Artery

Arteries are muscular tubes that carry blood flow away from the heart to the tissues and organs of the body ( by contrast, veins are the return path tubes).

The arterial layer that is in direct contect with the flow of blood is the tunica intima, commonly called the intima. This layer is made up of mainly endothelial cells. Just deep to this layer is the tunica media, known as the media. This "middle layer" is made up of smooth muscle cells and elastic tissue. The outermost layer (furthest from the flow of blood) is known as the tunica adventitia or the adventitia. This layer is composed of connective tissue.

The arterial system is the higher-pressure portion of the blood system. Since the heart output is pulsatile, arterial pressure varies between systolic, the peak pressure during heart contraction, and diastolic, the minimum pressure between heart contractions, values with each heart cycle. This pressure and blood volume variation within the artery produces the pulse which is palpable in any artery, reflecting the heart action.

The systemic arterial pressures, e.g 120/80 [mmHg, are generated by the forceful contractions of the heart's left ventricle. Similarly, the pulmonary arterial pressures, e.g 25/6 mmHg, are generated by the contractions of the heart's right ventricle.

Healthy resting arterial pressures, compared to many man-engineered system are relatively low, mean systemic pressures typically being under 100 mmHg, about 1.8 lbf/in², above surrounding atmospheric pressure (about 760 mmHg or 14.7 lbf/in² at sea level).

To withstand and adapt to the pressures within, the arteries are surrounded by a varying degree of smooth muscle which has extensive elastic and inelastic connective tissue and also exhibits muscular contraction or relaxation in response to adrenergic, cholinergic, other locally produced peptides, nitrous oxide, etc. (See epinephrine, norepinephrine, alpha and beta receptors.)

The effects of arterial responsiveness is most dramatic in the arterioles, the smallest end arteries, typically about 20,000 of them in a 150 lb (68 kg) individual. These arteries, the arterioles, have the greatest collective influence on both local blood flow and, collectively, on overall blood pressure. They are the primary "adjustable nozzles" in the blood system, across which the greatest pressure drop occurs. The combination of heart output (cardiac output) and total peripheral resistance, which refers to the collective average resistance of all the arterioles, are the principal determinates of the mean arterial blood pressure at any given moment. The pulse pressure, i.e. Systolic vs. Diastolic difference, is determined primarily by the amount of blood ejected by each heart beat, stroke volume, versus the volume and elasticity of the major arteries.

The pulmonary arteries deliver blood to the lungs, blood that has just returned from the body. In the pulmonary circulation blood oxygen saturation increases and carbon dioxide levels decrease. Hemoglobin molecules within red blood cells greatly increase the blood's oxygen carrying capability, about 20 fold. Each hemoglobin molecule can bind up to four molecules of elemental oxygen, termed 100% saturation if all four O2 binding sites have bound oxygen. Blood returning from the lungs, via the 4 pulmonary veins, to the left atrium typically has nearly 100% saturation.

The systemic arteries deliver blood to the capillaries of the body, where nutrients are released and hemoglobin releases oxygen molecules, one by one, as the blood cells enter the relatively oxygen-poorer environments of the distal tissues and carbon dioxide & waste products are picked up. The hemoglobin oxygen release phenomenon is described by the oxyhemoglobin dissociation curve (a sigmoid curve illustrating hemoglobin's affinity for oxygen). Returning from the body to the right atrium, hemoglobin molecules have typically given up only 1 of the 4 oxygen molecules, a venous saturation of 75%. However, by contrast, the heart muscle, at all times, typically extracts 3 of the 4 bound oxygen molecules; coronary venous saturation typically being about 25%.

The aorta is the root systemic artery. It receives blood directly from the left ventricle of the heart via the aortic valve. As the aorta branches and these arteries branch in turn, they become successively smaller in diameter, successively down to the arteriole. The arterioles supply capillaries which in turn empty into venules.

Capillaries have no smooth muscle surrounding them and have a diameter less than that of a red blood cell; a red blood cell is typically 7 micrometers outside diameter, capillaries typically 5 micrometers inside diameter. The red blood cells partially fold up in order to pass through the capillaries, at least twice (pulmonary & systemic phases) each circle of the body. At the level of the capillaries oxygen is released to and carbon dioxide picked up from the surrounding tissue cells. The capillaries are continuous with venules. Venules pool together to form larger vessels, each helping to transport wastes, oxygen-poor red blood cells and surrounding carbon dioxide-elevated blood back through the lower-pressure return system of the veins. Typical venous pressure entering the right atria of the heart is 5 mmHg above atmospheric pressure.

Over time, higher arterial blood sugar (see Diabetes Mellitus), lipoprotein cholesterol, pressures (high blood pressure), smoking, possibly many other inflammatory agents vasculitis, and other known and unknown factors are all involved in damaging both the endothelium and walls of the arteries, resulting in atherosclerosis. Diabetes Mellitus also leads to capillary damage, in adition to the arterial damage.

The arterial system is extremely important in sustaining life. Its proper functioning is responsible for the delivery of oxygen and nutrients to all cells, as well as the removal of carbon dioxide, waste products, maintenance of optimum pH and mobility of the elements, proteins and cells, of the immune system. Barring trauma, infection and malignancy, it is most often the vascular system which determines whether we continue living or not. In First World countries the two leading causes of death, myocardial infarction and stroke, are each directly the result of an arterial system that has been slowly, progressively compromised by years of deterioration, see atherosclerosis.

See also: Vein -- Heart -- blood vessel

Cardiovascular system

Heart - Aorta - Arteries - Arterioles - Capillaries - Venules - Veins - Venae cavae - Pulmonary arteries - Lungs - Pulmonary veins - Blood