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Blood Circulation and Blood Vessels

Blood circulates continuously around the body, but its efficient description can be divided into two parts: pulmonary and Systemic circulation.
Blood circulation
Blood circulates continuously around the body, but its efficient description can be divided into two parts:
1. Pulmonary circulation
2. Systemic or general circulation

The right ventricle is responsible for pulmonary circulation, whereas the left ventricle is responsible for systemic circulation.

1. Pulmonary circulation
Blood from the right ventricle of the heart enters the left atrium, which returns it to the right ventricle. The lungs are responsible for excreting oxygen and carbon dioxide. The pulmonary trunk is where deoxygenated blood is evacuated from the right ventricle. The left and right pulmonary arteries diverge through this artery in the 5th thoracic vertebrae. Left and right pulmonary arteries branch out twice from it before passing through each lung lobe again. Right pulmonary artery branches out into two branches at the tail end of the right lung. In the middle and lower lobes, the larger branch carries blood, while the smaller branch goes to the upper lobe. Several arteries, arterioles, and capillaries are subdivided into smaller ones within the lung. The capillary blood of the lungs exchanges gases with the air in the alveoli. As oxygen-rich blood passes through the capillaries of each lung, it eventually forms the pulmonary veins. Each lung has two pulmonary veins that leave it as oxygenated blood returns to the left atrium. The left atrium is the first artery of the general circulation, providing oxygen-rich blood to the left ventricle during atrial systole, and the aorta during ventricular systole.

2. Systemic or general circulation
An artery leading into the aorta is the left ventricle, which is the largest in the body. As blood travels along the aorta, it is carried in capillary networks and arterioles throughout the body. A venule and a vein are formed when capillaries merge. In the lower vena cava, blood travels from the lower body; in the upper vena cava, blood travels from the upper body. Right atrium blood is returned by these two caval veins.

There are four types of aortae: ascending, aortic arch, thoracic, and abdominal, which are continuous vessels. Ascending aorta begins at the left ventricle's first inch. Upon crossing over the heart, an arch of the aorta curves posteriorly. As the thoracic aorta descends into the chest cavity and passes through the diaphragm, the blood is pumped towards the heart. Under the diaphragm, at the 4th lumbar vertebra, the abdominal aorta separates into two separate vessels. As the aorta travels through body parts and organs, it carries blood throughout. As the ascending aorta carries blood to the heart muscle, it is crossed by the right as well as the left coronary arteries.

In addition to brachiocephalic, common carotid, and left subclavian arteries carrying blood to the head and arms, three other arteries branch from the aortic arch. Short and narrow brachiocephalic arteries are the proximal arteries of the right common carotid artery and right subclavian artery. The external and internal carotid arteries supply the brain respectively from these two arteries as they reach the neck. A large number of arteries in the body change name as they pass through different spaces: the subclavian artery becomes the axillary artery, and the axillary becomes the brachial artery. The circle of Willis is a connection between the pituitary gland and the arterial supply to the brain.

Besides the carotid arteries on the right and left sides, the basilar and vertebral arteries (branches of the subclavian artery) are also part of the circle of Willis. Even while sleeping, the brain requires constant blood flow to supply oxygen and eliminate waste products. This is why the circle of Willis is supplied with blood by four vessels. Several arterial branches enter the brain as a result of this anastomosis (the cerebral arteries). As well as providing blood to the chest wall, the thoracic aorta supplies the organs located in the chest cavity.

Aortic arteries supply the abdominal wall and organs along with those that extend to the legs, known as the common iliac arteries. External iliac arteries become femoral arteries, which become popliteal arteries, which take different names based on location. Blood drains from organs or parts of the body through systemic veins, which are often parallel to their corresponding arteries.

Blood vessels
Veins, vessels, arterioles, capillaries, and venules are the different types of blood vessels, each with its characteristics, structures, and functions.

Arteries and arterioles
Blood is transported out of the heart through vessels like these. Each layer of tissue in the walls of these organisms varies greatly in size and composition:
  1. the outer layer of fibrous tissue or tunica adventitia
  2. the middle layer of smooth muscle or tunica media
  3. the inner lining of squamous epithelium or elastic tunica intima.
Arteries vary depending on their size and function in terms of how much muscular and elastic tissue they contain. Tunica media of larger arteries consists of more elastic tissue and less smooth muscle than tunica media of smaller arteries. Vascular walls stretch in response to the heart beating, which allows them to absorb pressure waves. However, as the arteries branch more times and get smaller, these proportions gradually change until the tunica media has almost entirely been replaced by smooth muscle next to the arterioles (the smallest arteries). By precisely controlling their diameter, their pressure can be adjusted within them. Resistance vessels are arteries that offer a substantial amount of resistance to blood flow, thus determining systemic blood pressure. Vascular walls are thinner than an artery, which allows arterial blood to withstand high pressure.

Capillaries and sinusoids
Several minute blood vessels are called capillaries, which are the smallest arterioles. An endothelial cell is a thin layer of basement membrane that permits the passage of water and other molecules in capillaries. In normal circumstances, plasma proteins are not permitted to pass through capillaries. There are thousands of capillaries connecting the smallest arteries to the smallest veins within the human body. Each of these cells is about the same size as an erythrocyte (7 μm). Capillary beds provide an exchange of substances between blood and tissues, bathing body cells with fluid. Circulating blood is controlled by smooth muscle rings at the entry of capillary beds (precapillary sphincters). In the presence of tissue wastes (hypoxia), blood flow increases due to the dilation of sphincters. There are several areas in the body where capillaries are much wider and leakier than normal, including the liver and bone marrow. The cells outside these capillaries, called sinusoids, are directly contacted by the blood because they have incomplete walls and large lumens. This results in the blood flowing through the sinusoids at a slower rate under less pressure and directly contacting the external cells. As a result, substances in the blood can be exchanged more quickly between the blood and tissues, such as in the liver, which helps to regulate the composition of blood coming from the GI tract.

Veins and venules
A vein is a blood vessel that delivers blood to the heart under low pressure. A vein's walls are thinner than that of an artery's, with the same layers of tissue as an artery. In the tunica media, there is less muscle and elastin, so veins have lower pressure than arteries. Venous walls collapse when veins are cut, but thicker-walled arteries remain open.

Blood flows away from the heart through valves in several veins, preventing blood from flowing backward. Located in the tunica intima, these tissues are bound together by connective tissue.

With concave edges towards the heart, the cusps are semilunar in shape. As a person stands, blood must travel a long distance against gravity, so there are many vasculatures in the legs, particularly those in the lower limbs. The thorax and abdomen lack them in the very small and the very large veins. A skeletal muscle surrounding the veins assists the valves in maintaining one-way flow.

Small veins are called venules. Vascular capacitance can be explained by the fact that vessels are distensible and therefore can concentrate a considerable amount of blood. The venous system carries about two-thirds of all the body's blood at any given time. Recoiling veins can absorb (to some extent) sudden drops in blood volume, like those caused by hemorrhage; this helps prevent blood pressure from dropping suddenly.
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