Of The Cardiovascular System

The principal components considered to make up the cardiovascular system include the blood, blood vessels, heart, and lymphatic system.

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

2.1. Blood

Blood is composed of formed elements (cells and cell fragments) suspended in the liquid (plasma) fraction. Blood, considered the only liquid connective tissue in the body, has three general functions: (1) transportation (e.g., O2, CO2, nutrients, wastes, hormones); (2) regulation (e.g., pH, temperature, osmotic pressures); and (3) protection (e.g., against foreign molecules and diseases, as well as for clotting to prevent excessive loss of blood). Dissolved within the plasma are many proteins, nutrients, metabolic waste products, and various other molecules traveling between the organ systems.

The formed elements in blood include red blood cells (erythrocytes), white blood cells (leukocytes), and the cell fragments known as platelets. All are formed in bone marrow from a common stem cell. In a healthy individual, the majority (~99%) of blood cells are red cells, which have a primary role in O2 exchange. Hemoglobin, the iron-containing heme protein that binds oxygen, is concentrated within the red cells; hemoglobin allows blood to transport 40 to 50 times the amount of oxygen that plasma alone could carry.

The white cells are required for the immune process to protect against infections and cancers. The platelets play a primary role in blood clotting. In a healthy cardiovascular system, the constant movement of blood helps keep these cells well dispersed throughout the plasma of the larger diameter vessels.

The hematocrit is defined as the percentage of blood volume occupied by the red cells (erythrocytes). It can be easily measured by centrifuging (spinning at high speed) a sample of blood, which forces these cells to the bottom of the centrifuge tube. The leukocytes remain on the top, and the platelets form a very thin layer between the cell fractions (other, more sophisticated methods are also available to do such analyses). Normal hematocrit is approx 45% in men and 42% in women.

The total volume of blood in an average-size individual (70 kg) is approx 5.5 L; hence, the average red cell volume would be roughly 2.5 L. Because the fraction containing both leukocytes and platelets is normally relatively small or negligible, in such an individual the plasma volume can be estimated as 3.0 L. Approximately 90% of plasma is water, which acts: (1) as a solvent, (2) to suspend the components of blood, (3) in absorption of molecules and their transport, and (4) in the transport of thermal energy. Proteins make up 7% of the plasma (by weight) and exert a colloid osmotic pressure.

Protein types include albumins, globulins (antibodies and immunoglobulins), and fibrinogen. To date, more than 100 distinct plasma proteins have been identified, and each presumably serves a specific function. The other main solutes in plasma include electrolytes, nutrients, gases (some O2, large amounts of CO2 and N2), regulatory substances (enzymes and hormones), and waste products (urea, uric acid, creatine, creatinine, biliru-bin, and ammonia).

2.2. Blood Vessels

Blood flows throughout the body tissues in blood vessels via bulk flow (i.e., all constituents together and in one direction). An extraordinary degree of branching of blood vessels exists within the human body, which ensures that nearly every cell in the body lies within a short distance from at least one of the smallest branches of this system—a capillary. Nutrients and metabolic end products move between the capillary vessels and the surroundings of the cell through the interstitial fluid by diffusion. Subsequent movement of these molecules into a cell is accomplished by both diffusion and mediated transport. Nevertheless, blood flow through all organs can be considered as passive and occurs only because arterial pressure is kept higher than venous pressure via the pumping action of the heart.

In an individual at rest at a given moment, approx 5% of the total circulating blood is actually in capillaries. Yet, this volume of blood can be considered to perform the primary functions of the entire cardiovascular system, specifically the supply of nutrients and removal of metabolic end products. The cardiovascular system, as reported by the British physiologist William Harvey in 1628, is a closed-loop system, such that blood is pumped out of the heart through one set of vessels (arteries) and then returns to the heart in another (veins).

More specifically, it can be considered that there are two closed-loop systems that both originate and return to the heart—the pulmonary and systemic circulations (Fig. 1). The pulmonary circulation is composed of the right heart pump and the lungs, whereas the systemic circulation includes the left heart pump, which supplies blood to the systemic organs (i.e., all tissues and organs except the gas exchange portion of the lungs). Because the right and left heart pumps function in a series arrangement, both will circulate an identical volume of blood in a given minute (cardiac output, normally expressed in liters per minute).

In the systemic circuit, blood is ejected out of the left ventricle via a single large artery—the aorta. All arteries of the

E Pulnionarv trunk t

Pulmonary arteries t

Capillaries of lungs t

Pulmonary veins ^

Pulmonary valve Lefl atrium

RigIK^eitlride Lift A^vaJve

RJgh t AV valve Left veil I rlcle

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Aorta Arteries Arterioles Capillaries Venules Veins m^m Venae cavae

Fig. 1. The major paths of blood flow through pulmonary and systemic circulatory systems. AV, atrioventricular.

systemic circulation branch from the aorta (this is the largest artery of the body, with a diameter of 2-3 cm) and divide into progressively smaller vessels. The aorta's four principal divisions are: the ascending aorta (begins at the aortic valve, where, close by, the two coronary artery branches have their origin), the arch of the aorta, the thoracic aorta, and the abdominal aorta.

The smallest of the arteries eventually branch into arterioles. They, in turn, branch into an extremely large number (estimated at 10 billion in the average human body) of vessels with the smallest diameter, the capillaries. Next, blood exits the capillaries and begins its return to the heart via the venules. Microcirculation is a term coined to describe collectively the flow of blood through arterioles, capillaries, and venules (Fig. 2).

Importantly, blood flow through an individual vascular bed is profoundly regulated by changes in activity of the sympathetic nerves innervating the arterioles. In addition, arteri-olar smooth muscle is very responsive to changes in local chemical conditions (i.e., those changes associated with increases or decreases in the metabolic rate of that given organ) within an organ.

From Heart

From Heart

Fig. 2. The microcirculation, including arterioles, capillaries, and venules. The capillaries lie between, or connect, the arterioles and venules. They are found in almost every tissue layer of the body, but their distribution varies. Capillaries form extensive branching networks that dramatically increase the surface areas available for the rapid exchange of molecules. A metarteriole is a vessel that emerges from an arteriole and supplies a group of 10 to 100 capillaries. Both the arteriole and the proximal portion of the metarterioles are surrounded by smooth muscle fibers, which elicit contractions and relaxations so as to regulate blood flow through the capillary bed. Typically, blood flows intermittently through a capillary bed as a result of the periodic contractions of the smooth muscles (5-10 times per min; vasomotion), which are regulated both locally (metabolically) and by sympathetic control. (Figure modified from Tortora and Grabowski, 2000.)

Fig. 2. The microcirculation, including arterioles, capillaries, and venules. The capillaries lie between, or connect, the arterioles and venules. They are found in almost every tissue layer of the body, but their distribution varies. Capillaries form extensive branching networks that dramatically increase the surface areas available for the rapid exchange of molecules. A metarteriole is a vessel that emerges from an arteriole and supplies a group of 10 to 100 capillaries. Both the arteriole and the proximal portion of the metarterioles are surrounded by smooth muscle fibers, which elicit contractions and relaxations so as to regulate blood flow through the capillary bed. Typically, blood flows intermittently through a capillary bed as a result of the periodic contractions of the smooth muscles (5-10 times per min; vasomotion), which are regulated both locally (metabolically) and by sympathetic control. (Figure modified from Tortora and Grabowski, 2000.)

Capillaries, which are the smallest and most numerous blood vessels in the human body (ranging from 5-10 ^m in diameter and numbering around 10 billion), are also the vessels with the thinnest walls; an inner diameter of 5 ^m is just wide enough for an erythrocyte to squeeze through. Further, it is estimated that there are 25,000 miles of capillaries in an adult; each capillary has an individual length of about 1 mm.

Most capillaries are little more than a single-cell-layer thick, consisting of a layer of endothelial cells and a basement membrane. This minimal wall thickness facilitates the capillary's primary function: to permit the exchange of materials between cells in tissues and the blood. As mentioned, small molecules (e.g., O2, CO2, sugars, amino acids, and water) are relatively free to enter and leave capillaries readily, promoting efficient material exchange. Nevertheless, the relative permeability of capillaries varies from region to region in the body with regard to the physical properties of their formed walls.

Based on such differences, capillaries are commonly grouped into two major classes: continuous and fenestrated. In the continuous capillaries, which are more common, the endothelial cells are joined such that the spaces between them are relatively narrow (i.e., narrow intercellular gaps). These capillaries are permeable to substances having small molecular sizes and/or high lipid solubilities (e.g., O2, CO2, and steroid hormones) and are somewhat less permeable to small water-soluble substances (e.g., Na+, K+, glucose, and amino acids). In fenestrated capillaries, the endothelial cells possess relatively large pores that are wide enough to allow proteins and other large molecules to pass through. In some such capillaries, the gaps between the endothelial cells are even wider than usual, enabling quite large proteins (or even small cells) to pass through. Fenestrated capillaries are primarily located in organs whose functions depend on the rapid movement of materials across capillary walls, e.g., kidneys, liver, intestines, and bone marrow.

If a molecule cannot pass between capillary endothelial cells, then it must be transported across the cell membrane. The mechanisms available for transport across a capillary wall differ for various substances depending on their molecular sizes and degree of lipid solubility. For example, certain proteins are selectively transported across endothelial cells by a slow, energy-requiring process known as transcytosis. In this process, the endothelial cells initially engulf the proteins in the plasma within capillaries by endocytosis. The molecules are then ferried across the cells by vesicular transport and released by exocytosis into the interstitial fluid on the other side. Endothelial cells generally contain large numbers of endocytotic and exocytotic vesicles, and sometimes these fuse to form continuous vesicular channels across the cell.

The capillaries within the heart normally prevent excessive movement of fluids and molecules across their walls, but clinical situations have been noted in which they may become "leaky." For example, "capillary leak syndrome," possibly induced following cardiopulmonary bypass, may last from hours to days. More specifically, in such cases, the inflammatory response in the vascular endothelium can disrupt the "gatekeeper" function of capillaries; their increased permeability will result in myocardial edema.

From capillaries, blood throughout the body then flows into the venous system. It first enters the venules, which then coalesce to form larger vessels, the veins (Fig. 2). Then veins from the various systemic tissues and organs (minus the gas exchange portion of the lungs) unite to produce two major veins: the inferior vena cava (lower body) and superior vena cava (above the heart). By way of these two great vessels, blood is returned to the right heart pump, specifically into the right atrium.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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