Concept 32.1   Circulatory Systems Can Be Open or Closed

• Animals with a high degree of division of labor need a circulatory system that transports respiratory gases, nutrients, hormones, and metabolic wastes throughout the body at high rates.
• O₂ transport is the most urgent function of the circulatory system in most types of animals. Because of this, animals with relatively high rates of O₂ consumption tend to have relatively high-performance circulatory systems. Insects are the exception that proves the rule. Although many insects have high rates of O₂ consumption, insects do not have high-performance circulatory systems because their tracheal breathing system, not their circulatory system, meets needs for O₂ transport.
• In closed circulatory systems—found in vertebrates, annelid worms, squid, and octopuses—the blood never leaves a system of vessels. In open circulatory systems—found in arthropods and most mollusks—the blood leaves vessels and bathes tissue cells directly. One advantage of closed circulatory systems is their ability to direct blood selectively to specific tissues.
• The blood in a closed circulatory system flows from the heart, through arteries, to the microscopic vessels of the microcirculation, which consists of arterioles, capillaries, and venules. Veins then carry blood back to the heart.
• Capillaries are the smallest vessels and are the principal sites of exchange of gases, nutrients, and other substances between the blood and the tissues. Arterioles control the rate of blood flow into the capillaries. Review Figure 32.2 and Figure 32.3
• In open circulatory systems, the heart typically pumps blood into arteries that carry the blood for at least a short distance. These vessels then end and the blood flows out into the animal’s tissues. The blood makes its way back to the heart through channels between the tissue cells called sinuses and lacunae. Review Figure 32.4
• In open circulatory systems there is no distinction between blood and interstitial fluid.

Concept 32.2   The Breathing Organs and Systemic Tissues Are Usually, but Not Always, in Series

• The systemic circuit consists of the blood vessels (or blood channels in the case of an open circulatory system) that carry blood to and from the systemic tissues. The breathing-organ circuit consists of the blood vessels that carry blood to and from the breathing organs (lungs or gills).
• Arteries are vessels that carry blood away from the heart. Veins carry blood toward the heart.
• In most animals the systemic circuit and breathing-organ circuit are connected in series: the blood flows from one circuit to the other sequentially. Circulation in series is the most efficient way to deliver O₂ to the systemic tissues. Review Figure 32.5A, Figure 32.5B and ACTIVITY 32.1
• Adult amphibians and non-avian reptiles have a partially divided ventricle that does not ensure series circulation. Some of these animals, such as turtles, can shunt blood away from the breathing-organ circuit at times when doing so may be advantageous, such as during prolonged diving. Review Figure 32.5C

Concept 32.3   A Beating Heart Propels the Blood

• Vertebrate hearts are multi-chambered and myogenic; heartbeats are initiated by specialized muscle cells within the heart.
• The simplest vertebrate heart, found in fish, is two-chambered. It has an atrium that receives blood from the body and a ventricle that pumps blood out of the heart. Amphibians and non-avian reptiles have a three-chambered heart with two atria and one partially divided ventricle.
• The heart of mammals and birds has four chambers—two atria and two ventricles—with valves to prevent backflow. Blood flows from the right atrium and ventricle to the lungs, then to the left atrium and ventricle, and then to the rest of the body. Review Figure 32.6 and ACTIVITY 32.2
• The cardiac cycle has two phases: systole, when the ventricles contract, and diastole, when the ventricles relax. Review Figure 32.7 and ANIMATED TUTORIAL 32.1
• The heartbeat in mammals is initiated by pacemaker cells in the sinoatrial node that spontaneously depolarize, triggering a wave of depolarization that spreads through the atria and then spreads with a brief delay into the ventricles via the atrioventricular node and the conducting system. Review Figure 32.8
• The autonomic nervous system controls heart rate by changing the rate of depolarization of the pacemaker cells. Norepinephrine from sympathetic nerves increases heart rate, and acetylcholine from parasympathetic nerves decreases it. Review Figure 32.9
• In mammals and many other vertebrates, a coronary circulatory system supplies O₂ to the cells of the heart muscle (myocardium).
• The depolarization of heart muscle cells can be detected on the surface of the body and recorded as an electrocardiogram (ECG or EKG). Review Figure 32.10
• The hearts of crustaceans have one chamber and are neurogenic; the muscle cells of the heart require nervous stimulation to beat. Review Figure 32.11

Concept 32.4   Many Key Processes Occur in the Vascular System

• Arteries are composed of muscle fibers and elastic fibers that enable them to dampen the surge in pressure when the heart beats and store in their stretched state some of the energy imparted to the blood by the heart. Veins have thinner walls and have a high capacity for storing blood. Review Figure 32.12 and ACTIVITY 32.3
• The pressure and linear velocity of blood vary greatly as it flows through the vascular system. Blood pressure is high in arteries and low in veins. Blood velocity is lowest in capillary beds. Review Figure 32.13
• Veins have one-way valves that assist in returning blood to the heart under low pressure. Review Figure 32.14
• Heat loss from certain regions of an animal’s body—such as the extremities of the arctic fox and the red swimming muscles of tuna—can be reduced by countercurrent heat exchange. Review Figure 32.15, Figure 32.16 and Figure 32.17
• High blood pressure forces fluid out of the blood as it travels through the initial part of a capillary bed; osmotic pressure draws some of the fluid back into the blood as blood pressure falls. Review Figure 32.18
• The excess fluid that does not reenter the blood as it travels through capillary beds is called lymph. The lymphatic system collects this fluid and returns it to the blood. Review Figure 32.19

Concept 32.5   The Blood Transports O₂ and CO₂

• In vertebrates, blood consists of a liquid plasma and cellular components (red blood cells, or erythrocytes; white blood cells, or leukocytes; and platelets). Red blood cells are produced in the bone marrow. Hemoglobin, the respiratory pigment in red blood cells, binds O₂ reversibly. Each hemoglobin molecule can carry a maximum of four O₂ molecules.
• Hemoglobin binds O₂ when the partial pressure of O₂ is high and releases it when the partial pressure is low. Therefore hemoglobin picks up (loads) O₂ as it flows through a breathing organ and gives up (unloads) O₂ in the systemic tissues. This chemical behavior can be summarized in an oxygen equilibrium curve. Review Figure 32.20, ACTIVITY 32.4, and ANIMATED TUTORIAL 32.2
• Hemocyanin is the respiratory pigment in mollusks and arthropods.
• CO₂ reacts with blood water to form bicarbonate ions (HCO₃^–) and is transported mostly in the form of bicarbonate.