The circulation is an evolutionary consequence of body size
Isolated single cells and small organisms do not have a circulatory system. They can meet their metabolic needs by the simple processes of diffusion and convection of solutes from the external to the internal milieu (Fig. 17-1A). The requirement for a circulatory system is an evolutionary consequence of the increasing size and complexity of multicellular organisms. Simple diffusion (see p. 108) is not adequate to supply nutrients to centrally located cells or to eliminate waste products; in large organisms, the distances separating the central cells from the external milieu are too long. A simple closed-end tube (see Fig. 17-1B), penetrating from the extracellular compartment and feeding a central cell deep in the core of the organism, would not be sufficient. The concentration of nutrients inside the tube would become very low at its closed end because of both the uptake of these nutrients by the cell and the long path for resupply leading to the cell. Conversely, the concentration of waste products inside the tube would become very high at the closed end. Such a tube represents a long unstirred layer; as a result, the concentration gradients for both nutrients and wastes across the membrane of the central cell are very small.
FIGURE 17-1 Role of the circulatory system in promoting diffusion. In C, nutrients and wastes exchange across two barriers: a surface for equilibration between the external milieu and blood, and another surface between blood and the central cell. Inset, Blood is the conduit that connects the external milieu (e.g., lumina of lung, gut, and kidney) to the internal milieu (i.e., extracellular fluid bathing central cells). In D, the system is far more efficient, using one circuit for exchange of gases with the external milieu and another circuit for exchange of nutrients and nongaseous wastes.
In complex organisms, a circulatory system provides a steep concentration gradient from the blood to the central cells for nutrients and in the opposite direction for waste products. Maintenance of such steep intracellular-to-extracellular concentration gradients requires a fast convection system that rapidly circulates fluid between surfaces that equilibrate with the external milieu (e.g., the lung, gut, and kidney epithelia) and individual central cells deep inside the organism (see Fig. 17-1C). In mammals and birds, the exchange of gases with the external milieu is so important that they have evolved a two-pump, dual circulatory system (see Fig. 17-1D) that delivers the full output of the “heart” to the lungs (see pp. 683–684).
The primary role of the circulatory system is the distribution of dissolved gases and other molecules for nutrition, growth, and repair. Secondary roles have also evolved: (1) fast chemical signaling to cells by means of circulating hormones or neurotransmitters, (2) dissipation of heat by delivery of heat from the core to the surface of the body, and (3) mediation of inflammatory and host defense responses against invading microorganisms.
The circulatory system of humans integrates three basic functional parts, or organs: a pump (the heart) that circulates a liquid (the blood) through a set of containers (the vessels). This integrated system is able to adapt to the changing circumstances of normal life. Demand on the circulation fluctuates widely between sleep and wakefulness, between rest and exercise, with acceleration/deceleration, during changes in body position or intrathoracic pressure, during digestion, and under emotional or thermal stress. To meet these variable demands, the entire system requires sophisticated and integrated regulation.
The heart is a dual pump that drives the blood in two serial circuits: the systemic and the pulmonary circulations
A remarkable pump, weighing ~300 g, drives the human circulation. The heart really consists of two pumps, the left heart, or main pump, and the right heart, or boost pump (see Fig. 17-1D). These operate in series and require a delicate equalization of their outputs. The output of each pump is ~5 L/min, but this can easily increase 5-fold during exercise.
During a 75-year lifetime, the two ventricles combined pump 400 million L of blood (enough to fill a lake 1 km long, 40 m wide, and 10 m deep). The circulating fluid itself is an organ, kept in a liquid state by mechanisms that actively prevent cell-cell adhesion and coagulation. With each heartbeat, the ventricles impart the energy necessary to circulate the blood by generating the pressure head that drives the flow of blood through the vascular system. On the basis of its anatomy, we can divide this system of tubes into two main circuits: the systemic and the pulmonary circulations (see Fig. 17-1D). We could also divide the vascular system into a high-pressure part (extending from the contracting left ventricle to the systemic capillaries) and a low-pressure part (extending from the systemic capillaries, through the right heart, across the pulmonary circulation and left atrium, and into the left ventricle in its relaxed state). The vessels also respond to the changing metabolic demands of the tissues they supply by directing blood flow to (or away from) tissues as demands change. The circulatory system is also self-repairing/self-expanding. Endothelial cells lining vessels mend the surfaces of existing blood vessels and generate new vessels (angiogenesis).
Some of the most important life-threatening human diseases are caused by failure of the heart as a pump (e.g., congestive heart failure), failure of the blood as an effective liquid organ (e.g., thrombosis and embolism), or failure of the vasculature either as a competent container (e.g., hemorrhage) or as an efficient distribution system (e.g., atherosclerosis). Moreover, failure of the normal interactions among these three organs can by itself elicit or aggravate many human pathological processes.