🦠 Excretory Products & Their Elimination Study Notes

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).

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):

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.

4.4 Function of the Tubules

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.

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.

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.