Circulation of blood
Oxygen (bound to hemoglobin in red blood cells) is the most critical nutrient carried by the blood.
It starts with the contraction of the two auricles (atria). The ventricles at this time are relaxing (or dilating) and are empty . Therefore, the blood from the auricles passes into the ventricles easily. When the ventricles contract, the auricles relax. The blood from the ventricles under pressure tends to return to the auricles, but the flaps of the two cuspid valves get tightened and puffed up, thus closing the passage and preventing the return of blood. The chordae tendinae hold the flaps of the valves in position and prevent their overturning into the auricles.
The only course left for the ventricular blood is to enter the pulmonary artery from the right ventricle and the aorta from the left ventricle. The mouths of the pocket ‐ like valves at the bases of these two blood vessels face away from the ventricles. Therefore, the blood leaving the ventricles presses the valves flat and gets a clear passage in between. When the ventricles dilate, the blood from the pulmonary artery and the aorta tends to return, the blood fills the pockets of the valves and closes the passage.
- Atrial (auricular) muscles contract.
- Openings of vena cava and pulmonary vein close.
- Blood enters ventricles by crossing through tricuspid and mitral valves.
- Semilunar valves at the roots of pulmonary artery and aorta are closed to prevent flow of blood back into ventricles.
- Ventricular muscles contract.
- Tricuspid and mitral valves close with a jerk producing the sound "LUBB".
- Blood passes into aorta and pulmonary artery through semilunar valves.
- Atria draw in blood through the openings of vena cava and pulmonary vein.
- Chordae tendinae hold the valves in position preventing their upturning due to pressure exerted by the contracting ventricles.