Kidneys, excretion and water balance explained
Homeostasis and response • Hormonal coordination in humans
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ADH, permeability of kidney tubules and negative feedback
Anti-diuretic hormone (ADH) is produced by the pituitary gland and circulates to the kidneys where it alters the permeability of collecting ducts and tubules. Increased ADH makes the kidney tubules more permeable to water, causing greater water reabsorption into the blood and producing smaller volumes of more concentrated urine. Reduced ADH makes the tubules less permeable, producing larger volumes of dilute urine. This mechanism restores blood water concentration toward normal and operates as negative feedback: a detected increase or decrease in blood water content triggers hormonal changes that correct the deviation .
Routes of water, ion and urea loss
Water leaves the body by urination, sweating and breathing out as water vapour. Ions are lost in urine and sweat. Urea, a nitrogenous waste, is removed mainly in urine after blood transport from tissues and the liver. The lungs release water vapour during exhalation; the skin releases water and some ions in sweat; the kidneys filter and excrete urea, excess salts and variable amounts of water depending on physiological needs . The relative loss by each route changes with conditions such as exercise, temperature and hydration status, causing adjustments in kidney reabsorption and ADH secretion.
Function of kidneys: filtration and selective reabsorption
Each kidney contains many nephrons that filter blood plasma under hydrostatic pressure. Filtration allows water, glucose, ions and urea to pass from blood into the nephron filtrate while larger cells and most proteins remain in the blood. Selective reabsorption then recovers all useful glucose, most required ions and variable amounts of water back into the blood via transport processes along the tubule. The remaining filtrate, containing all urea, excess salts and excess water, becomes urine and is excreted. This three-step summary of (1) filtration, (2) selective reabsorption and (3) excretion maintains internal concentrations of water and salts and removes toxic urea .
Osmotic changes in body fluids and effects on cells
Body-fluid osmolarity determines water movement across cell membranes by osmosis. An increase in blood solute concentration (hypertonic blood) draws water out of cells, causing cell shrinkage and impaired cellular processes. A decrease in blood solute concentration (hypotonic blood) causes water to move into cells, producing swelling and possible lysis if severe. Homeostatic control of plasma osmolarity, primarily via ADH-driven changes in renal water reabsorption, prevents harmful osmotic shifts that disrupt enzyme activity and cell function .
Deamination of excess amino acids and urea formation
Excess dietary or cellular amino acids undergo deamination in the liver: the amino group is removed to form ammonia, a toxic and highly soluble compound. The liver rapidly converts ammonia to urea, a less toxic, water-soluble compound, which enters the blood and is transported to the kidneys for excretion. This two-step route (deamination → ammonia → urea) safely removes nitrogen from the body while limiting ammonia accumulation in tissues and blood .
Interpreting concentrations before and after filtration
Filtration leaves plasma components in the nephron filtrate at similar concentrations to blood plasma for small solutes. Selective reabsorption then removes all glucose and selected ions and water, so concentrations measured in blood after filtration differ accordingly. Tables or bar charts showing glucose, ions and urea concentrations before and after filtration demonstrate: glucose present before filtration and reabsorbed to normal levels after filtration; ions partly reabsorbed (some decrease after filtration); urea present before filtration and removed (lower in blood after filtration). Simple comparisons of bar heights or table values indicate which substances are retained, reabsorbed or excreted by the kidney .
Basic principles of kidney dialysis
Dialysis replaces some kidney filtration functions by circulating the patient's blood alongside dialysis fluid across a selectively permeable membrane in a machine. Dialysis fluid is formulated to have physiological concentrations of glucose and salts so there is no net diffusion of these useful substances, while the fluid contains no urea. Urea diffuses from the blood into the dialysis fluid down its concentration gradient and is removed. Dialysis therefore clears waste solutes and helps restore correct ion and fluid balance, but requires repeated sessions and does not replace all kidney functions permanently .
Treating kidney failure: dialysis versus transplant (evaluation)
Dialysis advantages: immediate life-sustaining treatment, availability without donor matching, avoids major surgery for some patients. Dialysis disadvantages: frequent long sessions, dietary and fluid restrictions, risk of infection at access sites and reduced quality of life; dialysis is not a cure. Transplant advantages: potential permanent restoration of kidney function, freedom from regular dialysis and improved quality of life. Transplant disadvantages: shortage of donor organs, risk of immune rejection, lifetime immunosuppressant drugs with side effects, and major surgery risks. Clinical choice depends on patient health, donor availability and long-term risks and benefits .
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