Good day everyone , Today we are discussing about circulation of blood;
Learning outcomes
Alter studying this section you should be able to:
- describe the chiculation of the blood through the lungs, naming the main vestris involved.
- list the arteries supplying blood to all major body structures
- describe the venous drainage involved in returning blood to the neart from the body
- describe the arrangement of blood vessels relating to the portal circulation.
Although circulation of blood round the body is continuous it is convenient to describe it in two parts:
pulmonary circulation systemic or general circulation.
Pulmonary circulation
This is the circulation of blood from the right ventricle of the heart to the lungs and back to the left atrium. In the lungs, carbon dioxide is excreted and oxygen is absorbed.
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The pulmonary artery or trunk, carrying deoxygenated blood, leaves the upper part of the right ventricle of the heart. It passes upwards and divides into left and right pulmonary arteries at the level of the 5th thoracic vertebra.
The left pulmonary artery runs to the root of the left lung where it divides into two branches, one passing into each lobe.
The right pulmonary artery passes to the root of the right lung and divides into two branches. The larger branch carries blood to the middle and lower lobes, and the smaller branch to the upper lobe.
Within the lung these arteries divide and subdivide into smaller arteries, arterioles and capillaries. The exchange of gases takes place between capillary blood and air in the alveoli of the lungs. In each lung the capillaries containing oxygenated blood merge into progressively larger venules, and eventually form two pulmonary veins.
Two pulmonary veins leave each lung, returning oxygenated blood to the left atrium of the heart. During atrial systole this blood is pumped into the left ventricle, and during ventricular systole it is forced into the aorta, the first artery of the general circulation.
Systemic or general circulation
The blood pumped out from the left ventricle is carried by the branches of the aorta around the body and returns to the right atrium of the heart by the superior and inferior venae cavae.
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The circulation of blood to the different parts of the body will be described in the order in which their arteries branch off the aorta.
Major blood vessels
The aorta is the largest artery of the body. The two hast veins, the superior and inferior venae cavae, return od from all body parts to the heart.
Aorta
The aorta begins at the upper part of the left ventricle and, after passing upwards for a short way, it arches backwards and to the left. It then descends behind the heart through the thoracic cavity a little to the left of the thoracic vertebrae. At the level of the 12th thoracic vertebra it passes behind the diaphragm then downwards in the abdominal cavity to the level of the 4th lumbar vertebra, where it divides into the right and left common iliac arteries.
Throughout its length the aorta gives off numerous branches. Some of the branches are paired, i.e. there is a right and left branch of the same name, for instance, the right and left renal arteries supplying the kidneys, and some are single or unpaired, e.g. the coeliac artery. The aorta will be described here according to its location:
- thoracic aorta
- abdominal aorta
Ascending aorta. This is the short section of the aorta that rises from the heart. It is about 5 cm long and lies well protected behind the sternum.
The right and left coronary arteries are its only branches. They arise from the aorta just above the level of the aortic valve and supply the myocardium.
Arch of the aorta. This is a continuation of the ascending aorta. It begins behind the manubrium of the sternum and runs upwards, backwards and to the left in front of the trachea. It then passes downwards to the left of the trachea and is continuous with the descending aorta.
Three branches arise from its upper aspect:
- brachiocephalic artery or trunk
- left common carotid artery
- left subclavian artery.
The brachiocephalic artery is about 4 to 5 cm long and passes obliquely upwards, backwards and to the right. At the level of the sternoclavicular joint it divides into the right common carotid artery and the right subclavian artery.
Descending aorta in the thorax. This part is continuous with the arch of the aorta and begins at the level of the 4th thoracic vertebra. It extends downwards on the anterior surface of the bodies of the thoracic vertebrae to the level of the 12th thoracic vertebra, where it passes behind the diaphragm to become the abdominal aorta.
The descending aorta in the thorax gives off many paired branches which supply the walls and organs of the thoracic cavity.
