Blood Vessels: Structure and Function of Blood Vessels

Blood Vessels

Blood is pumped from the heart and then circulates via a closed system of blood vessels throughout the entire body before returning to the heart. Nearly every cell in the body has a channel through which blood can go that is made up entirely of these veins. Blood flow through the vessels is controlled so that metabolically active tissues receive more blood when they need it, without denying other tissues. As a result, every cell receives nutrients and eliminates wastes in accordance with its needs.

Three groups of blood vessels can be distinguished based on how they function:

• Arteries: Transport blood to other organs so it can leave the heart. Atherosclerotic arteries branch out into progressively smaller arteries until they reach their tiniest branches. Arterioles are the tiniest arteries.
• Veins:  They are the blood arteries that carry blood back to the heart from the tissues. The venules are the tiniest veins. When capillaries combine, venules are created.
• Capillaries: Arterioles branch out into numerous tiny capillaries as they enter a tissue. They enable substance exchange between blood and body cells or between blood and air in the lung tissues.

Structure of Blood Vessels

Except for capillaries, all blood vessels have walls made up of three separate coatings or layers.

• Tunica externa (adventitia): It is the outermost layer and is made of connective tissue that is fibrous. The blood vessels are shielded by the loosely woven collagen fibers that also hold it to the nearby structures.
• Tunica media: It is the middle layer, which is made up of fibers of smooth muscle and elastic connective tissue. It surrounds the vessel lumen in a circle.
• Tunica interna (intima): The endothelium is a single layer of epithelial cells that makes up the innermost layer of the skin. The blood in the vessel lumen is in close contact with this layer.

In arteries compared to veins, the muscle layer is significantly thicker. As a result, the arterial wall's larger muscle layer can withstand the high pressures caused by ventricular systole. The tunica media in arteries is essential for maintaining blood pressure and managing blood flow throughout the body. Since it is a smooth muscle, the sympathetic branch of the autonomic nervous system is in charge of controlling it. Vasoconstriction (decrease in lumen diameter) is caused by smooth muscle contraction in response to an increase in sympathetic activation. On the other hand, vasodilation occurs when smooth muscle fibers relax as a result of less sympathetic activation (widening of the lumen diameter).

Veins differ structurally from arteries in a special way. They have one-way valves that stop blood from flowing backward through them.

The capillary membrane is made up of just one layer of flat, endothelial cells, which is its most distinctive structural characteristic. The capillary wall only has one layer, called the tunica interna, as opposed to having three layers or coats. Many substances, including glucose, oxygen, and wastes, can quickly pass through it on their way to or from the cells due to its thinness. However, large protein molecules and red blood cells typically do not leave the capillaries.

Vascular Anastomosis

Blood is delivered to the majority of bodily tissues by many arterial branches. An anastomosis is the union or interconnection of the branches of two or more arteries feeding the same body part. Blood travels multiple routes to critical organs in this way. Thus, receiving blood even when one of its supply vessels is blocked helps a tissue. The Willis circle and the mesenteric arches, as described below, are a couple of examples of these anastomoses.

• The basilar artery, which is made up of the junction of two vertebral arteries, supplies blood to the circle of Willis in addition to the two internal carotid arteries. The cerebrum and other regions of the brain get branches from this artery circle, which is located directly beneath the brain's center.
• The communications between the vessels' branches that supply blood to the intestines make up the mesenteric arches.

Functions of the Blood Vessels

• All body tissues are supplied with oxygen-rich blood that is carried via arteries out from the heart.
• It delivers blood that is low in oxygen to the lungs, with the exception of the pulmonary arteries.
• Blood with low oxygen content is returned to the heart by veins from bodily tissues. However, the left atrium receives oxygenated blood from the lungs via the pulmonary veins.
• Additionally, veins transport waste materials away from the organs and tissues, and veins connected to the small intestine transport digested food to the inferior vena cava via the liver.
• Gaseous exchange between the capillaries and the tissue fluids is made possible by the capillaries.
• The blood and tissue cells exchange nutrients and waste products through capillaries.
• Capillaries' blood is delivered to veins by venules, which collect it.