Excretion and Homeostasis

The body constantly produces waste products from its chemical reactions. Excretion removes these before they accumulate to toxic levels. At the same time, the body maintains a stable internal environment — a process called homeostasis.

Excretion vs Egestion¶

These two terms are often confused:

Faeces is not simply an excretory product — it largely consists of undigested material. However, it does contain bilirubin, which is a genuine excretory product: it is formed in the liver from the breakdown of haemoglobin in old red blood cells and is excreted via bile into the gut, giving faeces their brown colour.

Metabolic Wastes and Where They Leave¶

Urea formation: When the body has more amino acids than it needs, the liver removes the nitrogen-containing amino group (deamination), converts it to ammonia, then combines it with CO₂ to form urea. Urea passes into the blood and is filtered by the kidneys.

Excretion in Plants¶

Plants also produce metabolic wastes and have their own methods of dealing with them:

Plants also release CO₂ (from respiration) and O₂ (from photosynthesis) through stomata — gases are exchanged, not strictly stored as waste.

The Urinary System and Kidney Structure¶

The kidneys filter blood, regulate water balance, and excrete urea in urine.

Urine passes from kidneys → ureters → bladder → urethra (out of the body).

Each kidney has:

• cortex — outer region; contains glomeruli and Bowman's capsules
• medulla — inner region; contains loops of Henle and collecting ducts
• pelvis — central space where urine collects before draining into the ureter

Key1.Renal pyramid2.Interlobular artery3.Renal artery4.Renal vein5.Renal hilum6.Renal pelvis7.Ureter8.Minor calyx9.Fibrous capsule (inferior)10.Fibrous capsule (superior)11.Interlobular vein12.Nephron13.Minor calyx14.Major calyx15.Renal papilla16.Renal column

The Nephron¶

The nephron is the functional unit of the kidney. Each kidney contains about one million nephrons. The nephron filters blood and selectively recovers useful substances.

Key1.Interlobular artery2.Afferent arteriole3.Juxtaglomerular apparatus4.Renal corpuscle5.Glomerulus6.Bowman's capsule7.Efferent arteriole8.Proximal convoluted tubule9.Loop of Henle10.Distal convoluted tubule11.Connecting tubule12.Interlobular vein

Ultrafiltration¶

Blood enters the glomerulus (a tight knot of capillaries inside the Bowman's capsule) under high pressure. Small molecules are forced out of the blood into the Bowman's capsule. Large molecules (proteins, red blood cells) are too big to pass through and stay in the blood.

What is filtered into the capsule: water, glucose, urea, mineral ions, amino acids.

Selective Reabsorption¶

As the filtrate travels along the nephron, useful substances are recovered back into the blood:

The remaining fluid is urine — mostly water, urea, and mineral salts.

Osmoregulation and ADH¶

Osmoregulation is the control of the water concentration of body fluids. This is critical because cells shrink or swell if the surrounding fluid concentration changes.

The hypothalamus monitors blood water concentration and controls the release of ADH (antidiuretic hormone) from the pituitary gland.

This is a negative feedback mechanism — the response opposes the change, returning the system toward normal.

Kidney Failure and Dialysis¶

If the kidneys fail, urea and other wastes accumulate in the blood to toxic levels. Treatment options:

• Dialysis (haemodialysis) — blood is passed through a machine containing a selectively permeable membrane bathed in dialysis fluid. Urea and excess ions diffuse out of the blood across the membrane; glucose, useful salts, and blood cells are retained. The patient typically needs dialysis several times per week.
• Kidney transplant — a healthier long-term solution; donor organ must be well-matched to avoid rejection.

The Skin¶

The skin has two main layers:

• Epidermis — outer layer; tough, protective; contains melanocytes that produce melanin (pigment responsible for skin, hair, and eye colour)
• Dermis — deeper layer; contains blood capillaries, sweat glands, hair follicles, sense receptors, and fat cells

Functions of the skin¶

• protection against pathogens, dehydration, UV radiation, and physical damage
• temperature regulation
• sensation (touch, pressure, pain, temperature)
• vitamin D synthesis (using sunlight)
• minor excretion of salts and urea in sweat

Melanin and UV Protection¶

Melanin absorbs UV radiation from sunlight, protecting the DNA in skin cells from damage that can lead to skin cancer. The more melanin in the skin, the darker the complexion. Production increases with UV exposure (tanning).

SPF (Sun Protection Factor) is a measure of how well a sunscreen blocks UV radiation. An SPF of 30 blocks about 97% of UVB rays. SPF is relevant because UV radiation can cause sunburn, premature skin ageing, and skin cancer — particularly in individuals with lower melanin levels.

Skin bleaching refers to the use of chemical agents (e.g. hydroquinone, mercury compounds) to reduce melanin production, lightening skin tone. While widely practised in some Caribbean communities, skin bleaching carries significant health risks: mercury is toxic to the kidneys and nervous system, and reduced melanin leaves the skin more vulnerable to UV damage and skin cancer.

Temperature Regulation¶

The body maintains core temperature at about 37°C — the optimum for enzyme activity. The hypothalamus acts as the thermostat, detecting temperature changes in the blood and sending signals to effectors in the skin.

FLASHCARD SET • 17 CARDSExcretion and Homeostasis