Nephron Anatomy and Physiology

Anatomy and Physiology of the Nephron

Pressure Filtration

• Afferent Arteriole: Blood going into the glomerulus.

• Efferent Arteriole: Blood leaving the glomerulus, going to peritubular capillaries.

• Filtrate: Comes out of the glomerulus and is collected by the Bowman's capsule. Plasma and filtrate are nearly identical in concentrations at this stage.

• Process: Blood pressure in the glomerulus forces plasma (less proteins) out of the blood into the Bowman's capsule.

• Composition of Filtrate: Water, salts, glucose, amino acids, and nitrogenous wastes.

• Concentration Differences: The afferent arteriole has less plasma compared to the efferent arteriole because fluid has been filtered out, concentrating the formed elements in the afferent arteriole.

Selective Reabsorption

• Materials are moved from the filtrate back into the blood.

• Location: Filtrate from Bowman's capsule moves into the proximal convoluted tubule (PCT), then to the loop of Henle.

• PCT Cells: Highly specialized with lots of mitochondria and microvilli to increase surface area which facilitates efficient active transport.

• Reabsorption Efficiency: Glucose and amino acids are 100% removed from the PCT and actively pumped back into the blood.

• Water Reabsorption: 65% of the water is reabsorbed in the PCT via a three-step process (explained next).

Three-Step Process for Water Reabsorption

1. Sodium Ions: Actively pumped into the blood.

2. Chlorine Ions: Follow sodium ions due to charge attraction.

3. Water Movement: Moves by osmosis into the blood because the blood becomes hypertonic relative to the filtrate due to the movement of sodium and chlorine ions.

Tubular Excretion

• Location: Distal convoluted tubule (DCT).

• Materials are moved from the blood into the tubule.

• Filtrate flow Filtrate comes from the loop of Henle, travels through the DCT, and then to the collecting duct.

• Once the filtrate leaves the collecting duct, it is classified as urine.

• Fine-Tuning: The DCT is where fine-tuning of blood composition is performed to make adjustments.

Fine Tuning Adjustments in the Distal Convoluted Tubule

1. Water Reabsorption: 10-15% is reabsorbed under hormonal control.

   • Urine can be dilute or concentrated based on the body's hydration needs.

2. Sodium Ion Reabsorption: Controlled by aldosterone to regulate blood pressure.

3. pH Balance: Kidneys remove excess substances like hormones (HCG), vitamins, and drugs (e.g., penicillin) from the blood.

Water Reabsorption Summary

• Proximal Convoluted Tubule (PCT): Reabsorbs approximately 65% of the water.

• Distal Convoluted Tubule (DCT): Reabsorbs 10-15% of the water, regulated by hormonal control.

• Loop of Henle: Reabsorbs about 15% of the water.

• Collecting Duct: Reabsorbs 5-10% of the water, depending on the body's needs.

• All locations utilize the three-step process for water reabsorption.

Loop of Henle and Medullary Hypertonicity

• Descending Loop: Permeable to water only.

  • Water is reabsorbed as the loop descends into the increasingly hypertonic medulla.

• Ascending Loop: Impermeable to water.

  • Sodium chloride (NaCl) is actively transported out, maintaining high tonicity in the medulla.

  • The removal of NaCl helps maintain the concentration gradient, with the medulla being more concentrated at the bottom and less concentrated at the top.

• This hypertonicity facilitates water and urea extraction in the collecting duct.

• The three-step process (sodium, chlorine, water) is utilized.

Countercurrent Mechanism in the Loop of Henle

• The medulla becomes increasingly hypertonic, causing water to leave the descending loop.

• As filtrate ascends, sodium and chloride leave – some by active transport, some by osmosis.

• The ascending loop pumps sodium ions out, causing water to exit the descending loop via the three-step process.

• The ascending loop is impermeable to water, so water moves into the blood.

Variations in Loop of Henle Length

• Mammals in the desert have longer loops of Henle that extend deeper into the renal medulla which allows for more extended water and sodium reabsorption to conserve water.