Blood Vessels

The vessels of the cardiovascular system form a closed system of tubes that carry the blood from the heart to the body cells and back again. These vessels include arteries, arterioles, capillaries, venules, and veins. The structure of these vessels and their role in circulation is described below, including a list of vessels you need to know!


ARTERIES AND ARTERIOLES

Arteries are strong, elastic vessels that are designed to carry blood away from the heart under fairly high pressure. These vessels branch into progressively thinner tubes and eventually give rise to fine branches called arterioles.

The wall of an artery consists of three distinct layers. The innermost layer, tunica intima, is composed of simple squamous epithelium, called endothelium, resting on a connective tissue membrane, which is rich in elastic and collagen fibers.

The middle layer, tunica media, makes up the bulk of the arterial wall. It includes smooth muscle fibers, which encircle the vessel, and a thick layer of elastic connective tissue.

The outer layer, tunica adventitia, is relatively thin and consists chiefly of connective tissue with irregularly arranged elastic and collagen fibers. This layer attaches the artery to the surrounding tissues, be it muscle, adipose, or other tissue types.

The smooth muscles in the walls of arteries and arterioles are innervated by the sympathetic branches of the autonomic nervous system (ANS). Impulses on these vasomotor fibers cause the smooth muscles to contract, reducing the diameter of the vessles. This action is called vasoconstriction. If such vasomotor impulses are inhibited, the muscle fibers relax and the diamter of the vessels increases. In this case, the vessels are said to undergo vasodilation. Changes in the diameters of arteries and arterioles greatly influence the flow and pressure of blood.
Arterioles, which are microscopic continuations of arteries, join capillaries. Although the walls of the larger arterioles have three layers, similar to those of arteries, these wall become thinner and thinner as the arterioles approach the capillaries. The wall of a very small arteriole consists only of an endothelial lining and some smooth muscle fibers, surrounded by a small amount of connective tissue.


CAPILLARIES

Capillaries are the smallest blood vessels. They form connections between the smallest arterioles and the smallest venules. Capillaries are essentially extensions of the inner linings of these larger vessels in their walls consists of endothelium - a single layer of squamous epithelial cells. These thin walls form the semipermeable membranes through which substances in the blood are exchanged for substances in the tissue fluid surrounding body cells.

The pores in capillary walls are thin slits that occur where two adjacent endothelial cells overlap. The sizes of such openings, and consequently the permeability of the capillary walls, vary from tissue to tissue. For example, the capillaries in muscle tissues are less permeable than those of the liver, spleen, or bone marrow.

The density of the capillaries within the tissue varies directly with the tissues' rates of metabolism. Thus, muscle and nerve tissues, which utilize relatively large quantities of oxygen and nutrients, are richly supplied with capillaries; while tissues of cartilage, the epidermis, and the cornea, whose metabolic rates are very slow, lack capillaries.

The patterns of capillary arrangement also differ in various body part. For example, some capillaries pass directly from arterioles to venule, but others lead to highly branched networks. Such arrangements make it possible for the blood to follow different pathways through a tissue and to meet the varying demands of its cells. During periods of exercise, for example, the blood can be directed into the capillary networks of the skeletal muscles, where the cells are experiencing an increasing need for oxygen and nutrients. At the same time, the blood can bypass some of the capillary nets in the tissues of the digestive tract, where the demand for blood is less critical.

The distribution of blood in the various capillary pathways is regulated mainly by the smooth muscles that encircle the capillary entrances. These muscles form precapillary sphincters, which may close a capillary by contracting or open it by relaxing. How precapillary sphincters are controlled is not clear, but they seem to respond to the demands of the cells supplied by their individual capillaries. When the cells are low in concentration of oxygen and nutrients, the sphincter relaxes; when the cellular needs are met, the sphincter may contract again.


EXCHANGES IN THE CAPILLARIES

The vital function of exchanging gases, nutrients and metabolic by-products between the blood and the tissue fluid surrounding the body cells occurs in the capillaries. The substances exchanged move through the capillary walls primarily by the processes of diffusion, filtration, and osmosis. Of these processes, diffusion provides the most important means of transfer.

It is by diffusion that molecules and ions move from regions where they are more highly concentrated toward regions where they are in lower concentration. Because the blood entering the capillaries of the tissues outside the lungs generally carries relatively high concentrations of oxygen and nutrients, these substances diffuse through the capillary walls and enter the tissue fluid. Conversely, the concentrations of carbon dioxide and other wastes are generally greater in these tissues, and the wastes tend to diffuse into the capillary blood.

The plasma proteins generally remain in the blood because their molecular size is too great to permit diffusion through the membrane pores between the endothelial cells of most capillaries.

Filtration involves the forcing of molecules through a membrane by hydrostatic pressure. In the capillaries, the force is provided by the blood pressure generated by contractions of the ventricular walls.

Blood pressure is also responsible for moving blood through the arteries and arterioles. Pressure tends to decrease, however, as the distance from the heart increases, because friction (peripheral resistance) between the blood and the vessel walls slows the flow. For this reason, blood pressure is greater in the arteries than in the arterioles and greater in the arterioles than in the capillaries. It is similarly greater at the arteriole end of a capillary than at the venule end. Therefore, the filtration effect occurs primarily at the arteriole ends of capillaries.

The plasma proteins, which remain in the capillaries, help to make the osmotic pressure of the blood greater (hypertonic) than that of the tissue fluid. Although the capillary blood has a greater osmotic attraction for water than does the tissue fluid, this attraction is overcome by the greater force of the blood pressure. As a result, the net movement of water and dissolved substances is outward at the arteriole end of the capillary by filtration.

Since the blood pressure decreases as the blood moves through the capillary, however, the outward filtration force is less than the osmotic pressure of the blood at the venule end. Consequently, there is a net movement of water and dissolved materials into the venule end of the capillary by osmosis.

Normally, more fluid leaves the capillaries than returns to them, and the excess is collected and returned to the venous circulation by lymphatic vessels.


VENULES AND VEINS

Venules are microscopic vessels that continue from the capillaries and merge to form veins. The veins, which carry the blood back to the atria, follow pathways that roughly parallel those of the arteries.

The walls of veins are similar to those of arteries in that they are composed of three distinct layers. Because the middle layer of the venous wall is poorly developed, however, veins have thinner walls and contain less smooth muscle and less elastic tissue than comparable arteries.

Many veins, particularly those in the arms and legs, contain flaplike valves, which project inward from their linings. These valves are usually composed of two leaflets that close if the blood begins to back up in a vein. In other words, the valves aid in returning blood to the heart, since the valves open as long as the flow is toward the heart, but close if flow is in the opposite direction.

In addition to providing pathways for the blood returning to the heart, the veins function as blood reservoirs, which can be drawn on times of need. For example, if a hemorrhage accompanied by a drop in arterial blood pressure occurs, the muscular walls of the veins are stimulated reflexly by sympathetic nerve impulses. The resulting venous constrictions help raise the blood pressure. This mechanism ensures a nearly normal blood flow even when as much as 25 percent of the blood volume has been lost.


PATHS OF CIRCULATION

The blood vessels of the cardiovascular system can be divided into two major pathways - a pulmonary circuit and a systemic circuit. The pulmonary circuit consists of those vessels that carry the blood from the heart to the lungs and back to the heart. The systemic circuit is responsible for carrying the blood from heart to all other parts of the body and back again.

The circulatory pathways described in the following sections are those of an adult:

PULMONARY CIRCUIT

The blood enters the pulmonary circuit as it leaves the right ventricle through the pulmonary trunk. The pulmonary trunk extends upward and posteriorly from the heart, and at about 5 cm above its origin, it divides into the right and left pulmonary arteries. These branches penetrate the right and left lungs, respectively. After repeated divisions, they give rise to the arterioles that continue into the capillary networks associated with the walls of the alveoli, where gas exchanges occur between the blood an the air.

From the pulmonary capillaries, the blood enters the venules, which merge to form small veins. They, in turn, converge to form still larger ones. Four pulmonary veins, two from each lung, return the blood to the left atrium, and this completes the vascular loop of the pulmonary circuit.

SYSTEMIC CIRCUIT

The freshly oxygenated blood received by the left atrium is forced into the systemic circuit by contraction of the left ventricle. This circuit includes the aorta and its branches that lead to all the body tissues, as well as the companion system of veins, which returns the blood to the right atrium.

Blood leaves the ventricle and the first branch is the Coronary artery, which feeds the heart tissue first. The first branch from the aorta before it descends to the torso and legs is the Brachiocephalic, which delivers blood to the head and arms. The next branch carries blood to the head through the Carotid artery, with the rest heading through the Subclavian artery. Through numerous branches, blood will flow through the Axillary then the Brachial artery, where it will branch into Radial and Ulnar arteries. The descending aorta has numerous branches feeding the spinal cord and the tissues of the torso. The Celiac feeds the stomach and other organs of the upper digestive tract, while the Superior/Inferior mesenteric arteries feed the intestines. Finally the aorta splits into the Iliac arteries, diving into the legs via the Femoral artery, Popliteal (behind the knee) and Tibial artery. Blood flows back to the heart in reverse flow after passing through the capillaries. The vessels are named, for the most part, the same as their arterial counterparts. Some notable exceptions include the Jugular vein, which returns blood from the heart to the Superior Vena cava.

A unique part of the venous flow is the collection of veins refered to as the Hepatic Portal system. The oxygen poor, nutrient rich blood from the stomach, intestine and spleen flow together into the Portal vein, and through the liver first, before returning to the heart. The liver works on the sugars and nutrients absorbed (part of it's regulatory function), destroys bacteria which have entered the blood from the intestine, and then returns it to the Hepatic vein, and into the Inferior Vena cava.

Arteries/Veins Accesory /Alternative Veins
Coronary A/V
Brachiocephalic A/V
Common Carotid A Jugular V
Subclavian A/V
Axillary A/V
Brachial A/V
Radial A/V Cubital V
Ulnar A/V
Celiac A Portal and Hepatic V
Renal A/V
Sup/Inf. Mesenteric A
Gonadal A/V
Iliac A/V
Femoral A/V
Popliteal A/V
Tibial A/V