The baroreceptor system effectively minimizes short-term fluctuations in the arterial blood pressure. Over the longer term, however, the ability to sustain a constant blood pressure depends on maintenance of a constant blood volume. This dependency arises because alterations in blood volume affect central venous pressure (CVP) and therefore cardiac output (CO) (see Chapter 17). Changes in CO also ultimately lead to adaptive effects of the vasculature which increase peripheral resistance, and therefore blood pressure (see Chapter 39).

Blood volume is affected by changes in total body Na⁺ and water, which are mainly controlled by the kidneys. Maintenance of blood pressure therefore involves mechanisms that adjust renal excretion of Na⁺ and water.

Role of Sodium and Osmoregulation

Alterations in body salt and water content, caused for example by variations in salt or fluid intake or perspiration, result in changes in plasma osmolality (see Chapter 5). Any deviation of plasma osmolality from its normal value of ∼290 mosmol/kg is sensed by hypothalamic osmoreceptors, which regulate thirst and release of the peptide antidiuretic hormone (ADH, or vasopressin) from the posterior pituitary. ADH enhances reabsorption of water by activating V2 receptors in principal cells of the renal collecting duct. This causes aquaporins (water channels) to be inserted into their apical membranes, so increasing their permeability to water. Urine is therefore concentrated and water excretion reduced. ADH also affects thirst. Thus, an increase in plasma osmolality due to dehydration causes increased thirst and enhanced release of ADH. Both act to bring plasma osmolality back to normal by restoring body water content (Figure 29a). Opposite effects are stimulated by a reduction in osmolality. ADH secretion is inhibited by alcohol and emotional stress, and strongly stimulated by nausea. Osmoregulation is extremely sensitive to small changes in osmolality (Figure 29b), and normally takes precedence over those controlling blood volume because of the utmost importance of controlling osmolality tightly for cell function (see Chapter 5).

An important consequence of the above is that blood volume is primarily controlled by the Na⁺ content of extracellular fluid (ECF), of which plasma is a part. Na⁺ and its associated anions Cl⁻ and HCO₃⁻

Stay updated, free articles. Join our Telegram channel

Join