Structure-Function Relationships in the Cardiovascular System

Circulation is the movement of blood throughout the body through one continuous circuit of blood vessels. Different organs have unique functions and, therefore, have different requirements, circulatory patterns, and regulatory mechanisms. Several of the most vital organs (including the brain, heart, and kidneys) have autoregulatory properties, meaning that they are able to maintain a relatively constant blood flow despite fluctuations in mean arterial pressure (MAP). In other cases, locally produced factors (such as adenosine, CO2, or NO) can produce local vasoconstriction or vasodilation, regulating blood flow under specific physiologic conditions.

Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular resistance is directly related to the diameter of the vessel (smaller vessels have higher resistance). Mean arterial pressure (MAP) is the average systemic arterial pressure and is directly related to cardiac output (CO) and systemic vascular resistance (SVR). The SVR and MAP are affected by the vascular anatomy as well as a number of local and neurohumoral factors.

The brain stem is a stalk-like structure that connects the cerebrum with the spinal cord and consists of the midbrain, pons, and medulla oblongata. The brain stem contains many nerves, pathways, reflex centers, and nuclei and serves as a major relay station for sensory, motor, and autonomic information. All cranial nerves, except I and II, originate from the brain stem. The brain stem also plays a critical role in the control of cardiovascular and respiratory function, consciousness, and the sleep-wake cycle.

The heart is a 4-chambered muscular pump made primarily of cardiac muscle tissue. The heart is divided into 4 chambers: 2 upper chambers for receiving blood from the great vessels, known as the right and left atria, and 2 stronger lower chambers, known as the right and left ventricles, which pump blood throughout the body. Blood flows through the heart in 1 direction, moving from the right side of the heart, through the lungs, and then returning to the left side of the heart, where it is pumped out to the rest of the body. As blood moves through the heart, 4 important valves prevent backflow. The heart muscle itself is supplied by the coronary arteries. The heart also has its own conduction system, triggering its own rhythmic contractions.

Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. The venous circulation is a low-pressure system with much lower amounts of smooth muscle and elastic tissue, thinner walls, and larger lumens than arteries. Veins are capacitance vessels with significant compliance and the ability to distend and hold up to 70%–80% of blood volume at rest.

Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries are organized into capillary beds, which are extensive networks of branches and anastomoses. Blood flows from the metarterioles, into the capillaries, out the thoroughfare channel, and into venules. Continuous, fenestrated, and sinusoid (discontinuous) capillaries are the 3 primary types, and each has a slightly different structure. Capillary dysfunction can occur either as a result of or a contribution to the clinical manifestation of many clinical conditions.

Muscle tissue is one of the basic tissue types. Histologically, the muscles of the body can be classified into 3 types: skeletal, smooth, and cardiac. The 3 types of muscle tissue are based on the morphologic and functional properties of the cells. One of the defining characteristics of muscle tissue is its contractility, which generates forces that move the musculoskeletal system as well as cause movement in the vasculature and multiple organ systems. This contractility is due to specialized proteins known as myofilaments, which create organized structures that have the ability to lengthen and contract.

Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Each of these types of arteries contain 3 primary layers: the tunica intima, tunica media, and tunica adventitia. In particular, the tunica media contains smooth muscle cells (allowing for vasoconstriction), and in the larger vessels, a significant amount of elastin (allowing these vessels to stretch and recoil in response to changes in pressure during systole and diastole).