The arterial system carries oxygenated blood away from the heart to tissues throughout the body. It is composed of arteries that decrease in size from the aorta to arterioles and then to capillaries. Arteries have several layers including the outer tunica externa, middle tunica media made of muscle and elastic tissue, and inner tunica intima of endothelial cells. The arterial system is divided into pulmonary arteries carrying blood to the lungs and systemic arteries delivering blood throughout the body. Capillaries allow for the exchange of gases, nutrients and waste between blood and tissues.

1. Arteries are muscular blood vessels that carry blood away from the heart (as opposed to veins, blood
vessels carrying blood toward the heart). All arteries, with the exception of the pulmonary and umbilical
arteries, carry oxygenated blood.
The circulatory system is extremely important for 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 and waste
products, maintenance of optimum pH, and the mobility of the elements, proteins and cells of the immune
system. In developed countries, the two leading causes of death, myocardial infarction and stroke each may
directly result from an arterial system that has been slowly and progressively compromised by years of
deterioration. (See atherosclerosis).
The arterial system is the higher-pressure portion of the circulatory system. Arterial pressure varies between
the peak pressure during heart contraction, called the systolic pressure, and the minimum, or diastolic
pressure between contractions, when the heart rests between cycles. This pressure variation within the artery
produces the pulse which is observable in any artery, and reflects heart activity.
The anatomy of arteries can be separated into gross anatomy, at the macroscopic level, and microscopic
anatomy, which must be studied with the aid of a microscope.
The arterial system of the human body is divided into systemic arteries, carrying blood from the heart to the
whole body, and pulmonary arteries, carrying blood from the heart to the lungs. The pulmonary arteries
carry blood from the heart to the lungs. They are the only arteries (other than umbilical arteries in the fetus)
that carry deoxygenated blood.In the human heart, the pulmonary trunk (pulmonary artery or main
pulmonary artery) begins at the base of the right ventricle. It is short and wide - approximately 5 cm (2
inches) in length and 3 cm (1.2 inches) in diameter. It then branches into two pulmonary arteries (left and
right), which deliver deoxygenated blood to the corresponding lung.
Histology
The outermost layer is known as the tunica externa formerly known as "tunica adventitia" and is composed
of connective tissue. Inside this layer is the tunica media, or media, which is made up of smooth muscle
cells and elastic tissue. The innermost layer, which is in direct contact with the flow of blood is the tunica
intima, commonly called the intima. This layer is made up of mainly endothelial cells. The hollow internal
cavity in which the blood flows is called the lumen

2. Types of arteries
Pulmonary arteries
The pulmonary arteries carry deoxygenated blood that has just returned from the body to the lungs, where
carbon dioxide is exchanged for oxygen.
Systemic arteries
Systemic arteries can be subdivided into two types; muscular and elastic; according to the relative
compositions of elastic and muscle tissue in their tunica media as well as their size and the makeup of the
internal and external elastic lamina. The larger arteries >1cm diameter are generally elastic and the smaller
ones 0.1-10mm tend to be muscular. Systemic arteries deliver blood to the arterioles, and then to the
capillaries, where nutrients and gasses are exchanged.
The Aorta
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, down to the arteriole. The arterioles supply capillaries which in turn empty into venules.
Arterioles
Arterioles, the smallest of the true arteries, help regulate blood pressure by the variable contraction of the
smooth muscle of their walls, and deliver blood to the capillaries.
Arterioles and blood pressure
Arterioles have the greatest collective influence on both local blood flow and 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 systemic vascular resistance, which refers to
the collective resistance of all of the body's arterioles, are the principal determinants of arterial blood
pressure at any given moment.
Capillaries
The capillaries are where all of the important exchanges happen in the circulatory system. The capillaries
are a single thick cell to aid fast and easy diffusion of gases, sugars and other nutrients to surrounding
tissues.
Functions of capillaries
To withstand and adapt to the pressures within, arteries are surrounded by varying thicknesses of smooth
muscle which have extensive elastic and inelastic connective tissues.
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