The vascular system, an integral part of the circulatory system, comprises various blood vessels that play crucial roles in maintaining the body's homeostasis. These blood vessels form a complex and efficient circulatory network. The three primary categories of blood vessels are the arteries, veins, and capillaries.

Arteries

Arteries circulate oxygenated blood from the heart, except the pulmonary artery, which transports deoxygenated blood to the lungs. Large arteries, such as the aorta, have thick walls mainly composed of elastic tissue. This elasticity helps cushion the pressure from ventricular contractions and propels blood forward. The endothelium within arteries plays a critical role in regulating vascular tone, blood pressure, angiogenesis, and inflammation control.

Artery walls are thick and consist of three layers:

1. Tunica Intima: The innermost layer, made of endothelial cells, provides a smooth lining, minimizing blood flow resistance. It also includes a subendothelial connective tissue layer.
2. Tunica Media: The middle layer, made of smooth muscle cells and thick elastic fibers in arteries, allows them to withstand and regulate high blood pressure. Large arteries have more elastic fibers to handle blood surges during each heartbeat.
3. Tunica Externa (Adventitia): The outermost layer, made of connective tissue, collagen fibers, and elastic fibers in larger arteries, provides structural support and protection.

Next, arterioles, smaller branches of arteries, contain more smooth muscle and less elastic tissue, allowing them to control blood pressure and blood flow distribution. These are the primary sites of vascular resistance, significantly contributing to blood pressure regulation.

Capillaries

Capillaries, the smallest blood vessels, comprise a single layer of endothelial cells. This thin structure facilitates the exchange of nutrients, oxygen, carbon dioxide, waste matter, and hormones between the blood and surrounding tissues. Capillaries connect arterioles to venules, forming a network that permeates all body tissues. The efficient exchange process in capillaries ensures that cells receive essential nutrients and oxygen while removing metabolic waste.

Veins and Venules

Venules are small vessels that receive blood from capillary beds and channel blood into larger veins. Veins transport deoxygenated blood to the heart, except the pulmonary veins, which transport oxygenated blood from the lungs to the heart.

1. Tunica Intima: Similar to arteries, it consists of endothelial cells but is less smooth to accommodate lower pressure.
2. Tunica Media: Thinner than in arteries, with fewer smooth muscle cells and elastic fibers, reflecting lower pressure in veins.
3. Tunica Externa (Adventitia): The thickest layer in veins, made of connective tissue and collagen fibers, provides support and prevents overexpansion.

The venous system operates under low pressure and high volume, with veins possessing thinner walls and larger diameters than arteries. The largest vein, such as the superior vena cava, delivers blood from the head, neck, and arms to the heart, along with the inferior vena cava, which brings back blood to the heart from the lower body. Pressure in the right side of the heart affects these large vessels. Skeletal muscle contractions in the legs and arms compress veins, propelling blood toward the heart via the muscle pump mechanism. This process is essential for maintaining blood flow in the low-pressure venous system, especially in the extremities.