Heterometric regulation of the heart

Preload

Is the degree of tension on the muscle when it begins to contract (filling pressure). In cardiac contraction, the force developed by cardiac muscle is considered as End Diastolic Pressure

• Right Ventricle EDP affects Central Venous Pressure
• Left Ventricular EDP affects Peripheral Venous Pressures

Afterload

Is load against which the muscle exerts its contractile force (arterial pressure). Afterload is related to aortic pressure (LV) and pulmonary artery pressure (RV). It Increases if blood pressure rises and if outflow valves narrow and an increase in afterload will reduce output

Cardiac Output

Defined as the quantity of blood pumped into the aorta each minute. It varies with:

• Body metabolism
• Exercise
• Age
• Body size

In man it is about 5,6 L/min and in women is about 4,9 L/min

• Cardiac output= heart rate x stroke volume

Venous Return

Defined as the quantity of blood flowing from the veins into the right atrium each minute Venous return and cardiac output equal each other

Frank-Starling Mechanism

When increased quantities of blood flow into the heart, the increased blood stretches the walls of the heart chambers, as a consequence cardiac muscle contract with increased force to release the extra blood The main reason peripheral factors are more important than the heart itself in controlling cardiac output is that the heart has a built-in mechanism that allows it to pump automatically whatever amount of blood flows into the right atrium from the veins "... a strong heart that is filled with blood empties itself more or less completely, in other words, [filling of the heart with blood] changes the extent of contractile power." Carl F.W. Ludwig CFW. Lehrbuch der Physiologie des Menschen. Vol 2. Lepzig, Germany: CF Winter;1852–1856:p.73. Stretchiness of the heart causes the heart to pump faster, increasing heart rate

• Cardiac Output= Stroke volume x Heart Rate

Effects

Pulmonary Oedema

Is due to failure of the left ventricle of the heart to adequately remove blood from the pulmonary circulation ("cardiogenic pulmonary oedema"). If the output of the right ventricle is greater than the left ventricle, blood will accumulate in the lungs, pulmonary blood pressure would rise and fluid will be forced into the lung interstitium and alveoli.

Postural Hypotension

Gravity causes blood to pool in the legs and Central Venous Pressures (CVP) falls. On standing from a prone position, gravity causes blood to pool in the legs and CVP falls. This, in return, causes a fall in cardiac output (due to starling’s law) and consequently a fall in blood pressure. This postural hypotension is rapidly corrected by the baroreceptor reflex which causes venoconstriction (raising CVP) increases the heart rate and output and restores the blood pressure.

Bibliography

GUYTON, Arthur C – HALL, John E. Textbook of Medical Physiology. 12th edition. 2011. ISBN 978-1-4160-4574-8.

WARD, Jeremy P. T – CLARKE, Robert W – LINDEN, Roger. Physiology at a Glance. 2nd edition. 2007. ISBN 1405177233.