Homeostasis

Homeostasis is the maintenance of a stable internal environment despite fluctuations in the external one. The PMDC MDCAT 2026 syllabus expects you to handle excretion (especially nitrogenous waste), the structure and function of the human kidney, and thermoregulation in mammals. Expect 3-5 MCQs from this chapter.

PMC Table of Specifications. This chapter covers three PMDC subtopics — Excretion, Homeostasis (Kidney) and Thermoregulation. Most papers carry one nephron-function MCQ and one ADH/RAAS MCQ.

Excretion

Excretion is the elimination of metabolic wastes from the body. The most important excretory product in mammals is urea, formed in the liver via the ornithine (urea) cycle from the breakdown of amino acids. Carbon dioxide, bile pigments and excess water/salts are also excreted.

Ammonotelic: Organisms that excrete ammonia directly. Highly toxic but very water-soluble — only feasible for aquatic animals (bony fish, amphibian larvae).
Ureotelic: Excrete urea. Less toxic, moderately soluble — mammals, adult amphibians, cartilaginous fish.
Uricotelic: Excrete uric acid as a semi-solid paste. Almost insoluble — conserves water. Birds, reptiles, insects.

Organs of excretion in humans

Kidneys remove urea, excess water, salts and toxins. Lungs excrete CO₂ and water vapour. Skin loses water, salts and small amounts of urea via sweat. Liver excretes bile pigments (bilirubin, biliverdin) into the gut.

Homeostasis (Kidney)

The kidney is the master regulator of plasma composition. Each human kidney contains roughly 1 million nephrons — the structural and functional units that filter blood and form urine.

Gross structure

A kidney shows an outer cortex, an inner medulla divided into pyramids, and a central pelvis that drains urine into the ureter. Blood enters via the renal artery and leaves via the renal vein.

The Nephron

A nephron has six parts in sequence: Bowman’s capsule (with glomerulus inside) → proximal convoluted tubule (PCT) → descending limb of loop of Henle → ascending limb → distal convoluted tubule (DCT) → collecting duct. Cortical nephrons are short; juxtamedullary nephrons have long loops that dip deep into the medulla and create the osmotic gradient needed for urine concentration.

Three steps of urine formation

1. Ultrafiltration: Occurs in the glomerulus. High hydrostatic pressure (~55 mmHg) forces water and small solutes through the basement membrane into Bowman’s capsule. Cells, platelets and large proteins are retained. Glomerular filtration rate (GFR) is about 125 mL/min (180 L/day).
2. Selective reabsorption: ~99% of the filtrate is reabsorbed. The PCT reclaims all glucose and amino acids (active transport via Na⁺ co-transporters), most Na⁺, Cl⁻, HCO₃⁻ and water (osmosis). The descending limb is permeable to water only; the ascending limb pumps out Na⁺/Cl⁻ but is impermeable to water (counter-current multiplier).
3. Tubular secretion: The DCT and collecting duct actively secrete K⁺, H⁺, ammonia, creatinine and certain drugs into the filtrate. This fine-tunes blood pH and removes substances that escaped filtration.

Nephron parts — what each region does
Region	Location	Permeability	Main function	Key transports
Bowman's capsule (glomerulus)	Cortex	Highly permeable to small solutes	Ultrafiltration	Water, glucose, urea, ions filter out; cells/proteins retained
Proximal convoluted tubule (PCT)	Cortex	Highly permeable to water	Bulk reabsorption (~65% of filtrate)	All glucose, all amino acids, Na⁺/Cl⁻, HCO₃⁻, water (osmosis)
Descending limb of Loop of Henle	Medulla (deep)	Permeable to water only	Water leaves → filtrate becomes hypertonic	Passive water exit
Ascending limb of Loop of Henle	Medulla (returning)	Impermeable to water; permeable to ions	NaCl pumped out → filtrate becomes hypotonic	Active Na⁺/Cl⁻/K⁺ efflux (loop diuretics target this)
Distal convoluted tubule (DCT)	Cortex	Variable (hormone-controlled)	Fine-tuning of Na⁺, K⁺, pH, Ca²⁺	Aldosterone → Na⁺ in / K⁺ out; H⁺ secretion; PTH → Ca²⁺ reabsorption
Collecting duct	Cortex → medulla	Permeability set by ADH	Final water reabsorption — concentrates urine	ADH → aquaporin-2 insertion; aldosterone → Na⁺ reabsorption

Nitrogenous waste excretion across animal groups
Type	Waste excreted	Toxicity	Water needed	Examples
Ammonotelic	Ammonia (NH₃)	Very high	Lots (must be diluted)	Bony fish, amphibian larvae, aquatic invertebrates
Ureotelic	Urea	Moderate	Moderate	Mammals, adult amphibians, cartilaginous fish
Uricotelic	Uric acid	Low (insoluble paste)	Very little	Birds, reptiles, insects, snails

ADH (Antidiuretic hormone / Vasopressin)

Released from the posterior pituitary when osmoreceptors in the hypothalamus detect rising blood osmolarity. ADH increases water permeability of the collecting duct by inserting aquaporin-2 channels. Result: more water reabsorbed, concentrated urine, blood volume restored. Lack of ADH causes diabetes insipidus (large volumes of dilute urine).

RAAS — Renin-Angiotensin-Aldosterone System

Triggered by a fall in blood pressure or Na⁺. Juxtaglomerular cells release renin, which converts angiotensinogen to angiotensin I. ACE in the lungs converts it to angiotensin II — a potent vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. Aldosterone makes the DCT/collecting duct reabsorb more Na⁺ (and water follows), restoring pressure.

Common trap. ADH acts on the collecting duct, NOT on the loop of Henle. Aldosterone acts on the DCT and collecting duct for Na⁺ reabsorption. Confusing the targets is the most frequent kidney MCQ error.

Counter-current multiplier

The loop of Henle establishes a steep osmotic gradient in the medulla (300 → 1200 mOsm). The descending limb is water-permeable; the ascending limb pumps out NaCl. This makes the medulla hypertonic, so when urine flows down the collecting duct (under ADH influence) water is osmotically pulled out, producing concentrated urine.

Thermoregulation

Mammals are endotherms — they maintain core body temperature (~37°C) regardless of ambient conditions. The hypothalamus acts as the body’s thermostat, comparing actual temperature with the set point and triggering corrective responses.

Role of the skin

The skin contains thermoreceptors, sweat glands, hair follicles with erector pili muscles, and a network of arterioles. It is the major effector for heat loss and heat conservation.

Responses when too hot

Vasodilation of skin arterioles → more blood near surface → more heat radiated.
Sweating → evaporation of water absorbs latent heat from the skin.
Hair lies flat (relaxation of erector pili) → trapped insulating air layer is reduced.
Behaviour: seeking shade, reduced activity.

Responses when too cold

Vasoconstriction of skin arterioles → blood diverted away from surface.
Shivering — rapid involuntary muscle contractions release heat.
Hair erection (piloerection) → thicker insulating air layer (more visible in furry mammals; in humans we call them goosebumps).
Increased metabolic rate via thyroxine and adrenaline; non-shivering thermogenesis in brown adipose tissue (in infants).

Mnemonic for the nephron sequence. "Bowman Plays Loops Down And Carefully" — Bowman’s capsule, PCT, Loop of Henle (Descending then Ascending), DCT, Collecting duct.

Worked MCQs

Five MCQs covering the high-yield testing patterns for homeostasis. Read every explanation — the deeper concept lives there.

Q1. ADH (vasopressin) increases water reabsorption primarily by acting on which part of the nephron?

Proximal convoluted tubule
Descending limb of loop of Henle
Ascending limb of loop of Henle
Collecting duct

ADH inserts aquaporin-2 channels into the apical membrane of the collecting duct, making it permeable to water. Water moves osmotically into the hypertonic medulla, producing concentrated urine. The PCT reabsorbs water constitutively (not ADH-dependent).

Q2. Which of the following is the chief nitrogenous excretory product of mammals?

Ammonia
Urea
Uric acid
Creatine

Mammals are ureotelic. Urea is synthesised in the liver via the ornithine (urea) cycle and excreted by the kidneys. Ammonia is too toxic; uric acid is the main waste in birds and reptiles.

Q3. A drop in blood pressure activates the RAAS pathway. Which hormone, released as a result, increases Na⁺ reabsorption from the DCT?

ADH
Renin
Aldosterone
Angiotensinogen

Renin is an enzyme, not a hormone target. Angiotensin II stimulates the adrenal cortex to release aldosterone, which acts on the DCT and collecting duct to reabsorb Na⁺ (water follows). ADH conserves water but not Na⁺.

Q4. Which response would you expect when a mammal is exposed to cold?

Vasodilation of skin arterioles
Increased sweating
Shivering and vasoconstriction
Hair lies flat against skin

In cold conditions the hypothalamus triggers heat-conserving and heat-generating responses: skin arterioles vasoconstrict (less heat loss), erector pili contract (hair stands up to trap air), and shivering generates heat through muscular activity.

Q5. Glucose normally appears in urine of healthy adults at what level?

Same concentration as plasma
About half of plasma concentration
Essentially absent — fully reabsorbed
Higher than plasma due to secretion

Glucose is freely filtered at the glomerulus but is then completely reabsorbed in the PCT by Na⁺-glucose co-transporters (SGLT2). Glucose only appears in urine when the renal threshold (~180 mg/dL) is exceeded, as in untreated diabetes mellitus.

Quick Recap

Mammals = ureotelic; birds/reptiles = uricotelic; bony fish = ammonotelic.
Nephron = Bowman’s capsule → PCT → loop of Henle → DCT → collecting duct.
Three steps: ultrafiltration (glomerulus), selective reabsorption (mostly PCT), tubular secretion (DCT).
ADH from posterior pituitary → collecting duct → water reabsorption.
RAAS: low BP → renin → angiotensin II → aldosterone → Na⁺ & water retention.
Thermoregulation = hypothalamus + skin (vasodilation/constriction, sweat, shivering, piloerection).

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.