Transport

Transport in animalsThe cardiac cycle

The cardiac cycle refers to the sequence of events which takes place during the completion of one heartbeat. It involves repeated contraction and relaxation of the heart muscle. Contraction is called systole and relaxation is called diastole. It occurs as follows:

Atrial diastole. During the time when the atria and the ventricles are both relaxed, blood returning to the heart under low pressure in the veins enters the two atria. Oxygenated blood enters the left atrium and deoxygenated blood enters the right atrium. At first the bicuspid and tricuspid valves are closed but, as the atria fill with blood, pressure in them rises. Eventually it becomes greater than that in the relaxed ventricles and the valves are pushed open. Semi-lunar valves are closed. Atrial systole. When atrial diastole ends, the two atria contract simultaneously. This is termed atrial systole and results in blood being pumped into the ventricles. Ventricular systole.

Almost immediately (about 0.1 to 0.2 seconds later) the ventricles contract. This is called ventricular systole. When this occurs the pressure in the ventricles rises and closes the atrioventricular valves, preventing blood from returning to the atria. When the pressure of ventricles exceeds that in the arteries, it forces open the semi-lunar valves of the aorta and pulmonary artery and the blood enters these vessels. The closing of the atrioventricular valves during ventricular systole produces the first heart sound, described as lub. Ventricular diastole. Ventricular systole ends and is followed by ventricular diastole. Pressure in the relaxing ventricles drops below that in the arteries. The high pressure developed in the aorta and pulmonary artery tends to force some blood back towards the ventricles and this closes the semi-lunar valves of the aorta and pulmonary artery. Hence backflow into the heart is prevented. The closing of the valves causes the second heart sound, dub. The two heart sounds are therefore:Ventricular systole = lub

Ventricular diastole = dub

The heart contracts, or beats, about seventy times a minute, and each heartbeat last about 0.85 second.

Cardiac cycle

TimeAtriaVentricles

0.15 secSystoleDiastole

0.30 secDiastoleSystole

0.40 secDiastoleDiastole

Ventricular systole, and the elastic recoil of the arteries as blood at high pressure is forced through them, causes a pulse.

Myogenic stimulation of heart rate

When a heart is removed from a mammal and placed in a well-oxygenated salt solution at 37 0C it will continue to beat rhythmically for a considerable time, without stimuli from the nervous system or hormones. This demonstrates the myogenic nature of the stimulation of the heart.

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