MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY, AND PATHOLOGY - Ya.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

CHAPTER 5. BLOOD AND LYMPH CIRCULATION AND ITS PATHOLOGY

CARDIAC ACTIVITY

1. PHASES OF CARDIAC ACTIVITY

By contracting, The Heart pumps Blood into the Arteries. The ventricles of the heart perform this function.

The right and left atria serve as reservoirs for incoming blood, thus functioning as temporary blood depots.

Heart Valves ensure that blood flows in only one direction.

The activity of the atria and ventricles consists of their systole (contraction) and diastole (relaxation). The cyclical alternation of systole and diastole in the Chambers of the heart is called the cardiac cycle. At a heart rate of 75 beats per minute, the duration of the cardiac cycle is 0.8 s

Heart contractions begin with atrial systole, which lasts for 0.1 s. At this time, the cuspid valves are open toward the ventricles, and blood flows from the atria into the ventricles. During atrial systole, blood cannot flow back into the Veins because the venous orifices are compressed by circular Muscles. Atrial systole is followed by atrial diastole and simultaneous ventricular systole, which lasts 0.33 s

Ventricular systole is divided into two periods: tension and blood ejection. At the beginning of ventricular systole, Muscle tone increases, the atrioventricular valves close, and the semilunar valves of the aorta and pulmonary artery—which were already closed—remain shut (period of tension). As a result, blood pressure in the ventricles exceeds the pressure in the aorta and pulmonary artery, causing the semilunar valves to open and blood to rush into the vessels (period of blood ejection).

Ventricular systole is followed by ventricular diastole, which consists of relaxation and filling periods. During the relaxation period, ventricular muscle tone drops, and the semilunar valves close (due to the increased blood pressure in the aorta and pulmonary artery). The atrioventricular valves are not yet open. This leads to a drop in intraventricular pressure, which results in the opening of the atrioventricular valves and the flow of blood from the atria back into the ventricles. In a working heart, atrial diastole partially overlaps with ventricular diastole, a phase known as the cardiac pause. During this pause, blood flows from the venae cavae into the right atrium and from the Pulmonary veins into the left atrium. The pause is followed by atrial systole, during which the ventricles undergo active blood filling.

The strict sequence of atrial and ventricular systole and diastole, along with the heart's valve apparatus, facilitates unidirectional blood flow.