The arteries, veins, and capillaries make up the circulatory system’s blood vessels. The major vessels are the large arteries and veins that carry blood to and from the heart.
When blood leaves the left ventricle, it passes via the aortic semilunar valve and enters the systemic circulation. The aorta, a huge, thick-walled artery, is the first portion of the systemic circulation. After passing through the aortic hole of the diaphragm at the level of the thoracic ten vertebra, the aorta arches and gives branches feeding the upper half of the body, it enters the belly. It then lowers, supplying branches to the abdomen, pelvis, perineum, and lower limbs.
The aorta’s walls are pliable. This flexibility aids in blood pressure regulation throughout the body. The aorta recoils and is responsible for pulsing blood pressure when it receives nearly five litres of blood from the heart. The elasticity of the aorta decreases as it branches into smaller arteries, but compliance increases.
Arteries branch into arterioles, which are narrow passageways that lead to capillaries. To carry blood into the venous system, the capillaries join together.
Capillaries become venules, which then become veins. The venous system feeds into two major veins: the superior vena cava, which drains tissues above and below the heart, and the inferior vena cava, which drains tissues below the heart. These two big veins drain into the heart’s right atrium.
Generally speaking, arteries from the heart branch out into capillaries, which then gather into veins that return to the heart. Only portal veins are an exception to this rule. The hepatic portal vein combines capillaries around the gastrointestinal tract where blood absorbs the various products of digestion; rather than leading directly back to the heart, the hepatic portal vein branches into a second capillary system in the liver, which is the only significant example in humans.
Circulation in the heart
The heart is supplied with oxygen and nutrients via a short “loop” of the systemic circulation and receives very little oxygen and nutrients from the blood within the four chambers. The coronary circulatory system is responsible for supplying blood to the heart muscle. Two coronary arteries, the right coronary artery and the left coronary artery, start the coronary circulation at the aorta’s origin. Blood returns through the coronary veins into the coronary sinus and from there into the right atrium after nourishing the heart muscle. Thebesian valve prevents backflow of blood through its opening during atrial systole. The tiniest cardiac veins enter the heart chambers directly.
Circulation in the brain
The brain has a dual blood supply, with arteries in the front and rear providing an anterior and posterior circulation. The internal carotid arteries supply the front of the brain with the anterior circulation. The vertebral arteries supply the posterior circulation, which supplies the rear of the brain and the brainstem. At the Willis circle, the circulation from the front and back meet.
Circulation in the kidneys
The renal circulation provides blood to the kidneys and has several specialised blood arteries. It receives around 20% of the cardiac output. The ascending vena cava receives blood from the abdominal aorta and returns it to the ascending vena cava.