Biology VI: Animal Form and Function I (Nutrition, Circulation, Gas Exchange, and Excretion)

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Unit 5 is the largest unit of the Biology syllabus (about 195 periods) and the most important for students of medicine (NIE, n.d.-a). The human body is studied system by system (Tortora & Derrickson, 2017; Guyton & Hall, 2020).

Nutrition and Digestion

• Nutrients: carbohydrates, lipids, proteins, vitamins, minerals, water, and fibre. Malnutrition, deficiency diseases, and obesity.
• Digestive tract: mouth (salivary amylase begins starch digestion), oesophagus (peristalsis), stomach (HCl at pH about 2 and pepsin begin protein digestion), small intestine (duodenum, jejunum, ileum; pancreatic juice with amylase, lipase, and proteases; bile from the liver emulsifies fats; brush-border enzymes), large intestine (water absorption, bacterial flora).
• Absorption occurs in the villi (large surface area, capillaries, lacteal): glucose and amino acids enter blood; fatty acids and glycerol enter the lacteals.
• Liver functions: bile production, glycogen storage, deamination and urea formation, detoxification, plasma protein synthesis.
• Disorders: peptic ulcer (Helicobacter pylori), lactose intolerance, diarrhoeal disease, hepatitis, gallstones.

Circulation

• Double circulation: pulmonary (heart to lungs and back) and systemic (heart to body and back).
• Heart: four chambers, valves (tricuspid, bicuspid, semilunar), coronary arteries. The cardiac cycle (atrial systole, ventricular systole, diastole) is initiated by the SA node (pacemaker), conducted through the AV node and the Purkinje fibres. Normal blood pressure is about 120/80 mmHg. The ECG records the heart's electrical activity.
• Blood vessels: arteries (thick elastic walls), veins (valves, thin walls), capillaries (one-cell-thick walls for exchange).
• Blood: plasma (about 55 percent); red blood cells (haemoglobin, no nucleus in mammals); white blood cells (defence); platelets (clotting). Clotting: thromboplastin, prothrombin to thrombin, fibrinogen to fibrin.
• Blood groups: ABO system (antigens A and B; antibodies in plasma) and the Rh factor; compatibility in transfusion (group O negative is the universal red-cell donor; AB positive the universal recipient).
• Lymphatic system: returns tissue fluid, absorbs fats, supports immunity.
• Disorders: atherosclerosis, hypertension, heart attack (myocardial infarction), stroke, anaemia, leukaemia.

Gas Exchange

• Human respiratory system: nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli (large surface area, thin walls, moist, rich blood supply).
• Ventilation: inspiration (diaphragm contracts and flattens; external intercostal muscles raise the ribs; volume increases, pressure falls) and expiration (passive in quiet breathing). Control by the medulla in response to blood CO₂ and pH. Lung volumes: tidal volume, vital capacity.
• Oxygen transport: oxyhaemoglobin; the oxygen dissociation curve is sigmoid; the Bohr effect (higher CO₂ and lower pH shift the curve to the right, releasing more oxygen to active tissues). Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.
• CO₂ transport: about 70 percent as bicarbonate (carbonic anhydrase in red cells), about 20 percent as carbaminohaemoglobin, about 7 percent dissolved.
• Disorders: asthma, bronchitis, emphysema, tuberculosis, lung cancer (smoking), pneumonia.
• Compare gas exchange in insects (tracheae), fish (gills with countercurrent flow), and amphibians.

Excretion and Osmoregulation

• The kidney removes nitrogenous waste, regulates water, ions, and pH. Each kidney contains about a million nephrons.
• Nephron function: (1) ultrafiltration at the glomerulus (high pressure forces small molecules into Bowman's capsule); (2) selective reabsorption in the proximal tubule (all glucose and amino acids, most water and salts); (3) the loop of Henle builds a concentration gradient (countercurrent multiplier); (4) distal tubule and collecting duct adjust water and salts under hormones; (5) tubular secretion of H⁺, K⁺, and drugs.
• Hormones: ADH (antidiuretic hormone) increases water reabsorption; aldosterone increases Na⁺ reabsorption.
• Urea is formed in the liver from ammonia (ornithine cycle). Birds and reptiles excrete uric acid; fish excrete ammonia.
• Disorders: kidney stones, urinary tract infection, kidney failure; treatment by dialysis (haemodialysis, peritoneal) and transplantation. Chronic kidney disease is an important public health issue in parts of Sri Lanka, including the dry zone, and has been widely studied.

Common Mistakes

• Mixing up the sequence of heart valves and blood flow.
• Confusing filtration with reabsorption.
• Forgetting that arteries do not always carry oxygenated blood (the pulmonary artery does not).

CHAPTER 9