Physiology
Renal Physiology
Renal physiology for MBBS and NEET-PG: nephron function, glomerular filtration, tubular reabsorption, countercurrent mechanism and hormonal control, mapped to NMC codes PY7.
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Renal Physiology
Renal physiology for MBBS and NEET-PG: nephron function, glomerular filtration, tubular reabsorption, countercurrent mechanism and hormonal control, mapped to NMC codes PY7.
Renal physiology covers how the kidney filters blood and regulates body fluids. It deals with the nephron and glomerular filtration, tubular reabsorption and secretion, the countercurrent mechanism of urine concentration, and the hormonal control of water, sodium and acid-base balance.
High-yield: Renal Physiology
- The nephron is the functional unit of the kidney and comprises the glomerulus and the tubule.
- The glomerular filtration rate is normally about 125 millilitres per minute.
- Filtration depends on the balance of hydrostatic and oncotic pressures across the glomerular membrane.
- The proximal tubule reabsorbs about two-thirds of the filtered sodium and water along with glucose and amino acids.
- Glucose is normally fully reabsorbed, and glycosuria appears once the transport maximum is exceeded.
- The loop of Henle sets up the medullary concentration gradient by the countercurrent mechanism.
- The ascending limb is impermeable to water and actively pumps out sodium, potassium and chloride.
- Antidiuretic hormone increases water reabsorption in the collecting duct by inserting aquaporins.
- Aldosterone acts on the distal tubule to reabsorb sodium and excrete potassium.
- The renin-angiotensin-aldosterone system defends blood pressure and sodium balance.
- Renin is released by the juxtaglomerular cells in response to low renal perfusion pressure.
- Clearance of a substance freely filtered and neither secreted nor reabsorbed, such as inulin, equals the GFR.
- The kidney regulates acid-base balance by reabsorbing bicarbonate and excreting hydrogen ions.
- Erythropoietin and the activation of vitamin D are important endocrine functions of the kidney.
Hormones acting on the nephron
- **Antidiuretic hormone:** Inserts aquaporins in the collecting duct to reabsorb water.
- **Aldosterone:** Distal tubule: reabsorbs sodium, excretes potassium.
- **Renin-angiotensin:** Defends blood pressure and sodium balance; renin from JG cells.
- **Inulin clearance:** Equals GFR because it is filtered but not secreted or reabsorbed.
NMC competencies in this chapter
- **PY7.1:** Functional anatomy of the kidney and the nephron
- **PY7.2:** Renal blood flow and its regulation
- **PY7.3:** Glomerular filtration and its measurement
- **PY7.4:** Tubular reabsorption and secretion
- **PY7.5:** Concentration and dilution of urine
- **PY7.6:** Regulation of water and electrolyte balance
- **PY7.7:** Role of the kidney in acid-base balance
- **PY7.8:** Micturition and its neural control
Frequently Asked Questions
What is the glomerular filtration rate?
The glomerular filtration rate is the volume of plasma filtered by all the glomeruli each minute, normally about 125 millilitres per minute, and it is a key measure of kidney function.
How does the kidney concentrate urine?
The loop of Henle uses a countercurrent mechanism to build a high solute concentration in the medulla, and antidiuretic hormone then lets water leave the collecting duct to concentrate the urine.
What does aldosterone do in the kidney?
Aldosterone acts on the distal tubule and collecting duct to reabsorb sodium and water and to excrete potassium and hydrogen ions.
Why is inulin clearance used to measure GFR?
Inulin is freely filtered at the glomerulus but is neither reabsorbed nor secreted by the tubule, so the amount cleared exactly reflects the filtration rate.
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