Blood volume is determined by the amount of water and sodium ingested, excreted by the kidneys into the urine, and lost through the gastrointestinal tract, lungs and skin. The amounts of water and sodium ingested and lost are highly variable. To maintain blood volume within a normal range, the kidneys regulate the amount of water and sodium lost into the urine. For example, if excessive water and sodium are ingested, the kidneys normally respond by excreting more water and sodium into the urine. The details of how the kidneys handle water and sodium are beyond the scope of this cardiovascular web site; therefore, the reader is encouraged to consult general medical physiology textbooks to learn more about this topic. The following paragraphs briefly describe how renal excretion of water and sodium are regulated and how blood volume affects cardiovascular function.
Regulation of Blood Volume by Renal Excretion of Water and Sodium
The primary mechanism by which the kidneys regulate blood volume is by adjusting the excretion of water and sodium into the urine. There are several mechanisms by which this regulation occurs. For example, increased blood volume increases arterial pressure, renal perfusion, and glomerular filtration rate. This leads to an increase in renal excretion of water and sodium that is termed pressure natriuresis. In certain types of renal disease, the pressure natriuresis relationship is altered so that the kidneys retain more sodium and water at a given pressure, thereby increasing blood volume.
Activation of the renin-angiotensin-aldosterone system causes increased sodium retention which also leads to reduced water loss into the urine. Both angiotensin and aldosterone, although by different mechanisms, stimulate distal tubular sodium reabsorption and decreases sodium and water loss by the kidney. Activation of the renin-angiotensin-aldosterone system occurs in renal artery stenosis, which is one cause of secondary hypertension. Drugs that block the formation of angiotensin II (i.e., angiotensin converting enzyme inhibitors), or block aldosterone receptors (e.g., spironolactone) enhance sodium and water loss, and thereby reduce blood volume. Therefore, any mechanism or drug that alters the activity of the renin-angiotensin-aldosterone system will affect blood volume.
Another important hormone in regulating water balance is vasopressin (antidiuretic hormone; ADH). This hormone is released by the posterior pituitary. One of its actions is to stimulate water reabsorption in the collecting duct of the kidney, thereby decreasing water loss and increasing blood volume.
How Blood Volume Affects Blood Pressure
Changes in blood volume affect arterial pressure by changing cardiac output. An increase in blood volume increases central venous pressure. This increases right atrial pressure, right ventricular end-diastolic pressure and volume. This increase in ventricular preload increases ventricular stroke volume by the Frank-Starling mechanism. An increase in right ventricular stroke volume increases pulmonary venous blood flow to the left ventricular, thereby increasing left ventricular preload and stroke volume. An increase in stroke volume then increases cardiac output and arterial blood pressure.