BLOOD CIRCULATION Dr.Sisara Bandara Gunaherath MBBS

Pulmonary Circulation Deoxygenated (used) blood is pumped out of the right ventricle. It travels through the pulmonary valve into the pulmonary artery, leaving the heart. Deoxygenated (used) blood reaches the lungs. Here, carbon dioxide is removed from the blood and oxygen is added to it. The fresh blood leaves the lungs.

Fresh blood enters the left atrium through the pulmonary vein. Then, it is pumped through the mitral valve into the left ventricle. This ends the pulmonary circulation.

Systemic Circulation Oxygenated (fresh) blood is pumped out of the left ventricle. It travels through the aortic valve into the aorta, leaving the heart. Oxygenated (fresh) blood reaches the head and the body (gut, kidney, muscles), where the oxygen in it is used and replaced by carbon dioxide.

Deoxygenated blood is collected through the vena cava into the right atrium and is pumped through the tricuspid valve into the right ventricle. This ends the systemic circulation.

Diffusion The exchange of molecules between cells and blood occurs at the capillary level. Capillaries are very small blood vessels with very thin walls. Oxygen and nutrients diffuse from the blood into the cell and carbon dioxide and waste diffuse from the cell into the blood.

Cardiac Cycle The cardiac cycles refers to the repeating pattern of contraction and relaxation of the heart The phase of contraction is called systole and the phase of relaxation is called diastole

The right and left atria contract almost simultaneously , followed by contraction of the right and left ventricles 0.1 to 0.2 seconds later During the time when both the atria and ventricles are relaxed, the venous return of blood fills the atria The build up of pressure that results causes the AV valves to open and blood flow from atria to ventricles

The ventricles are about 80% filled with blood even before the atria contract Contraction of atria adds the final 20% to the end diastolic volume , which is the total volume of blood in the ventricles at the end of diastole Contraction of the ventricles in systole ejects about 2/3 of the blood they contain ( Stroke volume ) leaving 1/3 of blood in the ventricles as end systolic volume.

Pressure changes during cardiac cycle When the heart is in diastole, pressure in the systemic arteries is about 80mmHg During systole pressure in the systemic arteries is about 120mmHg.