Human Physiology

Sliding filament model of striated muscle. Each sarcomere (the contractile unit between Z-lines) contains:

• Thin filaments of actin (with tropomyosin and troponin blocking myosin-binding sites).
• Thick filaments of myosin (with protruding heads).

Contraction steps:

1. Action potential from motor neuron arrives at neuromuscular junction → acetylcholine released.
2. AP spreads along sarcolemma and into T-tubules → opens Ca²⁺ release channels in sarcoplasmic reticulum.
3. Ca²⁺ binds troponin → tropomyosin moves → exposes myosin-binding sites on actin.
4. Myosin head binds actin (using ATP energy stored from prior cycle).
5. Power stroke — myosin head pivots, sliding actin toward sarcomere centre.
6. New ATP binds myosin → head detaches.
7. ATP hydrolysed → head re-cocks.
8. Cycle repeats while Ca²⁺ present.

Relaxation: Ca²⁺ pumped back into sarcoplasmic reticulum; tropomyosin returns; binding sites blocked.

Kidney structure. Each kidney contains ~1 million nephrons.

Nephron structure:

• Glomerulus in Bowman's capsule — filters blood.
• Proximal convoluted tubule (PCT) — reabsorbs glucose, amino acids, most water and salts (active + passive).
• Loop of Henle — establishes osmotic gradient in medulla (counter-current multiplier).
• Distal convoluted tubule (DCT) — fine-tunes salt and pH.
• Collecting duct — final water reabsorption controlled by ADH.

ADH (antidiuretic hormone) from posterior pituitary increases water permeability of collecting duct → more water reabsorbed → concentrated urine.

When blood is dilute (low solute), ADH decreases → less water reabsorbed → dilute urine.

Liver functions:

• Glucose homeostasis — stores excess glucose as glycogen; releases glucose during fasting (gluconeogenesis from amino acids and lactate).
• Deamination of amino acids → ammonia → converted to urea (urea cycle) for safe excretion.
• Detoxification of alcohol, drugs, and toxins via cytochrome P450 enzymes.
• Plasma protein synthesis (albumin, clotting factors).
• Bile production — for fat emulsification.
• Storage of vitamins (A, D, K) and iron.
• Erythrocyte breakdown — Kupffer cells phagocytose old red blood cells; haem recycled to bile pigments (bilirubin).

The liver's vast blood supply (hepatic artery + hepatic portal vein) brings nutrients and toxins for processing.

Bohr shift in haemoglobin. Lower pH (high CO₂) shifts the oxygen-dissociation curve to the RIGHT — haemoglobin releases more O₂ at the same partial pressure. This means:

• In actively respiring tissues (high CO₂, low pH) → MORE O₂ released where needed.
• In lungs (low CO₂, higher pH) → haemoglobin saturates with O₂ efficiently.

Carbon dioxide is transported in blood:

• ~70% as bicarbonate (HCO₃⁻) in plasma.
• ~25% bound to haemoglobin (carbaminohaemoglobin).
• ~5% dissolved.

Tobacco effects revisited. Carbon monoxide in cigarette smoke binds haemoglobin ~200× more tightly than O₂ → carboxyhaemoglobin reduces O₂ transport → tissue hypoxia.

Foetal haemoglobin has higher O₂ affinity than adult — allows efficient O₂ transfer across the placenta from mother to foetus.