The Human Heart and its function.

The human heart is about the size of a clenched fist. It contains four chambers: two atria and two ventricles. Oxygen-poor blood enters the right atrium through a major vein called the vena cava. The blood passes through the tricuspid valveinto the right ventricle. Next, the blood is pumped through the pulmonary artery to the lungs for gas exchange. Oxygen-rich blood returns to the left atrium via the pulmonary vein. The oxygen-rich blood flows through the bicuspid (mitral) valve into the left ventricle, from which it is pumped through a major artery, the aorta to the rest of the body. . Two valves called semilunar valves are found in the pulmonary artery and aorta.The heart then pumps blood again to the rest of the body to supply oxygen to the cells and collect carbon dioxide to be delivered to the lungs for removal.

 

Gas exchange in the lungs

 

Inhalation and Exhalation

Inhalation is the process of taking air into the lungs. For this to occur, the air pressure inside the lungs must be lower than that of the external atmosphere as air flows from areas of higher pressure to lower pressure. This is achieved by the contraction of the external intercostal muscles and the relaxation of the internal intercostals which acts to pull the ribcage upwards and outwards, hence, increasing the volume of the thoracic cavity. The diaphragm also contracts to extend the cavity further downwards. Due to the function of the pleural membrane, the size of the lungs is increased which, in turn, creates an area of lower air pressure inside the lungs; hence, air is drawn into the lungs.

Exhalation is the process of expelling air out of the lungs. For this to occur, the air pressure inside the lungs must be higher than that of the external atmosphere as air flows from areas of higher pressure to ones of lower pressure. This is achieved by the relaxation of the external intercostal muscles which bring the rib cage inwards and the diaphragm bulges upwards (recoiling to its original shape) respectively. The internal intercostal muscles contract so as to pull the rib cage further inwards, hence, decreasing the volume of the thoracic cavity and that of the lungs which, in turn, creates high pressure in the lungs; hence air is pushed out of lungs. In passive breathing, the movements of the diaphragm are responsible – the ribs aren’t needed.

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