Review Figure 49.12

• Abundant smooth muscle cells allow vessels to change their diameter, altering their resistance and thus blood flow. Arteries have elastic fibers that enable them to withstand high pressures. Review Figure 49.13, Activity 49.3

• Capillary beds are the site of exchange of materials between blood and tissue fluid.

• Starling’s forces suggest that blood volume is maintained in the capillary beds by an exchange of fluids driven by both blood hydrostatic pressure and osmotic pressure. Review Figure 49.15

• Bicarbonate ions in the blood plasma contribute to the osmotic forces that draw water back into capillaries.

• The ability of a specific molecule to cross a capillary wall depends on the architecture of the capillary and the chemical characteristics of the molecule.

• Veins have a high capacity for storing blood. Aided by gravity, by contractions of skeletal muscle, and by the actions of breathing, they return blood to the heart. Review Figure 49.16

• The Frank–Starling law describes forces that increase cardiac output, such as stretch of the cardiac muscles cells caused by increased venous return.

• The lymphatic system returns the interstitial fluid to the blood.

• Blood flow through capillary beds is controlled by local autoregulatory mechanisms, hormones, and the autonomic nervous system. Review Figure 49.18, Activity 49.4

• Blood pressure is controlled in part by the hormones ADH and angiotensin, which stimulate contraction of blood vessels. Review Figure 49.19

• Heart rate is controlled by the autonomic nervous system, which responds to information about blood pressure and blood composition that is integrated by regulatory centers in the medulla. Review Figure 49.20