Renal Reabsorption Notes

The Nephron

The nephron is the functional unit of the kidney responsible for reabsorption and filtration.

Sodium-Potassium Pump:
- Located at the base membrane of the PCT.
- Actively transports Na^+ into the interstitial space.
- This active transport establishes concentration gradients that drive other transport processes.
Sodium Entry:
- Na^+ enters the cell at the apical membrane.
Reabsorption Mechanisms:
- Water Reabsorption: Occurs via osmosis through aquaporins.
  - Water reabsorption increases the concentration of remaining solutes.
- Solute Reabsorption: These concentrated solutes are reabsorbed later.
- Paracellular Route: Various ions (e.g., Cl^-, Ca^{2+}, K^+) and urea diffuse through the paracellular route (between cells).
- Cotransport: Reabsorption of organic nutrients and specific ions occurs via cotransport at the apical membrane.
- Transcellular Route: Lipid-soluble substances diffuse through the transcellular route (across cells).

Glucose Transporters:
- Glucose transporters are present.
Water Permeability:
- The PCT and the descending limb of the loop of Henle are permeable to water.
Solute Gradient:
- Sodium and glucose are pumped out of the tubule, creating a solute gradient.
Water Movement by Osmosis:
- Water follows the solutes by osmosis, a passive transport mechanism.
Osmolarity:
- Since water and solutes leave in proportion, there is minimal change in osmolarity throughout the length of the PCT, remaining around 300 mOsM.

Location of DCT:
- Chemicals removed from the DCT do not enter the medulla because the DCT is located within the cortex.
- Instead, these chemicals enter nearby blood vessels.
Aldosterone:
- A steroid hormone that regulates the Na-K pump in the distal tubule.
- Increased aldosterone leads to increased sodium reabsorption into the blood.
ADH (Antidiuretic Hormone):
- Usually rises when aldosterone levels are high.
- Increases the permeability of the membrane and collecting duct to water.
- More water follows sodium into the blood.
- Reduces the amount of water in urine.
Dehydration:
- Increases aldosterone levels, causing sodium and water retention in the body.
ANP (Atrial Natriuretic Peptide):
- A hormone that down-regulates sodium transport into the blood.
- Secreted by the heart when blood pressure is high.
- Increased ANP reduces sodium in the blood.
- Causes more water to remain in the urine, increasing urine volume.

Triggers for ADH Release:
- ADH levels increase when dehydrated, plasma osmolarity is high, or blood pressure is low.
Mechanism of Action:
- When ADH binds to its receptors, aquaporins (water channels) open, allowing water to move out of the collecting duct and into the blood.
Nature of ADH:
- ADH is not a lipid-based hormone.
Urine Osmolarity Control:
- To produce urine with low osmolarity, ions and urea are secreted near the bottom of the collecting duct, adding solutes to the medulla.
Alternative Name:
- In the USA, ADH is called vasopressin based on its role in lifting blood pressure.
Diuresis:
- Refers to increased urine production, sometimes indicating excessive urine production.
Antidiuretic Hormone's Function:
- Antidiuretic hormone (ADH) works against diuresis, reducing urine production.
Effect on Urine Volume:
- Without ADH, a large amount of dilute urine is produced.
- With ADH, water is retained and less, more concentrated urine is produced.

Key Parts: Renal Corpuscle, Proximal Convoluted Tubule, Loop of Henle (Descending and Ascending Limb), Distal Convoluted Tubule, Collecting Duct.
Processes: Filtration, Reabsorption, Secretion.
Legend Abbreviations:
- Aa. - Amino acids
- ADH - Antidiuretic hormone
- Ald. - Aldosterone
- ANP - Atrial natriuretic peptide
- AT II - Angiotensin II
- UroD - Urodilatin
Permeability: High permeability to water (H2O+).
Reabsorption in Red, Secretion in Green

An interactive exercise is available online for labeling the kidney nephron diagram: https://wordwall.net/resource/6185367/biology/kidney-nephron