Nephron Structure, Function, and Reabsorption

Capillary Bed Composition

• Capillary beds are composed of capillaries.
• Capillaries are made of simple squamous epithelial tissue known as endothelium.
• Bowman's capsule is also lined with simple squamous epithelial tissue.

Filtration Layers

• Filtrate moving from the glomerulus into Bowman's capsule crosses two layers of simple squamous epithelial tissue.
  • One layer from the capillaries.
  • One layer from Bowman's capsule.

Glomerulus and Bowman's Capsule Structure

• The endothelium of the glomerulus capillaries is composed of simple squamous epithelial tissue.
• Endothelium refers to the simple squamous epithelial tissue lining blood vessels.
• Bowman's capsule is also composed of simple squamous epithelial tissue.
• All epithelial tissue sits on top of a basement membrane.
• There are two basement membranes:
  • One for the glomerulus.
  • One for Bowman's capsule.
• Filtrate passes through two layers of simple squamous epithelial tissue and two basement membranes.

Filtrate Composition

• Filtrate consists of blood plasma components:
  • Water.
  • Ions.
  • Waste products.
  • Hormones.
  • Nutrients.
• It does not include:
  • Red blood cells.
  • White blood cells.
  • Platelets.
  • Plasma proteins.

Epithelial Tissue in the Nephron

• Proximal convoluted tubule: Simple cuboidal epithelial tissue.
• Distal convoluted tubule: Simple cuboidal epithelial tissue.
• Collecting duct: Simple cuboidal epithelial tissue.
• Loop of Henle:
  • Thick portions: Simple cuboidal epithelial tissue.
  • Thin segments (descending and ascending limbs, bottom of the loop): Simple squamous epithelial tissue.

Nephron Processes: Filtration, Reabsorption, and Secretion

• Filtration: Occurs in the renal corpuscle (Bowman's capsule and glomerulus).
  • Involves moving components of the blood plasma into Bowman's capsule.
  • Approximately 180 liters of filtrate are produced daily.
• Reabsorption: Moves substances from the nephron back into the blood.
  • Occurs along the nephron, including the convoluted tubules, loop of Henle, and collecting duct.
  • Does not occur in the renal corpuscle.
• Secretion: Moves substances from the blood into the nephron.
  • Occurs in the proximal and distal convoluted tubules, and the loop of Henle.

Direction of Movement:

• Filtration: From the blood into the nephron (specifically, Bowman's capsule).
• Reabsorption: From the nephron back into the blood.
• Secretion: From the blood into the nephron.

Importance of Reabsorption

• The body reabsorbs substances that are filtered but still needed, such as glucose.
  • If glucose gets filtered, it is reabsorbed back into the blood for energy use.

Filtration vs. Secretion

• Both move substances from the blood into the nephron, but they occur in different locations.
• Filtration: Only happens in the renal corpuscle.
• Secretion: Happens in the proximal convoluted tubule, loop of Henle, or distal convoluted tubule (mainly the proximal convoluted tubule).

Net Result of Processes

• Blood plasma is filtered into Bowman's capsule multiple times a day.
• The nephron and brain determine which substances to reabsorb back into the blood.
• Additional substances are secreted from the blood into the nephron for excretion.

Excretion Equation

• Excretion is what leaves the body in urine.
• Excretion = Filtration - Reabsorption + Secretion
  • Filtration: Movement from blood into nephron.
  • Reabsorption: Movement from nephron into blood.
  • Secretion: Movement from blood into nephron.

Reabsorption and Secretion Location

• Secretion:
  • Can happen in the proximal convoluted tubule, loop of Henle or distal convoluted tubule, but primarily happens in the proximal convoluted tubule.
• Reabsorption:
  • Can happen in the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and even the collecting duct, but primarily happens in the proximal convoluted tubule.

Physiology Primer: Sodium and Potassium Gradients

• High concentration of sodium (Na^+) outside of cells.
• Low concentration of sodium (Na^+) inside of cells.
• High concentration of potassium (K^+) inside of cells.
• Low concentration of potassium (K^+) outside of cells.

Ion Movement

• Sodium (Na^+) wants to move into the cell due to its chemical gradient (high to low concentration).
• Potassium (K^+) wants to move out of the cell due to its chemical gradient (high to low concentration).
• Ions require channels to move through the membrane.

Action Potentials

• In neurons or muscle cells:
  • Sodium (Na^+) influx causes depolarization.
  • Potassium (K^+) efflux causes repolarization.

Maintaining Chemical Gradients

• Sodium (Na^+) and potassium (K^+) continually move in and out of cells, but their gradients must be maintained.
• The sodium-potassium pump restores chemical gradients.

Active vs. Passive Processes

• Diffusion (high to low) is a passive process.
• Moving ions against their gradients (low to high) is an active process that requires energy.

Energy Types

• Chemical energy: ATP.
• Kinetic energy: Energy of movement.

Sodium-Potassium Pump

• Pumps sodium (Na^+) out of the cell and potassium (K^+) into the cell against their gradients.
• Requires ATP.
• Ratio: 3 sodium (Na^+) out for every 2 potassium (K^+) in.

Diffusion and Kinetic Energy

• Passive movement of ions down their gradient creates kinetic energy.
  • Sodium (Na^+) moving into the cell.
  • Potassium (K^+) moving out of the cell.

Reabsorption in the Proximal Convoluted Tubule (PCT)

• The peritubular capillary is located near the proximal convoluted tubule.

Lumen and Extracellular Fluid (ECF)

• Lumen: The open space in a blood vessel or nephron tubule.
• Filtrate in the lumen has a similar chemical makeup to the extracellular fluid (ECF).
• Extracellular fluid (ECF) includes interstitial fluid and blood plasma.
• Intracellular fluid is inside cells.

Reabsorption Process

• Reabsorption is the movement from the lumen back into the peritubular capillary (blood).

Membrane Crossing During Reabsorption

• Substances being reabsorbed must cross two membranes:
  • The apical surface of the tubule cell.
  • The basal surface of the tubule cell.

Sodium Reabsorption

• Sodium (Na^+) diffuses into the cell through a channel on the apical surface (passive process).
• Sodium (Na^+) is then pumped out of the cell by the sodium-potassium pump on the basal surface (active process).
• Nephrons use a lot of ATP to actively pump substances during reabsorption.

Glucose Reabsorption

• Glucose needs to be reabsorbed for energy (cellular respiration).
• The distribution of glucose:
  Lower concentration both in the lumen and capillary
  Higher concentration inside thecell.

Active Transport of Glucose

• Active transport is required to move glucose from the lumen into the cell because it's moving from an area of low to high concentration.
• Kinetic energy from sodium (Na^+) moving down its gradient drives glucose transport.

Sodium-Glucose Transporter (SGLT)

• The SGLT transports both sodium (Na^+) and glucose.
• Sodium (Na^+) moves passively into the cell, creating kinetic energy.
• The kinetic energy is used to move glucose against its gradient into the cell.
  The SGLT is located on the apical side of the tubule cell.

Glucose Movement

• Glucose passively moves across the basolateral membrane from the tubule cell into the peritubular capillary due to the concentration gradient (high to low).

Diabetes and Glucose Reabsorption

• In diabetes, high blood glucose leads to a high glucose concentration in the filtrate.
• Excess glucose saturates the SGLT transporters, and some glucose is not reabsorbed.
• Unabsorbed glucose remains in the filtrate and travels through the nephron.

Water Follows Glucose

• Water follows solutes, including glucose.
• In diabetes, excess glucose in the urine draws water with it, leading to frequent urination.
  Diabetics may experience:
  Frequent urination.
  Sweet-smelling urine (due to the presence of glucose).