Ultrafiltration: In the glomerulus, high blood pressure forces small molecules (water, urea, glucose, salts) through a basement membrane into the Bowman's capsule, creating glomerular filtrate.
Selective Reabsorption: In the Proximal Convoluted Tubule (PCT), 100% of glucose is actively transported back into the blood; this requires energy from numerous mitochondria in the PCT cells.
Water Recovery: As the filtrate travels through the Loop of Henle and the Collecting Duct, water is reabsorbed via osmosis back into the bloodstream to maintain osmotic balance.
Homeostatic Balance: Osmoregulation is the control of water and salt levels in the blood; it prevents cells from bursting (lysis) or shrinking (crenation) due to osmotic pressure changes.
ADH Mechanism: The hormone ADH (Antidiuretic Hormone) is released by the pituitary gland and increases the permeability of the collecting duct walls to water.
Negative Feedback Loop: When blood water potential is low, more ADH is released, causing more water reabsorption and producing a small volume of dark, concentrated urine.
| Feature | Excretion | Egestion |
|---|---|---|
| Source | Metabolic reactions in cells | Undigested food material |
| Substances | Urea, , water, salts | Faeces |
| Exit Point | Urethra, skin, lungs | Anus |
Ureter vs. Urethra: The ureter connects the kidney to the bladder (internal), whereas the urethra connects the bladder to the outside world (external).
Filtrate vs. Urine: Glomerular filtrate contains useful substances like glucose that must be reclaimed, whereas urine is the final waste product containing only substances the body intends to lose.
Terminology Precision: Always use the term 'permeability' when describing how ADH affects the collecting duct; examiners look for this specific word to award marks.
Glucose Check: Remember that in a healthy individual, the concentration of glucose in the urine should be zero; its presence is a diagnostic indicator of conditions like diabetes.
Pressure vs. Diffusion: Note that ultrafiltration is driven by high blood pressure, not simple diffusion; the diameter difference between afferent and efferent arterioles creates this pressure.
Common Trap: Never list faeces as an excretory product; it is the correct answer for egestion questions, but will lose marks in excretion contexts.
