Homeostasis: MDCAT Notes and MCQs
Urinary system
Two kidneys → ureters → urinary bladder → urethra. Kidney: outer cortex, inner medulla (renal pyramids), pelvis. Functional unit = nephron (≈ 1 million per kidney).
Nephron
- Bowman's capsule + glomerulus (= renal corpuscle / Malpighian body) → proximal convoluted tubule (PCT) → loop of Henle → distal convoluted tubule (DCT) → collecting duct.
- Afferent arteriole (wider) → glomerulus → efferent arteriole (narrower) → peritubular capillaries (and vasa recta around the loop).
Urine formation
- Glomerular filtration: high hydrostatic pressure (afferent wider than efferent) forces water, glucose, amino acids, salts, urea into Bowman's capsule. Blood cells and plasma proteins stay. GFR ≈ 125 ml/min (≈ 180 L/day); urine only ≈ 1.5 L/day.
- Selective reabsorption: mostly in PCT (≈ 65–80% of filtrate; all glucose and amino acids by active transport, Na+, water). Loop of Henle builds a concentration gradient in medulla (countercurrent multiplier).
- Tubular secretion: H+, K+, NH4+, creatinine, drugs move from peritubular capillaries into the tubule (mainly DCT) — also regulates blood pH.
Glomerular vs peritubular capillaries
| Glomerular | Peritubular |
|---|---|
| High pressure → filtration | Low pressure → reabsorption & secretion |
| Between two arterioles | Arise from efferent arteriole, drain into venules |
Excretion + osmoregulation
Kidneys remove nitrogenous wastes (excretion) and adjust water & salt balance (osmoregulation). ADH (from posterior pituitary) increases water permeability of DCT/collecting duct → concentrated urine; lack of ADH → diabetes insipidus. Aldosterone (adrenal cortex) → Na+ reabsorption.
Kidney stones
- Mostly calcium oxalate; also calcium phosphate, uric acid. Causes: low water intake, excess Ca/oxalate in diet, infection, hyperparathyroidism.
- Treatment: drinking plenty of water, medicines, lithotripsy (shock waves break stones), surgery.
Kidney failure
Causes: diabetes mellitus, hypertension, infections, kidney stones/obstruction, toxins, injury, blood loss. Treatment: dialysis (haemodialysis / peritoneal) or kidney transplant.
Thermoregulation
- Need: enzymes work best at an optimum temperature (≈ 37 °C in humans).
- Ectotherms (fish, amphibians, reptiles) depend on external heat; endotherms (birds, mammals) generate metabolic heat.
- Thermostat = hypothalamus. Hot: sweating, vasodilation of skin arterioles. Cold: shivering, vasoconstriction, hair erection, increased metabolism (thyroxine). Fever: pyrogens reset the thermostat.
Nitrogenous wastes
| Waste | Toxicity / water needed | Animals |
|---|---|---|
| Ammonia (ammonotelic) | Most toxic, needs most water | Aquatic animals, bony fish |
| Urea (ureotelic) | Less toxic; made in liver by ornithine (urea) cycle | Mammals, amphibians, sharks |
| Uric acid (uricotelic) | Least toxic, least water | Birds, reptiles, insects |
Other: creatinine, trimethylamine oxide, guanine, hippuric acid.
PMDC learning outcomes for this unit
- Urinary system: 15.1 Explain different organs of urinary system. Describe the structure of kidney and relate it with its function.
- Homeostasis (kidney specifically): 15.2 Explain the processes of glomerular filtration, selective re-absorption and tubular secretion as the events in kidney functioning.
- Homeostasis (kidney specifically): 15.3 Justify the functioning of kidneys as both excretion and osmoregulation.
- Homeostasis (kidney specifically): 15.4 Compare the function of two major capillary beds in kidney i.e. glomerular capillaries and peritubular capillaries.
- Homeostasis (kidney specifically): 15.5 Explain the causes and treatments of kidney stones.
- Homeostasis (kidney specifically): 15.6 Outline the causes of kidney failure.
- Thermoregulation: 15.7 Describe thermoregulation and explain its needs.
- Excretion: 15.8 List various nitrogenous compounds excreted during the process of excretion.