Question

How are the kidneys involved in controlling blood volume? How is the volume of blood in the body related to arterial pressure?

Answer 1

• The primary mechanism by which the kidneys regulate blood volume is by adjusting the amount of water and sodium lost into the urine. At different sites along the proximal tubules, thick ascending limb of the loop of Henle, distal and collecting tubules, sodium transport is regulated by angiotensin II (Ang II), which increases sodium transport thereby leading to sodium retention. In the collecting tubules, another hormone (aldosterone), stimulates sodium transport from the tubular fluid into the interstitium. Together, Ang II and aldosterone provide a powerful mechanism for increasing sodium retention and consequently fluid volume in the body. A third hormone, antidiuretic hormone (ADH), increases water permeability in the late distal tubules and collecting tubules. 
• 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 preloaded increases ventricular stroke volume. 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.

Answer 2

• When the volume of blood decreases the flow pressure, decreases.
• This can be sensed by the hypothalamus and osmoreceptors are stimulated and the antidiuretic hormone is secreted from the neurohypophysis.
• The aquaporins in the proximal convoluted tubules and collecting tubule reabsorb water. Hence the blood volume increases and the blood pressure increases.