The circulatory system- The pulmonary and systemic

The circulatory system consists of the heart and blood vessels which transport blood and other substances to all areas of the body. There are three main types of blood vessels:

1. Arteries (thick muscular and elastic vessels)
2. Capillaries (one cell thick and minute vessels)
3. Veins (have non return valves thinner and less elastic than arteries)

The blood is pumped from the heart to the main arteries which divide into smaller arterioles, the arterioles link with even smaller vessels called capillaries.

The blood travels from the capillaries into the venules, small veins, and then into larger veins before going back to the heart.

Arteries carry oxygenated blood away from the heart with the exception of the pulmonary which carries deoxygenated.

The circulatory system consists of two parts:

1. The systemic circulation, which supplies all the body except for the lungs. Oxygenated blood enters the left atrium of the heart from the pulmonary veins. It then enters the left ventricle via the bicuspid valve before being forced into the aorta by the contraction of the heart beat. From the aorta it subdivides into the left and right subclavian arteries to carry oxygenated blood and nutrients to the head and other major arteries, eg renal, mesenteric supply the organs in the abdominal cavity whilst the iliac arteries subdivide to supply the lower limbs.

The minute capillaries converge to form venules which then form larger vessels called veins. The blood is returned to the heart from the legs and trunk via the inferior vena cava and whilst the subclavian veins drain the arms, the jugular veins drain the head and neck into the superior vena cava. The blood then passes back to the right atrium of the heart.

1. The pulmonary circulation, which carries the blood to and from the lungs. The deoxygenated blood enters the right atrium and passes into the right ventricle which contracts to force the blood into the lungs via the pulmonary arteries. The pulmonary arteries divide into two to supply the right and left lungs. These vessels divide down to capillaries which are one cell thick where gaseous exchange occurs, with oxygen being picked up and carbon dioxide passed to the lungs for exhaling. The blood is now high in oxygen and low in carbon dioxide. The re-oxygenated blood returns to the heart via the pulmonary veins to circulate around the body supplying tissues with oxygen.

The arterial blood supply to the head and face travels from the heart to the common carotid arteries on either side of the neck. From here the blood supply divides into the internal and external carotid arteries. The internal carotid artery feeds into the brain. The external carotid artery feeds into the occipital, temporal and facial arteries which supply the head and face.

The venous blood supply to the face and head are fed by the internal jugular vein in the neck area, which carries blood from the face and head, whilst the external jugular carries blood from the scalp via the occipital and temporal veins and feed into the subclavian vein at the base of the neck /chest region.