13.1 Homeostasis
Homeostasis is the maintenance of a relatively constant internal environment (such as body temperature, blood glucose levels, blood pH, and osmotic pressure) despite changes in the external environment.
Negative Feedback Mechanism
Homeostasis operates via a negative feedback loop:
- Increase/Decrease in Set Value → Receptors Detect Change → Control Centre Integrates Signal → Effectors Trigger Compensatory Mechanism → Factor Restored to Normal Range.
Regulating Body Temperature (Thermoregulation)
Controlled by the hypothalamus via nervous and hormonal responses:
- When Body Temperature Rises: Vasodilation of cutaneous arterioles (increases heat loss by radiation), increased sweating (evaporative cooling), erector muscles relax (hair lies flat to trap less air), decreased metabolic rate and shivering.
- When Body Temperature Falls: Vasoconstriction of cutaneous arterioles (reduces heat loss), erector muscles contract (hair stands erect to trap an insulating layer of air), skeletal muscles shiver (generates heat), increased secretion of thyroxine and adrenaline (elevates metabolic rate).
Regulating Blood Glucose Levels
Controlled by hormones secreted by the Islets of Langerhans in the pancreas:
- High Blood Glucose (above normal): Beta (β) cells secrete insulin. Insulin stimulates muscle and liver cells to convert excess glucose into glycogen (glycogenesis) and increases cellular glucose uptake.
- Low Blood Glucose (below normal): Alpha (α) cells secrete glucagon. Glucagon stimulates the liver to convert stored glycogen back into glucose (glycogenolysis) and promotes non-carbohydrate conversion to glucose.
Regulating Partial Pressure of Carbon Dioxide ($pCO_2$) and Blood pH
- Elevated cellular respiration increases blood $pCO_2$, lowering blood pH (forming carbonic acid).
- Central chemoreceptors in the medulla oblongata and peripheral chemoreceptors (carotid and aortic bodies) detect pH drop.
- Impulses are sent to the respiratory centre in the medulla oblongata, increasing the rate and depth of breathing and intercostal/diaphragm muscle contractions to expel $CO_2$.
13.2 Structure and Function of the Kidney
The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The functional filtering unit of the kidney is the nephron.
Structure of a Nephron
- Renal Corpuscle: Bowman's capsule enclosing a knot of capillaries called the glomerulus.
- Renal Tubule: Proximal convoluted tubule, Loop of Henle, distal convoluted tubule, and collecting duct.
Urine Formation Processes
- Ultrafiltration: High hydrostatic pressure in the glomerulus (due to wider afferent arteriole vs narrower efferent arteriole) forces fluid, glucose, amino acids, urea, salts, and water across the glomerular membrane into Bowman's capsule to form glomerular filtrate. Blood cells and plasma proteins remain in capillaries.
- Reabsorption:
- Proximal Convoluted Tubule: 100% of glucose and amino acids are reabsorbed via active transport; water by osmosis; $Na^+$ and $Cl^-$ by active transport/diffusion.
- Loop of Henle: Water reabsorbed in descending limb; $Na^+$ and $Cl^-$ reabsorbed in ascending limb.
- Distal Convoluted Tubule & Collecting Duct: Water and salts selectively reabsorbed under hormonal control.
- Secretion: Waste substances (urea, uric acid, creatinine, $H^+$, $K^+$, drugs) pass from peritubular capillaries into the renal tubules via active transport/diffusion to adjust blood pH and clear toxins.
13.3 Osmoregulation and Mechanism of Urine Formation
Osmoregulation is the process of regulating water and salt solute concentrations in body fluids to maintain osmotic pressure.
Role of Hormones in Osmoregulation
- Anti-diuretic Hormone (ADH):
- Synthesized by the hypothalamus and released by the posterior pituitary gland.
- When blood osmotic pressure is high (dehydration/excessive sweating), osmoreceptors in the hypothalamus trigger ADH release.
- ADH increases permeability of distal convoluted tubule and collecting duct cells to water, increasing water reabsorption → small volume of concentrated urine.
- Aldosterone:
- Secreted by the adrenal cortex when blood $Na^+$ levels or blood pressure drops.
- Increases reabsorption of $Na^+$ ions (followed by water) in distal convoluted tubule and collecting duct → restores blood pressure and electrolyte balance.
13.4 Health Issues Related to the Urinary System
- Kidney Failure: Inability of kidneys to filter metabolic waste products and regulate water/electrolyte balance, caused by diabetes, hypertension, or kidney stones.
- Hemodialysis: Medical treatment where patient's blood is passed through a dialyzer across a semi-permeable membrane to remove urea and excess salts artificially.
- Kidney Stones: Hard mineral deposits (calcium oxalate, uric acid) formed in the renal pelvis or urinary tract due to inadequate water intake or high protein/salt diet.