Fundamentals: Obligate and Facultative Processes
To understand the core of regulation, nephron processes must be divided into obligatory and controllable ones.
Obligatory reabsorption occurs in the proximal convoluted tubule and the loop of Henle. These processes are genetically determined and virtually impervious to external control:
- In the proximal tubule, 100% of glucose, amino acids, and vitamins are reabsorbed. About 70% of water, sodium, and chloride ions are also reabsorbed here.
- In the descending limb of the loop of Henle, water exits passively, while in the ascending limb, sodium is actively reabsorbed (this limb 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 fine-tune the final urine composition in response to changes in blood pressure and blood osmolarity.
Nervous Regulation and Hemodynamics
The glomerular filtration rate depends directly on the hydrostatic blood pressure in the glomerular capillaries. The main regulator of renal vascular 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, thereby increasing filtration volume.
- Constriction of afferent arterioles: lowers capillary pressure and decreases the volume of primary urine.
Under extreme conditions, such as massive hemorrhage or severe painful shock, SNS tone increases so drastically that renal blood flow drops precipitously. Urine formation ceases entirely, resulting in pain-induced anuria.
Humoral Regulation: Three Groups of Hormones
Hormones and biologically active substances affect the kidneys via 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-action hormones (directly affect substance transport across 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 products rises in the blood, acting as an osmotic component that draws water along and increases diuresis. Adrenaline and thyroxine can also induce glucosuria (glucose in the urine), increasing the volume of excreted fluid.
Antidiuretic Mechanism: The RAAS Pathway
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.
RAAS Biochemical Cascade:
- Juxtaglomerular cells release the enzyme renin into the blood.
- In plasma, renin encounters angiotensinogen (synthesized by the liver) and cleaves a fragment from it to form inactive angiotensin I.
- In pulmonary blood vessels, angiotensin-converting enzyme (ACE) converts it into active angiotensin II.
Angiotensin II constricts arterioles (rapidly increasing blood pressure), triggers thirst via the hypothalamus, and stimulates the adrenal cortex to release aldosterone. Aldosterone enhances sodium and water reabsorption in the distal tubules. Concurrently, the pituitary gland releases vasopressin (antidiuretic hormone), rendering the collecting ducts permeable to water. Result: diuresis decreases, urine becomes concentrated, and blood volume and pressure are restored.
Natriuretic Mechanism
This system operates in reverse during an excess of water and salts. An increase in blood volume leads to atrial stretch. In response, cardiomyocytes secrete atrial natriuretic peptide (ANP).
This hormone is a direct antagonist of aldosterone. In the distal tubules and collecting ducts, it blocks the reabsorption of sodium and chloride ions while halting the secretion of potassium and protons. Water stops being reabsorbed into the blood. As a result, diuresis increases significantly, and excess fluid and sodium leave the body, lowering blood pressure.