4.1 Why & What We Excrete
Metabolism produces toxic nitrogenous wastes — chiefly ammonia, urea and uric acid — that must be removed to keep body fluids balanced (osmoregulation + excretion).
| Mode | Waste | Toxicity / water | Examples |
|---|
| Ammonotelic | Ammonia | Most toxic; most water | Bony fishes, aquatic amphibians & insects |
| Ureotelic | Urea | Less toxic; moderate water | Mammals, terrestrial amphibians, marine fishes |
| Uricotelic | Uric acid | Least toxic; least water | Reptiles, birds, land snails, insects |
Note
Urea is produced in the
liver and excreted by the kidneys; ammonia is mostly lost by diffusion across gills or the body surface.
4.2 The Human Excretory System
It consists of a pair of kidneys, two ureters, a urinary bladder and a urethra. Kidneys are reddish-brown, bean-shaped, lying between the last thoracic and third lumbar vertebra; each is ~10–12 cm long, 5–7 cm wide, 2–3 cm thick and ~120–170 g.
The notch on the inner surface — the hilum — admits the ureter, vessels and nerves. Inside is the renal pelvis with projections (calyces). The kidney has an outer cortex and an inner medulla with conical medullary pyramids; cortical tissue between them forms the columns of Bertini.
The functional unit is the nephron (over a million per kidney):
Glomerulus→Bowman's capsule→PCT→Loop of Henle→DCT→Collecting duct
Glomerulus = capillary tuft (afferent in, efferent out). Glomerulus + Bowman's capsule = the Malpighian body.
| Cortical nephron | Juxtamedullary nephron |
|---|
| Loop of Henle | Short, barely into medulla | Long, deep into medulla |
| Vasa recta | Reduced / absent | Well developed |
4.3 Urine Formation
Urine is formed in three steps:
- Glomerular filtration (ultrafiltration): blood is filtered through three layers — capillary endothelium, basement membrane and Bowman's capsule podocytes (slit pores). Everything except proteins passes through. GFR ≈ 125 mL/min ≈ 180 L/day.
- Reabsorption: the tubules reabsorb ~99% of the filtrate, so only ~1.5 L of urine forms daily. (Active: glucose, amino acids, Na⁺. Passive: water, urea.)
- Secretion: tubular cells secrete H⁺, K⁺ and NH₃ into the filtrate to maintain ionic and acid–base balance.
JGA
Where the DCT touches the afferent arteriole, the
juxtaglomerular apparatus forms; a fall in GFR makes its cells release
renin.
4.4 Function of the Tubules
| Region | Main role |
|---|
| PCT | Reabsorbs nearly all glucose, amino acids & ~70–80% of ions and water; maintains pH (secretes H⁺, NH₃; absorbs HCO₃⁻) |
| Loop of Henle | Little reabsorption, but maintains the medullary gradient; ascending limb impermeable to water, descending limb permeable |
| DCT | Conditional reabsorption of Na⁺ & water; secretes H⁺, K⁺, NH₃ (pH and Na⁺–K⁺ balance) |
| Collecting duct | Large water reabsorption → concentrated urine; lets urea into the medulla to keep the gradient |
4.5 Concentration of the Filtrate — Counter Current
The kidney concentrates urine using a counter-current mechanism formed by the loop of Henle (counter-current multiplier) and the vasa recta (exchanger). Flow in their two limbs is in opposite directions.
Descending limb (water out)→Ascending limb (NaCl out)→Medullary gradient→Vasa recta exchange→Concentrated urine
NaCl and urea maintain a gradient from ~300 mOsmol/L (cortex) to ~1200 mOsmol/L (inner medulla).
This lets the collecting duct reabsorb water and produce hypertonic (concentrated) urine, conserving water.
4.6 Regulation of Kidney Function
Kidney function is fine-tuned by the hypothalamus, the JGA and the heart, through hormones:
- ADH (vasopressin): on dehydration, osmoreceptors trigger ADH from the posterior pituitary → more water reabsorbed from distal parts (prevents diuresis); also constricts vessels, raising BP.
- RAAS: fall in GFR → renin → angiotensinogen → angiotensin I → angiotensin II (raises BP & GFR) → adrenal cortex → aldosterone → more Na⁺ & water reabsorption.
- ANF: a rise in BP makes the atrial wall release ANF → vasodilation → BP falls (a check on the RAAS).
4.7 Micturition
Urine collects in the bladder until a voluntary signal allows release. A full bladder stretches receptors that signal the CNS; motor signals then contract the bladder smooth muscle and relax the urethral sphincter, expelling urine — the micturition reflex.
About 1–1.5 L of urine forms daily — ~95% water, 2.5% urea, 2.5% other wastes; pale yellow, slightly acidic, with ~25–30 g of urea excreted per day.
Clinical
Glucose (glucosuria) and ketone bodies (ketonuria) in urine can indicate
diabetes mellitus.
4.8 Role of Other Organs in Excretion
- Lungs: remove large amounts of CO₂ (~200 mL/min) and water vapour.
- Liver: secretes bile with bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs (lost with faeces).
- Skin: sweat glands lose NaCl, urea, lactic acid and water (also cooling); sebaceous glands lose sebum (sterols, hydrocarbons, waxes).
4.9 Disorders of the Excretory System
- Uremia: malfunctioning kidneys let urea build up in the blood — harmful and possibly leading to kidney failure; removed by haemodialysis.
- Haemodialysis: blood + heparin is passed through a cellophane tube in dialysing fluid (like plasma but without nitrogenous wastes); wastes diffuse out and cleared blood returns via a vein (with anti-heparin).
- Renal calculi: stones — insoluble crystallised salts (e.g. oxalates) in the kidney.
- Glomerulonephritis: inflammation of the glomeruli.
- Kidney transplant: the ultimate treatment for acute renal failure, ideally from a close relative to limit rejection.