The Heart and Circulation

Blood Flow Round the Circulation * With each systole 70 ml. of blood are ejected from each ventricle. * This quantity is termed the stroke volume. * The aorta and large arteries are elastic vessels and they accommodate this blood with a relatively small rise in pressure. During the ensuing diastole, there is no output from the heart, and the pressure in the arteries falls. The blood now in the arteries is pulsatile, and if an artery is cut, the blood shoots out in spurts.

* The blood flow to the tissues is maintained by the elastic recoil of the arterial walls, and by the time the capillaries have been reached, the flow has ceased to be pulsatile; the steady flow of blood through these vessels shows no variation in systole or diastole.

The blood returns to the heart along the veins, and the venous flow is a steady one. If a vein is cut the blood oozes out from the distal end without any pulsation. On approaching the heart, the flow again becomes pulsatile as the ventricles are unable to receive the venous blood during systole

Function of the Atria: *The blood returning to the heart during ventricular diastole passes through the atria and enters the ventricles. *The blood returning during systole is unable to do this as the A-V valves are closed. *The atria act as a storage reservoir for this blood until the end of systole when the A-V valves open. With the opening of these valves the ventricles fill rapidly with the waiting blood.

Further filling occurs as blood returns during diastole *Further filling occurs as blood returns during diastole. One-tenth of a second before the onset of systole, when the ventricles are already 70% full of blood, the atria contract and complete the filling of the ventricles by adding the remaining 30%. *Atrial contraction is not essential for life, but the heart is a much more efficient pump when the atria are contracting. *The flow of blood through the lungs and tissues is continued but the flow is intermittent through the heart.

Pressure Changes In The Heart: Although the pressures could be expressed in dyne/cm.2 or lb. per in.2, it is usual to express blood pressure in terms of the height of a corresponding column of mercury measured in millimeters. The units are mm. Hg. These pressures are measured with reference to the atmospheric pressure.

The pressure in the intra-thoracic part of the circulation may be less than the atmospheric pressure. This is due to the elastic recoil of the lungs. The pressure in the atria, for example, may be 755 mm. Hg (absolute) whilst the outside barometric pressure is 760 mm. Hg. Such a pressure is denoted as -5 mm. Hg meaning 5 mm. Hg less than atmospheric. Such a pressure is referred to as a negative pressure.

The atria are thin-walled vessels and the pressure of blood in them is never very far from the pressure outside in the surrounding mediastinum. This pressure is slightly negative (-2 mm. Hg) and it becomes more negative when breathing in (-8 mm. Hg).

Respiratory variations in atrial pressure are seen in records of atrial pressure which are obtained with the chest closed by passing a catheter along the veins into the heart and into the right atrium. They disappear when the chest is opened as, for example, in thoracic surgery.

When the A-V valves (mitral and tricuspid) are open during diastole (up to 0.1 and from 0.45 to 0.9 seconds), the atrial pressure is very slightly greater than the ventricular pressure, otherwise the blood would not enter the ventricles. But to all intents and purposes the atrial and ventricular pressures are identical during this phase.

Similarly during ventricular systole, when the aortic and pulmonary valves are open, the pressures in the ventricles will be equal to those in the aorta and pulmonary artery (from 0.15 to 0.4 and from 0.95 seconds onwards).

Ventricular Pressure Changes: The left ventricle develops a maximum pressure of 120 mm. Hg during systole and therefore the aortic pressure reaches the same peak value. During diastole the ventricular pressure falls to that in the thorax which is approximately 0 mm. Hg (atmospheric pressure). The ventricular changes are thus 120/0 mm. Hg.

The pressure in the aorta, however, is maintained by the elastic recoil of the arterial walls. Due to this elastic recoil the pressure in the aorta has only fallen to 80 mm. Hg by the time the next systole occurs. Thus the pressure in the aorta fluctuates between 120 mm. Hg and 80 mm. Hg. These changes are denoted as 120/80 mm. Hg.

The right ventricle pumps out exactly the same quantity of blood as the left ventricle but at a much lower pressure. The right ventricle develops a maximum pressure of 25 mm. Hg. The pressure falls to 0 mm. Hg in diastole and the right ventricle pressure changes are 25/0 mm. Hg. The pressure in the pulmonary artery falls to 8 mm. Hg during diastole and the pressure changes in this vessel are, therefore 25/8 mm. Hg.

Atrial Pressure Changes The atrial pressure tracing follows a complex pattern due to the interplay of several factors. Atrial contraction (atrial systole) is associated with a pressure rise in the atria. It lasts for only 0.1 seconds (time 0 to o.1 and 0.8 to 0.9 seconds). For the reminder of the cardiac cycle the atrial muscle is in u state or relaxation (atrial diastole).

The isometric contraction of the ventricles causes the mitral and tricuspid valves to bulge into the atria causing a further increase in atrial pressure (time 0.1 to 0.15 and 0.9 to 0.95 seconds). However, as soon as the aortic and pulmonary valves open, the ventricular volumes decrease rapidly.

The A-V ring descends increasing the volume of the atria The A-V ring descends increasing the volume of the atria. The atrial pressure falls sharply. Blood returning to the heart cannot now enter the ventricles and the pressure in the atria builds up (time 0.2 to 0.45 seconds).

With the relaxation of the ventricles the atrioventricular ring moves upwards towards the base of the heart decreasing the volume of the atria and causing the atrial pressure to rise further (just before 0.45 seconds).

With the opening of the mitral and tricuspid valves at time 0 With the opening of the mitral and tricuspid valves at time 0.45 seconds the pressures in the atria and ventricles fall. Blood returning to the heart enters the atria and ventricles and their pressures build up together till time 0.8 seconds is reached when another atrial contraction occurs.