Functional blood volume system
The normal specific blood volume is 60.5–84.7 ml/kg. Any deviations in this parameter or changes in electrolyte concentration are instantly detected by volumoreceptors (volume sensors), baroreceptors, and osmoreceptors. Signals from them are relayed to the hypothalamus and cerebral cortex.
The body's response is implemented through several pathways:
- Behavioral regulation: activation of the thirst center and search for water, or, conversely, inhibition of drinking behavior.
- Autonomic and humoral regulation: redistribution of fluid between the intracellular, extracellular, and blood plasma spaces.
- Excretory changes: adjustment of the rate of urine formation, perspiration, respiratory evaporation, and water excretion via the gastrointestinal tract.
- Hemodynamic shifts: alteration of blood vessel tone and blood pooling or mobilization from blood depots.
Response to hypohydration and salt deficiency
A state of fluid and salt deficit (hypoosmolarity and hypovolemia) is accompanied by tissue dehydration and colloid desiccation. A drop in blood osmotic pressure causes decreased secretion of vasopressin (ADH). Initially, this leads to increased diuresis, which further reduces blood volume and blood pressure.
To compensate for the drop in pressure and renal blood flow, the renin-angiotensin-aldosterone system (RAAS) is activated:
- The kidneys secrete the enzyme renin.
- Renin cleaves the plasma protein angiotensinogen into angiotensin I.
- Under the action of the enzyme convertin (ACE), a potent regulator is formed — angiotensin II.
Angiotensin II triggers a cascade of compensatory responses. It stimulates the adrenal cortex to release aldosterone, which dramatically enhances sodium reabsorption in the kidneys, increasing osmotic pressure and secondarily reducing diuresis. Simultaneously, the thirst center in the central nervous system is stimulated (via the supraoptic and paraventricular nuclei of the hypothalamus), initiating copious water intake. Finally, direct vasoconstriction (vasoconstriction) occurs, blood is mobilized from reservoirs, and sweating and moisture evaporation are minimized.
Response to hyperhydration and salt excess
Excessive salt intake increases blood osmolarity, resulting in colloid hydration (tissues bind water). In response, vasopressin production increases, retaining fluid in the kidneys. As a result, blood volume increases, leading to elevated blood pressure and central venous pressure.
Atrial stretch caused by excess blood volume prompts the myocardium to release natriuretic hormone (ANP). This peptide triggers processes directly opposite to the effects of the RAAS:
- Inhibition of retention systems: the secretion of renin and aldosterone is blocked, and convertin activity is suppressed. The thirst center in the anterior hypothalamic nuclei is inhibited, eliminating the urge to drink.
- Direct effects: blood vessel relaxation (vasodilatation) occurs, which lowers blood pressure. In the kidneys, sodium reabsorption is inhibited, leading to profuse diuresis (elimination of excess water and salts).
- Additional mechanisms: perspiration, respiratory fluid evaporation, and GI water excretion are enhanced, while excess blood is redistributed into vascular depots, reducing the load on the cardiovascular system.
Briefly: answers to topical questions
In which specific parts of the renal tubules does aldosterone enhance sodium reabsorption?
Aldosterone enhances facultative sodium reabsorption in the distal convoluted tubules and collecting ducts of the nephron. In these segments, the hormone activates Na⁺,K⁺-ATPase, leading to enhanced return of sodium to the blood (followed by passive water reabsorption). Simultaneously, aldosterone increases the secretion of potassium, ammonia, and hydrogen ions from the blood into the urine. These processes are activated during salt depletion or decreased blood pressure.
From which zone of the adrenal cortex is aldosterone secreted?
Aldosterone is secreted from the zona glomerulosa (zona glomerulosa) of the adrenal cortex. This is the outermost zone of the cortex, located directly beneath the capsule. In this zone, secretory cortical endocrinocytes form specific clusters resembling rounded arches ("glomeruli"). It is in this zone that mineralocorticoids are synthesized, the principal representative being aldosterone—a steroid hormone derived from cholesterol.
What other types of natriuretic peptides exist in the body besides atrial natriuretic peptide?
In the cited sources, besides the atrial natriuretic factor/atriopeptin, brain natriuretic peptide (BNP) is mentioned. NT-proBNP—the N-terminal fragment of the brain natriuretic peptide prohormone—is also noted. BNP or NT-proBNP is tested to diagnose heart failure and monitor treatment; their levels are also discussed in the section on pulmonary hypertension.
To avoid confusing hormone functions, remember: Aldosterone Actively Accumulates (retains) sodium and water, whereas Natriuretic hormone is Aimed At excretion.
Class and exam questions
Where is the salt appetite center localized?
The salt appetite center is located in the anterior nuclei of the hypothalamus. It is activated when osmotic pressure decreases or when tissues have an increased demand for electrolytes.
How are vasopressin and osmotic pressure related?
With an increase in osmotic pressure (salt excess), vasopressin secretion rises, leading to water retention in the body. When osmotic pressure decreases, vasopressin production drops, and excess free water is excreted in the urine.
What role does convertin play in pressure regulation?
Convertin (angiotensin-converting enzyme) catalyzes the conversion of inactive angiotensin I into active angiotensin II, which causes vasoconstriction and stimulates aldosterone release, increasing blood pressure.
What else to study on this topic
- Mechanisms of vascular volumoreceptor and baroreceptor function
- Synthesis and functions of blood plasma angiotensinogen
- Features of tissue hydration and colloid dehydration
- Physiological role of blood depots in hemodynamics
- Effect of the supraoptic and paraventricular hypothalamic nuclei on drinking behavior
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