Excretory System and Homeostasis

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Excretion is the removal of metabolic waste products from the body. These waste products are toxic if allowed to accumulate. Egestion (defaecation) is different — it removes undigested food residues, which are NOT metabolic wastes.

Excretory Products:

Organism	Main Nitrogenous Waste	Form
Fish (aquatic)	Ammonia	Very toxic, diluted with water
Birds, reptiles	Uric acid	Non-toxic, paste (conserves water)
Mammals	Urea	Moderately toxic, dissolved in water
Insects	Uric acid	Non-toxic, solid

Why different forms? Ammonia requires lots of water to dilute and excrete safely — only practical for aquatic organisms. Urea is less toxic and requires less water. Uric acid is non-toxic and can be excreted as a solid — essential for animals whose eggs develop outside water (birds, reptiles, insects).

Human Excretory System: Kidneys → Ureters → Bladder → Urethra

Kidney Structure:

• Cortex: Outer region, contains glomeruli and Bowman’s capsules
• Medulla: Inner region, contains loops of Henle and collecting ducts
• Renal pelvis: Central cavity, collects urine before it leaves via ureter

The Nephron: Each kidney contains about 1 million nephrons. Each nephron:

1. Bowman’s capsule: Cups around glomerulus
2. Proximal convoluted tubule (PCT): Reabsorbs glucose, amino acids, water, salts
3. Loop of Henle: Creates concentration gradient in medulla
4. Distal convoluted tubule (DCT): Further fine-tuning of ions
5. Collecting duct: Collects urine, carries it to renal pelvis

⚡ WAEC Tip: The kidney’s main function is osmoregulation — regulating water and salt balance, not just waste removal. The nitrogenous waste (urea) is a byproduct of protein metabolism, not the primary function.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Urine Formation: Urine is formed in three stages:

1. Ultrafiltration (in Bowman’s capsule):

• Blood pressure forces fluid out of glomerulus into Bowman’s capsule
• Glomerular filtrate contains: water, glucose, amino acids, urea, salts, vitamins
• Blood cells and proteins are too large — stay in blood
• Filtrate rate: ~180 litres per day (but only ~1.5 litres urine produced)

2. Selective Reabsorption (in PCT and DCT):

• Proximal tubule: 85% of filtrate water reabsorbed by osmosis; all glucose and amino acids reabsorbed by active transport
• Loop of Henle: Creates hypertonic medulla (gradient from 300 to 1200 mOsm/kg)
• Distal tubule: Fine-tuning of Na⁺ and K⁺
• Collecting duct: Water reabsorbed if ADH present

3. Secretion (in DCT and collecting duct):

• Active transport of H⁺, K⁺, drugs (e.g., penicillin), creatinine from blood into tubule
• Important for removing foreign substances

Hormonal Control:

Antidiuretic Hormone (ADH / Vasopressin):

• Produced by hypothalamus, stored in posterior pituitary
• Increases water reabsorption in collecting ducts
• High ADH → concentrated urine (small volume)
• Low ADH → dilute urine (large volume)
• Diabetes Insipidus: Deficiency of ADH → large volumes of dilute urine → dehydration

Renin-Angiotensin-Aldosterone System (RAAS):

1. Low blood pressure → kidneys release renin
2. Renin converts angiotensinogen → angiotensin I
3. ACE (angiotensin-converting enzyme) → angiotensin II
4. Angiotensin II → vasoconstriction + aldosterone release
5. Aldosterone → increases Na⁺ reabsorption in DCT → water follows → blood pressure rises

Other Excretory Organs:

Lungs:

• Excrete CO₂ (produced by cellular respiration)
• Also exhaled water vapour
• ~400 mL water lost per day via lungs

Skin:

• Sweat glands: Excrete water, NaCl, urea (small amounts)
• Sebaceous glands: Produce sebum (oil) — not excretion
• Skin is NOT a major excretory organ for nitrogenous wastes

Liver:

• Deaminates excess amino acids → ammonia → urea (via ornithine cycle)
• Produces bile → contains bilirubin (haemoglobin breakdown product)
• Detoxifies alcohol, drugs, hormones

Homeostasis: The maintenance of a stable internal environment despite external changes.

What is Homeostasized?

Factor	Normal Range	Mechanism
Blood glucose	80-100 mg/100 mL	Insulin/glucagon
Body temperature	36.5-37.5°C	Sweating, shivering, vasodilation/constriction
Blood water potential	300 mOsm/kg	ADH
Blood pH	7.35-7.45	Buffers (bicarbonate), breathing rate
Blood pressure	120/80 mmHg	RAAS, heart rate

⚡ Common Student Mistakes: Confusing excretion with egestion. Egestion removes faeces (undigested food) — this material never entered the body’s cells. Excretion removes metabolic wastes (urea, CO₂, water) produced inside cells. Also confuse Bowman’s capsule with glomerulus — filtrate goes FROM glomerulus INTO Bowman’s capsule.

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

The Ornithine Cycle (Urea Cycle):

The liver converts toxic ammonia to less toxic urea: $$2NH_3 + CO_2 + 3ATP \rightarrow H_2N-CO-NH_2 + 2ADP + AMP + 2P_i$$

Steps:

1. CO₂ + NH₃ → carbamoyl phosphate (enzyme: carbamoyl phosphate synthetase)
2. Carbamoyl phosphate + ornithine → citrulline
3. Citrulline + NH₃ → argininosuccinate (ATP → AMP)
4. Argininosuccinate → arginine + fumarate
5. Arginine + H₂O → urea + ornithine

Countercurrent Mechanism (Loop of Henle):

The Loop of Henle creates a concentration gradient in the medulla by countercurrent exchange:

• Descending limb: Permeable to water, impermeable to NaCl → water leaves by osmosis → filtrate becomes more concentrated
• Ascending limb: Impermeable to water, actively pumps NaCl out → filtrate becomes less concentrated

Concentrations in Medulla:

• Cortex: 300 mOsm/kg (same as blood)
• Outer medulla: 600 mOsm/kg
• Inner medulla: 1200 mOsm/kg

This gradient allows the kidney to produce urine up to 4x more concentrated than blood.

Temperature Regulation:

Thermoregulation in mammals:

If too hot:

• Vasodilation: More blood to skin → heat radiates out
• Sweating: Evaporation cools skin
• Erector pili muscles relax → hair lies flat → less insulation
• Behavioural: Seek shade, reduce activity

If too cold:

• Vasoconstriction: Less blood to skin → less heat loss
• Shivering: Rapid muscle contractions → generate heat
• Erector pili muscles contract → hair stands up → traps air (goosebumps)
• Non-shivering thermogenesis: Brown fat in infants generates heat
• Behavioural: Seek warmth, curl up

Blood Glucose Regulation:

Pancreatic hormones:

• Alpha cells ( glucagon): Raise blood glucose

  • Glycogenolysis: Glycogen → glucose (liver)
  • Gluconeogenesis: Amino acids → glucose

• Beta cells (insulin): Lower blood glucose

  • Glucose → glycogen (liver and muscle)
  • Glucose → fat (adipose tissue)
  • Increases glucose uptake by cells

Adrenal hormones:

• Adrenaline: Fast response — increases blood glucose during stress/fight-or-flight
• Cortisol: Slow response — increases gluconeogenesis during prolonged stress

Diabetes Mellitus:

Type 1 (Insulin-dependent):

• Autoimmune destruction of beta cells
• Usually appears in childhood/adolescence
• Requires insulin injections
• Blood glucose high, glucose in urine

Type 2 (Non-insulin-dependent):

• Cells resistant to insulin
• Usually in adults, linked to obesity
• Managed with diet, exercise, medication (e.g., metformin)
• Rising in Nigeria due to changing lifestyles

Kidney Failure:

• Acute: Sudden failure (infection, drugs, low blood pressure)
• Chronic: Gradual loss of function (diabetes, hypertension)
• Treatment: Dialysis (artificial kidney) or kidney transplant
• Haemodialysis: Blood filtered through machine; 3x per week, 4 hours per session

Dialysis: A semipermeable membrane allows diffusion of urea, salts, excess water out of blood while retaining blood cells and proteins. Dialysis fluid has normal concentrations of salts to ensure proper diffusion.

Kidney Stones:

• Crystalline deposits (calcium oxalate, uric acid)
• Cause severe pain when passing through ureter
• More common in dehydration
• Treatment: Pain relief, ultrasound to break stones, or surgery

Homeostatic Imbalances:

Condition	Problem	Symptom
Addison’s disease	Low cortisol	Weight loss, fatigue, low blood pressure
Cushing’s syndrome	High cortisol	Weight gain, high blood pressure, moon face
Hyperthyroidism	High thyroid hormone	Weight loss, rapid heart rate, protruding eyes
Hypothyroidism	Low thyroid hormone	Weight gain, fatigue, cold intolerance

Liver and Homeostasis:

• Glycogen storage and release
• Urea formation
• Detoxification of drugs and alcohol
• Storage of fat-soluble vitamins (A, D, E, K)
• Synthesis of plasma proteins (albumin, fibrinogen)
• Production of bile

⚡ WAEC Examination Patterns: Draw and label the kidney and nephron. Explain the three processes in urine formation (ultrafiltration, reabsorption, secretion). Explain the role of ADH. Describe how the Loop of Henle creates a concentration gradient. Explain homeostasis with examples. Describe temperature regulation. Compare Type 1 and Type 2 diabetes.