Kidney Physiology:

1. Introduction:

The kidneys are vital organs in the urinary system, playing a crucial role in filtering blood, removing waste products, and maintaining the body’s internal balance. The kidneys also participate in blood pressure regulation, red blood cell production, and various other functions.

2. Kidney Functions:

• Endocrine Functions:

• Blood Pressure Regulation:

• The kidneys release renin, an enzyme involved in the renin-angiotensin-aldosterone system (RAAS).

• RAAS is a vital hormone system in blood pressure control.

• When blood pressure drops, the kidneys release renin, triggering a chain reaction that produces angiotensin II, a potent vasoconstrictor, increasing blood pressure.

• Angiotensin II also stimulates the adrenal cortex to secrete aldosterone, a hormone that enhances Na+ and water reabsorption in the distal convoluted tubule, contributing to increased blood volume and pressure.

• Red Blood Cell Production Stimulation:

• The kidneys produce erythropoietin (EPO), a hormone that stimulates the bone marrow to produce red blood cells.

• When blood oxygen levels decrease, the kidneys release EPO, enabling the body to increase red blood cell production for more efficient oxygen transport.

• Exocrine Functions:

• Urine Formation and Excretion:

• The kidneys filter blood, eliminating waste products like urea, creatinine, uric acid, toxins, and other excess substances.

• Urine is formed from the filtered blood fluid, following reabsorption and secretion processes in the renal tubules.

3. Structural Unit of the Kidney:

• Nephron: The fundamental functional unit of the kidney, composed of:

• Glomerulus: A specialized network of capillaries located within Bowman’s capsule.

• Bowman’s Capsule: A sac that envelops the glomerulus, capturing the filtered fluid.

• Renal Tubule: Includes the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

4. Nephron Location:

• Nephrons are primarily concentrated in the renal cortex, with a portion of the loop of Henle extending into the renal medulla.

5. Capillary System in the Medullary Region:

• Capillaries run parallel to the loop of Henle, forming straight vessels (vasa recta).

• Vasa recta play a vital role in maintaining the hypertonicity of the renal medulla, contributing to efficient water reabsorption.

6. Tissue Secreting Blood Pressure and Red Blood Cell Production Regulating Hormones:

• Juxtaglomerular apparatus: A structure located near the glomerulus, secreting crucial hormones like renin and erythropoietin.

7. Nephron Classification:

• Functionally, nephrons are categorized into two types:

• Cortical nephrons: Account for approximately 80% of total nephrons, primarily involved in secretion.

• Juxtamedullary nephrons: Account for about 20% of total nephrons, primarily responsible for reabsorption.

8. Vascular System within the Nephron:

• Afferent and glomerular capillary network: Functional capillaries involved in blood filtration.

• Peritubular capillary network: Nutrient-supplying capillaries nourishing the renal tubule cells.

9. Characteristic of the Efferent Arteriole of Juxtamedullary Nephrons:

• The efferent arteriole of juxtamedullary nephrons does not form a network as seen in cortical nephrons.

10. Blood Filtration Process in the Kidney:

• Blood filtration in the kidney is incredibly rapid due to the substantial blood flow through the kidney (approximately 20% of cardiac output).

• Renal blood flow through the straight vessels (vasa recta) is characterized by low flow and slow velocity.

11. Filtration Pressure:

• The capillary pressure in the renal tubules is suitable for the reabsorption function in the tubular system.

• When renal blood flow decreases, the renal cortex is more susceptible to functional impairment because the cortex receives 80-90% of the blood supply to the kidneys.

12. Factors Influencing Filtration Rate:

• Renal edema: The capsule limits kidney swelling -> increased interstitial pressure -> decreased glomerular filtration rate.

• Afferent arteriole dilation to the glomerulus:

• Dilation increases renal blood flow and filtration.

• Strong and prolonged constriction reduces filtration as plasma retention leads to increased filtration and protein accumulation, raising oncotic pressure and decreasing filtration pressure.

• Sympathetic nerve stimulation:

• Moderate stimulation causes afferent arteriole constriction and decreased filtration rate.

• Strong stimulation results in a significant reduction in renal blood flow, leading to temporary cessation of filtration.

13. Reabsorption Process in the Renal Tubule:

• Substances reabsorbed entirely or nearly completely: glucose, protein, lipids.

• Substances reabsorbed as needed: electrolytes, vitamins.

• Substances secreted entirely: metabolic byproducts.

• Substances secreted as needed: excess electrolytes.

14. Reabsorption Mechanism in the Renal Tubule:

• Proximal convoluted tubule:

• Primarily reabsorbs nutrients like glucose, proteins, amino acids, and vitamins.

• Reabsorbs Na+, Cl-, HCO3-, H2O, and urea.

• Secretes H+ and creatinine.

• Loop of Henle:

• Descending limb: Water reabsorption via diffusion.

• Ascending limb: Na+ and urea reabsorption.

• Distal convoluted tubule:

• Na+ reabsorption (dependent on aldosterone levels).

• Water reabsorption (dependent on ADH levels).

• K+ secretion (dependent on aldosterone levels).

• Secretion of substances like phenol, hippuric acid, creatinine, and drug metabolites.

• Active H+ secretion.

• Collecting duct:

• Water reabsorption (dependent on ADH levels).

• Na+ and urea reabsorption.

• Active H+ secretion.

15. Role of the Juxtaglomerular Complex:

• The juxtaglomerular complex plays a crucial role in regulating renal blood flow and red blood cell production.

• Renin-angiotensin system: Regulates renal blood flow.

• Erythropoietin: Stimulates red blood cell production in the bone marrow.

16. Note:

• Kidney function can be affected by various factors, including illnesses, diet, and the environment.

• Individuals should undergo regular health checkups to detect early kidney issues.

• Protect your kidney health by:

• Maintaining a healthy diet, limiting salt, fat, and sugar.

• Drinking plenty of water.

• Engaging in regular physical activity.

• Managing blood pressure and blood sugar levels.

17. Conclusion:

The kidneys are essential organs with numerous physiological functions.

Understanding kidney physiology helps us grasp the organ’s workings and how to safeguard its health.