Understanding CIRCULATION

BLOOD CIRCULATION
General Pulmonary Circulation Systematic Circulation Hepatic Portal System Circulation Coronary Circulation

General.
The blood system of vertebrates is a closed system, i.e. the blood vessels are not open -ended. In the organs the arteries divide to form arterioles (small arteries) which in turn divide to form numerous capillaries. The capillaries unite to form larger vessels, called the venules (small veins). The venules leave the tissues and join with other venules to form veins. Veins leave the organs and eventually join the venae cavae. It is thus clear that the arterial and venous blood link up by means of capillaries in the tissues.
In this section we will discuss the main circulatory system.

Pulmonary Circulation.
The pulmonary artery arises from the right ventricle and tranports deoxygenated blood (oxygen-poor) to the lungs, where the blood becomes oxygenated again. The four pulmonary veins return the oxygenated blood (oxygen-rich) to the left atrium of the heart. The pulmonary circulation is also referred to as the lesser circulation. The summary of the pulmonary circulation is thus:
Right Ventricle - pulmonary artery - lungs - pulmonary veins - left atrium - left ventricle.

Systemic Circulation.
Oxygenated blood is pumped from the left ventricle into the aorta. Branches of the aorta convey blood to all the tissues and organs of the body (except the lungs). The tissue cells are oxygenated and deoxygenated blood returned to the heart via the superior and inferior venae cavae. The blood then flows via the tricuspid valve into the right ventricle, from where it joins the pulmonary circulation. The systemic circulation is also referred to as the greater circulation. The summary of the systemic circulation is thus:
Left ventricle - aorta - organs - venae cavae - right atrium - left ventricle.

The Hepatic Portal System of Circulation.
This system serves the intestines, spleen, pancreas and gall bladder. The liver receives it blood from two main sources. The main sources are the hepatic artery, which as a branch of the aorta, supplies oxygenated blood to the liver and the hepatic portal vein, which is formed by the union of veins from the spleen, the stomach, pancreas, duodenum and the colon. The hepatic portal vein transports, inter alia, the following blood to the liver:
absorbed nutrients from the duodenum;
white blood cells (added to the circulation) from the spleen;
poisomous substances, such as alcohol which are absorbed in the intestines, and
waste products, such as carbon dioxide from the spleen, pancreas, stomach and duodenum.
The hepatic artery and hepatic portal vein open into the liver sinuses where the blood is in direct contact with the liver cells. The deoxygenated blood, which still retains some dissolved nutrients, eventually flows into the inferior vena cava via the hepatic veins.Coronary Circulation.
This circulation supplies the heart muscle itself with oxygen and nutrients and conveys carbon dioxide and other waste products away from the heart. Two coronary arteries lead from the aorta to the heart wall, where they branch off and enter the heart muscle. The blood is returned from the heart muscle to the right atrium through the coronary vein, which enters the right atrium through the coronary sinus.