Basics: obligatory and facultative processes
To understand the essence of regulation, it is necessary to divide nephron processes into mandatory and variable ones.
Obligatory reabsorption occurs in the proximal convoluted tubule and the loop of Henle. These processes are genetically determined and virtually unsusceptible to external control:
- Glucose, amino acids, and vitamins are 100% reabsorbed in the proximal tubule. About 70% of water, sodium, and chloride ions are also reabsorbed here.
- Water passively exits in the descending limb of the loop of Henle, while sodium is actively reabsorbed in the ascending limb (which is impermeable to water).
Facultative (regulated) reabsorption and secretion take place in the distal convoluted tubules and collecting ducts. It is here that the nervous and endocrine systems finely tune the final composition of the urine in response to changes in blood pressure and blood osmolarity.
Neural regulation and hemodynamics
The glomerular filtration rate depends directly on the hydrostatic blood pressure in the glomerular capillaries. The main regulator of renal vessel tone is the sympathetic nervous system (SNS).
The effect of the SNS depends on which vessels constrict:
- Constriction of efferent arterioles: leads to an increase in glomerular pressure, increasing the filtration volume.
- Constriction of afferent arterioles: reduces pressure in the capillaries and decreases the volume of primary urine.
Under extreme conditions, such as massive blood loss or severe pain shock, SNS tone increases so drastically that renal blood flow drops sharply. Urine production ceases entirely, leading to so-called pain-induced anuria.
Humoral regulation: three groups of hormones
Hormones and bioactive substances affect the kidneys through various pathways. They are divided into three groups:
- Vasoactive hormones (affect vascular tone, indirectly altering diuresis).
- Vasoconstrictors: adrenaline, serotonin, vasopressin.
- Vasodilators: histamine, kinins, acetylcholine.
- Direct-acting hormones (directly affect the transport of substances in the epithelium).
These include melatonin (from the pineal gland), corticotropin-releasing hormone, ACTH, and local renal hormones such as medullin.
- Hormones acting via metabolism.
Thyroid hormones and growth hormone enhance protein breakdown. The concentration of metabolic waste products rises in the blood, acting as an osmotic component that pulls water along and increases diuresis. Adrenaline and thyroxine can also cause glucose to appear in the urine (glucosuria), increasing the volume of excreted fluid.
Antidiuretic mechanism: RAAS function
In water deficiency, salt loss, or a drop in blood pressure (BP), the renin-angiotensin-aldosterone system (RAAS) is activated. The sensors are juxtamedullary nephrons: they lack myogenic autoregulation of blood flow, allowing them to accurately assess systemic blood pressure levels.
Biochemical cascade of the RAAS:
- Juxtaglomerular cells secrete the enzyme renin into the bloodstream.
- In plasma, renin encounters the protein angiotensinogen (synthesized by the liver) and cleaves a fragment from it to form inactive angiotensin I.
- In the pulmonary blood vessels, the enzyme converting enzyme (ACE) turns it into active angiotensin II.
Angiotensin II constricts arterioles (rapidly increasing blood pressure), triggers the sensation of thirst via the hypothalamus, and stimulates the adrenal cortex to release aldosterone. Aldosterone enhances sodium and water reabsorption in the distal tubules. Simultaneously, the pituitary gland releases vasopressin (antidiuretic hormone), which makes the collecting ducts permeable to water. Result: diuresis decreases, urine becomes concentrated, and blood volume and pressure are restored.
Natriuretic mechanism
The system operates in reverse during an excess of water and salts. An increase in blood volume leads to atrial stretch. In response, cardiomyocytes secrete natriuretic hormone.
This hormone is a direct antagonist of aldosterone. In the distal tubules and collecting ducts, it blocks the reabsorption of sodium and chloride ions and halts the secretion of potassium and protons. Water stops being reabsorbed into the blood. As a result, diuresis increases significantly, and the excess fluid volume and sodium leave the body, lowering blood pressure.
Briefly: answers to topical questions
What other factors besides the RAAS and natriuretic peptide regulate circulating blood volume?
Circulating blood volume, aside from the RAAS and natriuretic hormone, is regulated by the hormone vasopressin, behavioral thirst mechanisms, and hemodynamic redistribution reactions.
- Vasopressin (ADH) — sharply increases the water permeability of the collecting duct wall, enhancing its reabsorption.
- Central regulation — upon activation of osmo- and volume receptors, the thirst center in the hypothalamus is stimulated, triggering active drinking behavior.
- Hemodynamic mechanisms — blood is mobilized from blood depots (spleen, skin, lungs), and circulatory centralization occurs to maintain systemic pressure and circulating blood volume.
How does aldosterone act at the cellular level in the distal tubules?
Aldosterone penetrates the cell and interacts with intracellular receptors. The resulting complex acts on nuclear DNA, stimulating mRNA synthesis and subsequent translation of induced proteins.
These proteins exert their effects through three mechanisms:
- Sodium channel components — integrate into the apical membrane from the tubular lumen side, increasing the reabsorption of sodium ions from the urine into the cell.
- Tricarboxylic acid cycle enzymes — such as citrate synthase in mitochondria, ensuring the production of ATP required for active ion transport.
- Sodium-potassium pump (Na+, K+-ATPase) — localized on the basolateral membrane, maintains a low intracellular Na+ concentration and a high K+ concentration, pumping reabsorbed sodium into the interstitium/blood.
The RAAS cascade is easy to remember by organs: the Liver provides the substrate (angiotensinogen), the Kidneys provide the starter enzyme (renin), and the Lungs provide the activating enzyme (converting enzyme).
Class and exam questions
Why are regulatory mechanisms aimed at reabsorption rather than filtration?
Due to differences in efficiency. If glomerular filtration changes by 10%, the amount of final urine shifts by only 0.1%. But if tubular reabsorption changes by 10%, diuresis increases or decreases 10-fold.
Where are the sensors that trigger renin production located?
The sensors are juxtaglomerular cells of afferent arterioles in juxtamedullary nephrons. They react to a decrease in volumetric blood flow and a drop in blood pressure.
How does vasopressin work?
Vasopressin (antidiuretic hormone) is secreted by the posterior pituitary when there is a water deficit in the blood. It acts on the collecting ducts, drastically increasing the permeability of their walls to water, leading to water retention in the body.
What else to study on this topic
- Features of the blood supply to juxtamedullary nephrons
- Mechanisms of obligatory secretion of ammonia and hydrogen protons in the proximal tubules
- Central effects of angiotensin II on hypothalamic structures
- Effect of parathyroid hormone and calcitonin on urine composition
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