Kidney: Excretion & Osmoregulation
Ultrafiltration occurs in the glomerulus under high hydrostatic pressure, created by the difference in diameter between the wider afferent and narrower efferent arterioles.
Small molecules, including water, glucose, urea, and ions, are forced across the basement membrane and into the Bowman's capsule to form glomerular filtrate.
Large components such as plasma proteins and blood cells are physically too large to pass through the capillary pores and remain in the blood.
This process is non-selective for small molecules; both waste products (urea) and useful nutrients (glucose) enter the nephron at this stage.
Selective Reabsorption occurs in the Proximal Convoluted Tubule (PCT), where all glucose is actively transported back into the blood.
The cells of the PCT are rich in mitochondria, providing the ATP necessary for active transport against the concentration gradient.
Water Reabsorption happens throughout the nephron but is finalized in the collecting duct via osmosis, moving water back into the peritubular capillaries.
Under normal conditions, 100% of glucose is recovered, while urea is left in the tubule to be concentrated into urine.
The body regulates water potential through a negative feedback loop involving the hypothalamus and the pituitary gland.
When blood water potential is low (dehydration), the pituitary gland releases more Antidiuretic Hormone (ADH).
ADH Mechanism: ADH increases the permeability of the collecting duct walls to water, allowing more water to be reabsorbed by osmosis into the blood.
This results in a small volume of concentrated (hypertonic) urine, while high water potential leads to less ADH and a large volume of dilute (hypotonic) urine.
| Feature | Ureter | Urethra |
|---|---|---|
| Connection | Kidney to Bladder | Bladder to External Environment |
| Function | Transporting urine for storage | Excreting urine from the body |
| Number | Two (one per kidney) | One |
Filtrate vs. Urine: Filtrate is the fluid inside the nephron containing glucose and water; urine is the final waste product lacking glucose and containing concentrated urea.
Urea vs. Protein: Urea is a small metabolic waste filtered out, while protein is a large functional molecule that should never leave the blood capillaries during ultrafiltration.
Specific Terminology: Always use the phrase 'increases the permeability of the collecting duct to water' when describing the action of ADH.
Clinical Indicators: If glucose is found in urine, it often indicates diabetes, as blood glucose levels exceed the PCT's reabsorption capacity.
Proteinuria: The presence of protein in urine suggests damage to the glomerulus (often due to high blood pressure), allowing large molecules to leak through.
Source Check: Remember that urea is produced in the liver (from deamination) but excreted by the kidney; do not confuse the site of production with the site of excretion.