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Structure and Functions of Artery, Vein and Capillaries

During a heartbeat, blood travels from the heart to capillaries via arterioles, which are smaller arteries. Arteries are divided into three layers.

During a heartbeat, blood travels from the heart to capillaries via arterioles, which are smaller arteries. Arteries are divided into three layers (or tunics) of tissue, each of which has a specific function.

A vessel's only part in contact with blood is its innermost layer, the tunica intima. It is also known as endothelium, composed solely of simple squamous epithelium cells. This forms the same lining as it forms in the endocardium, the heart lining. It also serves the same purpose: its smoothness prevents abnormal blood clotting. In addition to endothelial production of nitric oxide (NO), the endothelium of vessels also acts as a vasodilator. In the middle layer, tunica media, which is composed of smooth muscles and elastin, there are smooth connective tissues. When the heart is relaxed, both tissues are involved in maintaining normal blood pressure, especially diastolic blood pressure.

Vasodilator NO acts on the smooth muscle, causing dilation of blood vessels by relaxing the muscle, Vasoconstriction are also caused by sympathetic nerve impulses in smooth muscle. The tunica external, the outermost layer, is composed of fibrous connective tissue. This tissue is also very strong, which is crucial to preventing artery ruptures or bursting caused by high pressure inside the body.

Functions - Blood is transported away from the heart by arteries, Low-oxygen blood is transported to the lungs from the right ventricle by pulmonary arteries, Throughout the body, oxygenated blood is transported using the systemic arteries.


Capillaries return blood to the heart, which is transported by veins; the smaller veins are called venules. Vessel walls are composed of three layers of tissue, although some differences exist when compared to arterial walls. Smooth endothelium lines the inside of veins, but it folds at intervals to form valves. The blood returns to the heart through the veins in the legs, where gravity forces blood back towards the heart.

Since veins do not control blood pressure and blood flow into capillaries as do arteries, they are thin. When a person experiences a severe haemorrhage, however, the ability to constrict veins becomes extremely important. Since blood pressure is so low, veins have a thin outer cover, and they require less fibrous connective tissue.

Functions - Almost all of your organs are supplied with deoxygenated blood that goes back to the heart through veins.


From the arterioles, blood travels to the venules via capillaries. Despite their small size, capillaries are connected to arteries and veins by simple squamous linings, which are the walls of their capillaries. Capillaries do not exist in certain tissues, including the epidermis, cartilage, lens, and cornea of the eye.

There are extensive capillary networks in many tissues. A capillary network's volume or number tells us how much metabolic activity is occurring within it. For example, kidneys function properly when they have a good blood supply. There are smooth muscle cells known as precapillary sphincters at the beginning of each capillary network that controls blood flow into those networks. The precapillary sphincter is not controlled by the nervous system. According to the tissues' requirements, it can dilate or constrict.

An organ can have a different type of capillary called a sinusoidal capillary, which is larger and more permeable. Protein molecules and blood cells can pass through sinusoids in particular. Red blood cells enter and leave the body through the bone marrow and spleen, and proteins are produced and secreted by organs like the liver and pituitary gland through the sinusoids. Functions - A capillary's primary function is to exchange material between tissue cells and the blood.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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