Summary

Hearts

• The output of a heart, known as the cardiac output, depends on the heart rate and stroke volume.
• The cells in the heart muscle, the myocardium, must have means of receiving O₂. In some hearts the myocardium is spongy, and blood flowing through the heart chambers flows through the spongy spaces, supplying O₂ to the cells. In other hearts, including those of mammals, the myocardium is compact and is supplied with blood and O₂ by means of coronary blood vessels.
• A heart is mywogenic if the depolarization impulses required for heartbeats originate in muscle cells or modified muscle cells. A heart is neurogenic if the impulses originate in neurons. Vertebrate hearts are myogenic. Hearts of adult decapod crustaceans are neurogenic.
• In the mammalian heart, the sinoatrial node in the wall of the right atrium acts as pacemaker, initiating waves of depolarization. Conduction of a wave of depolarization from the atria to the ventricles occurs through the conducting system, which ensures both that the ventricles contract later than the atria and that the entire ventricular myocardium contracts approximately at once.
• When a part of the myocardium is in the process of contracting, voltage differences in the extracellular fluids develop between regions of muscle cells that have already undergone depolarization and regions that have not. These differences can be detected on the body surface. The electrocardiogram is a recording of such differences as a function of time.
• The rate and force of heart contraction are governed by nervous, endocrine, and intrinsic controls.

Principles of Pressure, Resistance, and Flow in Vascular Systems

• Blood pressure is measured relative to environmental pressure; it is the extent to which the pressure in the blood exceeds that in the environment.
• During steady flow of blood through horizontal vessels or systems of vessels, the rate of blood flow is directly proportional to the difference of pressure between the inlet and outlet. It is also inversely proportional to vascular resistance. Thus the equation describing blood flow (Equation 25.3) is analogous to Ohm’s law. According to the Poiseuille equation (Equation 25.2), vascular resistance varies inversely with the fourth power of vessel radius.
• Blood pressure declines during the flow of blood through vessels because the potential energy represented by the pressure is converted to kinetic energy, which then is converted to heat in overcoming viscous resistance to flow. During steady flow through a horizontal system, this drop in blood pressure is a measure of the energy cost of perfusion.

Circulation in Mammals and Birds

• Mammals and birds, like virtually all other vertebrates, have closed circulatory systems, meaning that the blood always remains within blood vessels lined with vascular endothelium.
• The pulmonary and systemic circuits are connected in series. The left ventricle develops high pressures to force blood through the high-resistance systemic circuit. The right ventricle develops lower pressures to force blood through the low-resistance pulmonary circuit.
• In the systemic circuit, arteries convey blood over relatively long distances with little loss of blood pressure; arteries also perform pressure-damping and pressure-reservoir functions because of their elasticity. Arterioles in the systemic microcirculatory beds exert fine spatial and temporal control over blood flow by contraction and relaxation of the smooth muscles in their walls (vasomotor controls). The capillaries are the principal sites of exchange between the blood and systemic tissues because their walls consist of just a single layer of fenestrated endothelial cells rich in aquaporins, and because they are densely distributed.
• As blood flows through systemic capillaries, blood pressure tends to force fluid to pass outward through the capillary walls by ultrafiltration. The colloid osmotic pressure of the blood plasma tends to cause fluid movement into the blood. The net effect of this interplay is a loss of fluid, which is picked up by the lymphatic system. The lower blood pressures in the pulmonary circuit help to prevent pulmonary flooding (edema).
• During exercise, cardiac output is augmented by increases in both heart rate and stroke volume. Arterial blood pressure does not rise excessively because vascular resistance is decreased, mainly by vasodilation in active muscles.

Circulation in Fish

• In most fish, the heart pumps blood to the gills, after which the blood passes through the systemic circuit before returning to the heart. The gill and systemic circuits are arranged in series.
• In air-breathing fish, blood leaving the air-breathing organ (ABO) usually mixes with systemic venous blood. Thus the circulation of the ABO is in parallel with the systemic circuit. This arrangement decreases the efficiency of O₂ transport but may help oxygenate the myocardium.
• Lungfish have a modified central circulation in which blood from the lungs enters the left side of the atrium, and the atrium, ventricle, and conus arteriosus are partly divided. Deoxygenated and oxygenated blood can be kept relatively separate and pumped selectively to the lungs and systemic circuit. Redistribution of cardiac output is possible, and occurs in synchrony with the intermittent breathing cycle.

Invertebrates with Open Circulatory Systems

• Most invertebrates have open circulatory systems in which blood leaves discrete vessels and flows through systems of lacunae and sinuses, where it comes into contact with ordinary tissue cells (accounting for the fact the blood is sometimes called hemolymph).
• Animals with open circulatory systems typically have a heart, and they may have extensive systems of blood vessels, even including capillary beds. The blood ultimately leaves the vessels, however.
• Open circulatory systems tend to be characterized by relatively small changes of blood pressure across the systemic circuit compared with those in closed circulatory systems. Resistance is low in open systems because blood is not forced through capillary beds in most or all tissues. Thus the rate of blood flow may be high despite the small pressure changes.
• Little is known about the spatial and temporal control of blood flow in animals having open circulatory systems. Some control is known to be possible, however (as by cardioarterial valves in crustaceans), and control is probably more sophisticated than generally assumed in the past.