Biology: Human Physiology

Biology: Human Physiology

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Human Physiology maps the mechanical, physical, and biochemical functions of the eleven organ systems that keep the body alive. Two formulas dominate numerical questions in HAT-UG Subject Knowledge: Cardiac Output (CO) = Stroke Volume (SV) × Heart Rate (HR), and BMI = weight (kg) / [height (m)]². Memorise the cardiac cycle sequence — atrial systole → ventricular systole → diastole — and the reflex arc pathway: receptor → sensory neuron → interneuron → motor neuron → effector. High-yield terms for the HAT-UG: homeostasis, nephron, alveoli, peristalsis, synapse, and vital capacity. Expect 1–2 questions (≈4% weight) testing hormone–gland matching, blood-group antigens, or gas transport in hemoglobin (O₂ as oxyhemoglobin, CO₂ mainly as bicarbonate).

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Major Organ Systems and Their Functions

Human Physiology integrates eight core systems. The digestive system converts food into absorbable units through mechanical breakdown (mastication, churning) and chemical hydrolysis by enzymes: salivary amylase (starch → maltose), gastric pepsin at pH 1.5–2 (proteins → peptides), pancreatic trypsin (peptides → amino acids), and lipase (fats → fatty acids + glycerol). Peristalsis — involuntary, wave-like smooth muscle contraction — propels chyme; this is distinct from segmentation, which mixes contents without forward movement.

The respiratory system exchanges gases across the thin, moist alveolar membrane (≈300 million alveoli, total surface ~70 m²). Oxygen binds hemoglobin reversibly to form oxyhemoglobin; about 70% of CO₂ travels as bicarbonate ions (HCO₃⁻) via carbonic anhydrase in RBCs. Vital capacity = ERV + TV + IRV, and Pulmonary Ventilation = Tidal Volume × Respiratory Rate (≈500 mL × 12 = 6 L/min at rest).

The circulatory system runs two loops: pulmonary (right heart → lungs → left heart) and systemic (left heart → body → right heart). Blood pressure obeys BP = CO × TPR, meaning a doubled peripheral resistance doubles pressure at constant output.

The excretory system forms urine through three nephron processes: ultrafiltration at the glomerulus, selective reabsorption (≈99% of filtrate, mostly in the proximal tubule), and tubular secretion in the distal tubule. GFR = Kf × Net Filtration Pressure, normally ~125 mL/min.

The nervous system transmits signals through neurons; at the synapse, neurotransmitter vesicles release chemicals (acetylcholine, dopamine) into the cleft, binding postsynaptic receptors to generate a new impulse. The endocrine system uses slower hormonal signals — insulin (β-cells of pancreas) lowers blood glucose, glucagon (α-cells) raises it — typically through negative feedback loops.

HAT-UG Question Patterns

Most items are MCQs asking for the odd-one-out organ, the correct hormone–function pair, or a numeric apply of the CO/BMI formula.

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Homeostasis and Feedback Control

Homeostasis is the dynamic maintenance of internal conditions — temperature (≈37 °C), blood pH (7.35–7.45), and blood glucose (70–110 mg/dL fasting). Two control systems cooperate: rapid neural responses (e.g., baroreceptor reflex adjusting heart rate within seconds) and slower hormonal responses (e.g., insulin–glucagon axis restoring glucose over minutes). Most loops are negative feedback: a deviation triggers a corrective response that reverses the deviation. Positive feedback is rare and time-limited — oxytocin amplification during childbirth, or the nerve impulse itself (Na⁺ influx depolarises, opening more voltage-gated channels).

Common Traps and Misconceptions

Candidates often reverse insulin and glucagon — remember insulin is hypoglycaemic (lowers glucose by promoting cellular uptake and glycogen synthesis); glucagon is hyperglycaemic. A second trap: believing inhalation is an active “pull” by the lungs; in fact, diaphragm contraction increases thoracic volume, and the lungs expand passively because intrapleural pressure drops. A third: attributing insulin to the adrenal gland — it is secreted by the pancreatic islets of Langerhans, while adrenaline comes from the adrenal medulla.

Worked Example

A resting adult has HR = 72 bpm and SV = 70 mL. Compute CO and the time for one complete cycle.

• CO = 72 × 70 = 5040 mL/min ≈ 5 L/min.
• Cycle length = 60 s / 72 = 0.833 s. The cardiac cycle therefore occupies ~0.83 s, with systole ≈ 0.3 s and diastole ≈ 0.53 s — a frequently tested ratio.

Connections to Adjacent Topics

Link nephron physiology to osmoregulation (ADH from the posterior pituitary increases water reabsorption in the collecting duct) and to Cell Biology (Na⁺/K⁺ ATPase powering active reabsorption). Synaptic transmission connects directly to drug action (receptor agonists/antagonists) covered in applied biology.

Practice Prompts

1. If TPR rises from 1.0 to 1.4 units while CO is held at 5 L/min, by what factor does blood pressure change?
2. A person weighs 70 kg and is 1.75 m tall. Calculate BMI and classify the value.

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.