Renal Physiology: 

.

Introduction: Why Study Renal Physiology?

Infographic showing parts of the nephron (glomerulus, proximal tubule, loop of Henle, distal tubule, and collecting duct) with arrows indicating filtration, reabsorption, secretion, and hormonal control mechanisms like ADH and aldosterone.

Renal physiology is the study of how the kidneys function to maintain homeostasis in the body. The kidneys are more than just waste filters — they regulate fluid balance, electrolytes, acid-base levels, and even blood pressure. Without the kidneys, life would be chemically chaotic.

Structure of the Kidney: 

The kidney is a bean-shaped organ with a highly specialized internal structure designed for filtration and regulation of body fluids. Each kidney measures about 10–12 cm in length, 5–7 cm in width, and weighs around 120–150 grams. It is constituted with the following-

1. Renal Capsule: It is a thin, tough, fibrous outer membrane. Renal capsule protects the kidney from trauma and infection. It also gives the kidney its distinct shape.

2. Renal Cortex: The outer reddish-brown layer just beneath the capsule is known as renal cortex. It contains renal corpuscles (glomerulus + Bowman’s capsule) and convoluted tubules of nephrons. This is the primary site of blood filtration.

3. Renal Medulla: It is located beneath the cortex, appears as cone-shaped renal pyramids. Each pyramid has a base facing the cortex and a papilla pointing inward. It Contains loops of Henle and collecting ducts, responsible for concentration of urine.

4. Renal Columns: It is the cortical tissue extensions that lie between the pyramids. Renal columns provide structural support and carry blood vessels.

5. Renal Pelvis: A funnel-shaped cavity that collects urine from the collecting ducts. It receives urine through minor calyces → major calyces → renal pelvis. Urine is then passed to the ureter.

6. Hilum of the Kidney: A concave notch on the medial side of the kidney. Entry and exit point for: Renal artery (brings blood in), Renal vein (takes filtered blood out), Ureter (transports urine to bladder) and Lymphatic vessels and nerves.

7. Nephron: Each nephron is a microscopic unit inside the kidney that filters blood and forms urine. Here’s a breakdown of its key components.

1. Bowman’s Capsule:

Structure: A double-walled, cup-shaped structure that surrounds the glomerulus.

Function: Acts as the first filtration site in the nephron.

How It Works: Receives the filtrate (water, glucose, salts, urea) from the glomerulus. Large particles like proteins and blood cells do not pass through. It’s like a “catcher’s mitt” that collects the first filtered fluid from blood.

2. Glomerulus:

Structure: A network of capillaries tucked inside Bowman’s capsule.

Function: Performs ultrafiltration of blood under pressure.

How It Works: Blood pressure forces water and small solutes through the glomerular wall into Bowman’s capsule, forming the filtrate.

Filtration occurs due to high pressure and selective permeability of capillary walls.

3. Proximal Convoluted Tubule (PCT):

Structure: A twisted tube immediately after Bowman’s capsule.

Function: Reabsorbs essential nutrients like glucose, amino acids, water, and salts.

How It Works: Uses active and passive transport to return 65–70% of the filtrate’s valuable substances back into the blood. PCT helps prevent nutrient loss and begins urine concentration.

4. Loop of Henle:

Structure: U-shaped section with descending and ascending limbs.

Function: Concentrates urine and regulates water and salt balance.

How It Works:

Descending limb: Permeable to water but not salts → water moves out.

Ascending limb: Permeable to salts but not water → salts move out.

Works like a “counter-current exchanger” to create a salt gradient in the kidney medulla.

5. Distal Convoluted Tubule (DCT):

Structure: Another coiled section after the loop of Henle.

Function: Fine-tunes salt and pH levels of the filtrate.

How It Works: Reabsorbs sodium and calcium, secretes potassium and hydrogen ions. It is responsive to hormones like aldosterone and parathyroid hormone. DCT plays a key role in maintaining electrolyte balance and blood pH. 

6. Collecting Duct:

Structure: A long duct receiving fluid from many nephrons.

Function: Final site for water reabsorption and urine concentration.

How It Works: Water reabsorption is regulated by antidiuretic hormone (ADH). It sends the final urine to the renal pelvis. More ADH = more water reabsorbed = concentrated urine.

How Filtration Works:

1. Glomerular Filtration: It occurs in the renal corpuscle. Blood pressure pushes water and small solutes (like glucose, salts, and urea) into Bowman’s capsule. It filtrates large molecules (e.g., proteins, blood cells) remain in the bloodstream.

2. Tubular Reabsorption: It takes place in the proximal tubule and loop of Henle. Tubular reabsorption helps to save valuable nutrients like glucose, amino acids, and water via active and passive transport into blood.

3. Tubular Secretion: Hydrogen ions, potassium, creatinine, and drugs are secreted into the nephron. It helps maintain pH balance and remove toxins.

4. Excretion: Final urine is collected in the collecting duct, sent to the renal pelvis, and passed through ureters to the bladder.

Functions of the Kidneys

1. Filtration of blood

2. Regulation of electrolytes (Na⁺, K⁺, Ca²⁺)

3. Acid-base balance (pH homeostasis)

4. Blood pressure control (RAAS system)

5. Erythropoietin production (stimulates red blood cell formation)

6. Vitamin D activation

Hormonal Regulation in Renal Physiology

Our kidneys are more than just filters — they are finely tuned organs that respond to hormones to maintain the body’s internal balance (homeostasis). Several hormones play vital roles in controlling water, salts, pH, and mineral levels through the nephron. A brief description of hormonal regulation in renal physiology is given bellow-

1. Antidiuretic Hormone (ADH) – Increases water reabsorption.

2. Aldosterone – Increases Na⁺ reabsorption and K⁺ secretion.

3. Renin – Initiates the renin-angiotensin-aldosterone system (RAAS).

4. Parathyroid Hormone (PTH) – Regulates calcium and phosphate levels.

1. Antidiuretic Hormone (ADH): It is also known as Vasopressin. ADH is produced by Hypothalamus and stored and released by the posterior pituitary. Antidiuretic hormone Increases water reabsorption in the collecting ducts of the nephron. It makes the duct walls more permeable to water, allowing water to move back into the bloodstream. When you're dehydrated or have low blood pressure, ADH levels rise. This helps conserve water and concentrate urine.

💧 More ADH = Less urine, more water in blood
🚱 Less ADH = More urine, risk of dehydration

2. Aldosterone: It is produced by adrenal cortex. Aldosterone is stimulated by Angiotensin II or low blood sodium. It increases Na⁺ (sodium) reabsorption and K⁺ (potassium) secretion in the distal convoluted tubule (DCT) and collecting duct. Sodium reabsorption pulls water along with it, increasing blood volume and pressure and Potassium secretion prevents toxic build-up.

🧂 Sodium stays → water follows → blood pressure rises

3. Renin: It is secreted by Juxtaglomerular cells in the kidney. It starts the Renin-Angiotensin-Aldosterone System (RAAS) during low blood pressure or sodium and converts angiotensinogen (from liver) into angiotensin I, which is later converted to angiotensin II — a powerful vasoconstrictor. Angiotensin II raises blood pressure and stimulates aldosterone release. This system ensures fluid retention and blood pressure regulation.

🩸 Low BP → Renin → Angiotensin → Aldosterone → BP normal again

4. Parathyroid Hormone (PTH): Parathyroid Hormone is produced by Parathyroid glands. It regulates calcium and phosphate levels in the blood. In the kidney, Increases calcium reabsorption and Decreases phosphate reabsorption. It helps maintain strong bones and neuromuscular functions by keeping calcium levels in balance.

🦴 More PTH = more calcium saved, phosphate excreted

These hormones act like managers in a busy factory, ensuring the kidneys fine-tune how much water, salts, and minerals are retained or excreted. Without hormonal regulation, even small imbalances could lead to dehydration, hypertension, or mineral disorders.

FAQs on Renal Physiology

Q1. What is the role of ADH in renal physiology?
ADH increases the permeability of the collecting duct to water, reducing urine volume and conserving body water.

Q2. How do kidneys maintain acid-base balance?
They excrete hydrogen ions (H⁺) and reabsorb bicarbonate (HCO₃⁻) to stabilize blood pH.

Q3. What is GFR and why is it important?
GFR (Glomerular Filtration Rate) measures how much blood is filtered by the kidneys each minute. It’s a vital indicator of kidney health.

Q4. Can kidneys regenerate after damage?
Kidneys have limited regenerative ability. Mild injury may heal, but chronic or severe damage often leads to permanent loss of function.

References

1. Guyton, A.C., & Hall, J.E. (2021). Textbook of Medical Physiology (14th ed.). Elsevier.

2. Sherwood, L. (2015). Human Physiology: From Cells to Systems (9th ed.). Cengage Learning.

3. Tortora, G.J., & Derrickson, B. (2020). Principles of Anatomy and Physiology (16th ed.). Wiley.