Functional Histology of Kidney

• What are the two main parts of the nephron in the kidney?

  1. Renal Corpuscle

  • Composed of the glomerulus and Bowman's capsule.
  • Functions in filtration of blood to form the initial filtrate.

  1. Renal Tubule

  • Composed of the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.
  • Functions in the reabsorption and secretion of various substances.
• What is the structure and function of the glomerulus in the renal corpuscle?

  • Structure: A network of capillaries located within the Bowman’s capsule. It is made up of fenestrated endothelial cells.
  • Function: Responsible for filtering blood. The high-pressure system forces water, ions, and small molecules out of the blood to form the filtrate, while larger molecules like proteins and blood cells remain in the blood.
• Describe the Bowman’s capsule structure and its role in the nephron.

  • Structure: A double-walled, cup-shaped structure surrounding the glomerulus. It consists of an outer parietal layer and an inner visceral layer composed of podocytes.
  • Function: Collects the filtrate that is formed by the glomerulus. It funnels the filtrate into the proximal convoluted tubule (PCT).
• What is the role of podocytes in the glomerulus?

  • Structure: Specialised cells with foot-like extensions that wrap around the capillaries of the glomerulus.
  • Function: Form the filtration barrier. The foot processes create filtration slits that allow selective passage of small molecules while preventing larger proteins and cells from passing into the filtrate.
• What is the function and structure of the proximal convoluted tubule (PCT)?

  • Structure: The PCT is lined with a single layer of cuboidal epithelial cells with microvilli on their apical surface.
  • Function: Involved in reabsorption of water, ions (Na+, Cl-, K+), glucose, amino acids, and other essential substances from the filtrate back into the bloodstream. It also plays a role in secretion of waste products into the filtrate.
• Explain the structure and function of the loop of Henle in the nephron.

  • Structure: A U-shaped structure with a descending and ascending limb. The descending limb is permeable to water, while the ascending limb is impermeable to water but actively transports Na+ and Cl- ions.
  • Function: Establishes the countercurrent multiplier system in the kidney, which is essential for creating a concentration gradient in the renal medulla, allowing the kidney to concentrate urine and conserve water.
• What are the roles of the distal convoluted tubule (DCT) in the nephron?

  • Structure: Lined with cuboidal cells, but with fewer microvilli compared to the PCT.
  • Function: Involved in the reabsorption of sodium (Na+), calcium (Ca2+), and water, as well as the secretion of potassium (K+) and hydrogen ions (H+) into the filtrate. The DCT plays a key role in the regulation of electrolyte balance and pH.
• What is the role of the collecting duct in the nephron?

  • Structure: Lined with cuboidal to columnar epithelial cells. The collecting ducts run through the renal medulla and merge to form larger ducts.
  • Function: Involved in the final concentration of urine. It is regulated by hormones such as antidiuretic hormone (ADH) for water reabsorption and aldosterone for sodium reabsorption.
• What is the significance of the juxtaglomerular apparatus (JGA) in kidney function?

  • Structure: Located where the distal convoluted tubule (DCT) passes close to the afferent arteriole of the glomerulus. The JGA consists of macula densa cells, juxtaglomerular cells, and extraglomerular mesangial cells.
  • Function: The JGA plays a role in regulating glomerular filtration rate (GFR) through the renin-angiotensin-aldosterone system (RAAS). It helps maintain blood pressure and fluid balance.
• What is the renal corpuscle and how does it contribute to the nephron’s function?

  • Structure: Composed of the glomerulus and Bowman’s capsule.
  • Function: The renal corpuscle is where blood is filtered, and the filtrate (containing water, ions, and small molecules) is formed. It initiates the process of urine formation.
• What is the role of the afferent arteriole in the glomerulus?

  The afferent arteriole carries blood to the glomerulus. Its larger diameter creates pressure, aiding the filtration of blood.
• What is the role of the efferent arteriole in the glomerulus?

  The efferent arteriole drains blood from the glomerulus. Its smaller diameter increases resistance and glomerular pressure, which supports filtration.
• How does the structure of the glomerulus and arterioles support kidney function?

  The structure of the glomerulus and its arterioles ensures efficient filtration. The afferent arteriole increases pressure in the glomerulus, pushing small molecules through to the Bowman’s capsule, while retaining larger molecules in the blood.
• What are the three layers of the glomerular filtration barrier?

  1. Endothelial cells of capillaries (fenestrated).
  2. Basement membrane (repels negatively charged proteins).
  3. Podocytes (foot-like extensions forming filtration slits).
• What is the function of the glomerular filtration barrier?

  The barrier selectively filters plasma, allowing small molecules like water and electrolytes into the Bowman’s capsule while blocking larger proteins and cells from entering the filtrate.
• What is the glomerular filtration rate (GFR)?

  GFR is the rate at which blood is filtered through the glomerulus, typically measured in milliliters per minute.
• How is GFR regulated by the arterioles?

  GFR is regulated by constriction or dilation of the afferent and efferent arterioles. A dilated afferent arteriole or constricted efferent arteriole increases GFR, while the opposite decreases it.
• What are juxtaglomerular cells?

  Juxtaglomerular cells are specialised smooth muscle cells located in the walls of the afferent arteriole, near the glomerulus.
• What is the function of juxtaglomerular cells?

  Juxtaglomerular cells release renin in response to low blood pressure or low sodium levels. This activates the renin-angiotensin-aldosterone system (RAAS) to increase glomerular pressure and GFR.
• What are the two layers of Bowman’s capsule?

  1. Parietal layer: Simple squamous epithelium forming the outer wall.
  2. Visceral layer: Contains podocytes that wrap around the glomerular capillaries, forming filtration slits.
• How do podocytes in Bowman’s capsule contribute to filtration?

  Podocytes have foot-like extensions that form filtration slits, preventing large proteins and cells from passing into the filtrate while allowing small molecules to pass.
• What is the Bowman’s capsule’s function in the filtration process?

  The Bowman’s capsule collects the filtrate (water, electrolytes, glucose, waste) from the glomerulus while preventing large molecules and blood cells from entering
• What are the three layers of the glomerular membrane involved in ultrafiltration of plasma?

  1. Fenestrated Endothelium
  2. Basement Membrane
  3. Podocytes (Visceral Layer of Bowman's Capsule)
• What is the role of the fenestrated endothelium in ultrafiltration in the glomerulus?

  The fenestrated endothelium is the innermost layer of the glomerular membrane. It contains small pores (fenestrae) that allow the passage of water, small solutes, and plasma proteins but prevent larger molecules (like blood cells) from passing through. This contributes to the initial filtration of plasma.
• What is the role of the basement membrane in ultrafiltration?

  The basement membrane is a gel-like structure that acts as a selective barrier, primarily preventing the passage of large plasma proteins like albumin. It is negatively charged, which also repels negatively charged molecules, helping to ensure that only small and neutral solutes pass through into the filtrate.
• What is the role of podocytes in the filtration process?

  Podocytes are specialised epithelial cells of the visceral layer of Bowman's capsule. They have long, finger-like projections (pedicels) that interdigitate, creating filtration slits. These slits, along with the slit diaphragms, allow the passage of water and small solutes while blocking the movement of larger molecules, such as proteins. Podocytes also help to maintain the structural integrity of the filtration barrier.
• How do the three layers of the glomerular membrane work together to perform ultrafiltration?

  • The fenestrated endothelium allows small molecules to pass but blocks cells.
  • The basement membrane provides structural support and acts as a molecular sieve, preventing large proteins from passing.
  • The podocytes create filtration slits that further restrict the passage of large molecules while ensuring small solutes and water can pass into the Bowman's capsule.
• What are the factors that affect ultrafiltration across the glomerular membrane?

  1. Pressure differences (Hydrostatic pressure in the glomerular capillaries vs. the pressure in Bowman's capsule)
  2. Selectivity of the filtration barrier (pore size, charge, and membrane composition)
  3. Plasma protein concentration (influences oncotic pressure)
• What is the cellular structure of the Proximal Convoluted Tubule (PCT) and its relation to function?

  • The PCT is lined by simple cuboidal epithelial cells with tall microvilli (brush border) that increase the surface area for absorption.
  • The cells contain numerous mitochondria for active transport of ions and solutes.
  • The PCT is responsible for the reabsorption of water, electrolytes, glucose, amino acids, and other essential nutrients from the filtrate back into the bloodstream.
• What are the key features of the Thin Limb of the Loop of Henle and its functional role?

  • The Thin Limb is lined by simple squamous epithelium and has a thin membrane, which is highly permeable to water and ions.
  • It plays a key role in the passive movement of water out of the filtrate into the surrounding medullary interstitium due to the osmotic gradient, contributing to the concentration of urine.
• What is the cellular composition of the Ascending Thick Limb of the Loop of Henle and its function?

  • The Ascending Thick Limb is lined by cuboidal to columnar epithelial cells with no brush border.
  • These cells are specialised for the active transport of Na+, K+, and Cl- via the Na+/K+/2Cl- co-transporter, which helps in the formation of a hyperosmotic medullary interstitium that facilitates water reabsorption in other parts of the nephron.
  • This segment is impermeable to water but actively transports ions to contribute to urine concentration.
• What is the role of the Distal Convoluted Tubule (DCT) and its cellular features?

  • The DCT is lined by simple cuboidal epithelium without a brush border and contains fewer mitochondria compared to the PCT.
  • It plays a critical role in the fine-tuning of electrolyte balance, including Na+ reabsorption and K+ secretion.
  • The DCT is also involved in acid-base regulation by secreting hydrogen ions (H+) and conserving bicarbonate.
• How is the Collecting Tubule structured and what is its function in the renal tubule?

  • The Collecting Tubule is lined by simple cuboidal epithelium that can transition to columnar cells in the collecting ducts.
  • It has two types of cells: principal cells, which are involved in Na+ reabsorption and K+ secretion, and intercalated cells, which are involved in acid-base balance through the secretion of H+ or bicarbonate.
  • The Collecting Tubule also plays a significant role in the final concentration of urine under the influence of antidiuretic hormone (ADH), which increases water reabsorption.
• How do the structural features of the PCT, Loop of Henle, DCT, and Collecting Tubule contribute to their specific functions in the kidney?

  • PCT: Tall microvilli and numerous mitochondria aid in the active transport of nutrients and solutes from the filtrate back into the bloodstream.
  • Thin Limb of Loop of Henle: Thin, squamous epithelium allows passive water reabsorption due to the osmotic gradient.
  • Ascending Thick Limb of Loop of Henle: Cuboidal epithelium with active ion transport mechanisms contributes to the establishment of a medullary osmotic gradient.
  • DCT: Lack of brush border and fewer mitochondria indicate less active transport, focusing on fine-tuning ion balance and acid-base homeostasis.
  • Collecting Tubule: Principal and intercalated cells regulate water reabsorption and acid-base balance, particularly under hormonal regulation like ADH.
• Describe the importance of mitochondria in the Proximal Convoluted Tubule (PCT) and Ascending Thick Limb

  • In the PCT, mitochondria are abundant to support active transport mechanisms such as the Na+/K+ pump, which helps in the reabsorption of water and solutes.
  • In the Ascending Thick Limb, mitochondria provide energy for ion transport pumps such as the Na+/K+/2Cl- co-transporter, which aids in the formation of the osmotic gradient necessary for urine concentration.
• What is the structure of the collecting duct in the kidney?

  The collecting duct is a tube-like structure that extends from the distal convoluted tubule to the renal pelvis. It consists of two types of cells:

  1. Principal cells: These are the most abundant and are responsible for water and sodium reabsorption.
  2. Intercalated cells: These cells are involved in acid-base balance by secreting or absorbing hydrogen ions (H⁺) and bicarbonate (HCO₃⁻).
• How does the structure of the collecting duct support its function in water reabsorption?

  • The collecting duct's structure allows it to play a crucial role in water reabsorption due to the presence of aquaporin channels, specifically aquaporin-2 (AQP2), in the principal cells.
  • When antidiuretic hormone (ADH) is present, it stimulates AQP2 expression, making the duct permeable to water.
  • This facilitates the movement of water from the duct into the surrounding medullary interstitium, concentrating urine.
• How do the intercalated cells of the collecting duct contribute to acid-base balance?

  Intercalated cells in the collecting duct are involved in maintaining the body's acid-base balance by regulating hydrogen ions (H⁺) and bicarbonate (HCO₃⁻).

  • Type A intercalated cells secrete H⁺ ions into the urine and reabsorb bicarbonate, helping to correct acidosis.
  • Type B intercalated cells secrete bicarbonate into the urine and reabsorb H⁺, helping to correct alkalosis.
• What is the role of aldosterone in the function of the collecting duct?

  • Aldosterone is a steroid hormone that acts on the collecting duct, specifically on the principal cells.
  • It increases the reabsorption of sodium (Na⁺) and water, and promotes the secretion of potassium (K⁺) into the urine.
  • This helps regulate electrolyte balance and blood pressure by increasing sodium retention, which in turn increases water retention and blood volume.
• What is the importance of the collecting duct in the final concentration of urine?

  • The collecting duct plays a crucial role in the final concentration of urine.
  • It adjusts the volume and concentration of urine by selectively reabsorbing water (under the influence of ADH) and electrolytes.
  • This allows the kidneys to conserve water when necessary, leading to the production of concentrated urine in states of dehydration, or dilute urine when excess water needs to be excreted.