Human Physiology and Body Systems

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Human Physiology — Key Facts for Sri Lanka A/L Examination

The Key Body Systems:

System	Main Organs	Function
Circulatory	Heart, blood vessels, blood	Transport of gases, nutrients, wastes
Respiratory	Lungs, trachea, bronchi	Gas exchange (O₂ in, CO₂ out)
Digestive	Stomach, intestine, liver	Mechanical and chemical digestion
Excretory	Kidneys, bladder	Removal of nitrogenous wastes
Nervous	Brain, spinal cord, nerves	Rapid communication and coordination
Endocrine	Glands (pituitary, thyroid, pancreas)	Hormonal coordination

Circulatory System:

Feature	Pulmonary Circulation	Systemic Circulation
Route	Right heart → Lungs → Left heart	Left heart → Body → Right heart
Function	Oxygenation of blood	Delivery of O₂ to tissues
Blood	Deoxygenated → Oxygenated	Oxygenated → Deoxygenated

⚡ A/L Exam Tip: The heart has its own blood supply — coronary arteries branch from the aorta just above the aortic valve!

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Human Physiology — Detailed Study Guide

The Heart

Structure of the Heart:

Chamber	Function	Valves
Right atrium	Receives deoxygenated blood from body (via vena cava)	Tricuspid valve
Right ventricle	Pumps blood to lungs	Pulmonary valve
Left atrium	Receives oxygenated blood from lungs (via pulmonary vein)	Bicuspid/mitral valve
Left ventricle	Pumps blood to body (via aorta)	Aortic valve

Cardiac Cycle:

Phase	Atria	Ventricles	Action
Atrial systole	Contract	Relax	Atria pump blood into ventricles
Ventricular systole	Relax	Contract	Ventricles pump blood out
Diastole	Relax	Relax	All chambers fill with blood

Cardiac Output: $$\text{CO} = \text{Stroke Volume} \times \text{Heart Rate}$$

• Normal resting HR: ~70 bpm
• Stroke volume: ~70 mL
• Resting CO: ~5 L/min

Heartbeat Regulation:

• Sinoatrial (SA) node: Natural pacemaker in right atrium (~70 bpm)
• Atrioventricular (AV) node: Delays signal briefly
• Bundle of His: Carries signal to ventricles
• Purkinje fibres: Distributes to ventricular walls

⚡ A/L PYQ: “What is the function of the SA node?” Answer: The SA node is the natural pacemaker of the heart — it initiates each heartbeat by generating electrical impulses at approximately 70 bpm, causing the atria to contract.

Blood and Circulation

Blood Components:

Component	Function	Lifespan
Red blood cells (RBCs)	Carry O₂ (haemoglobin)	~120 days
White blood cells (WBCs)	Immune defence	Hours to years
Platelets	Blood clotting	~9 days
Plasma	Transport medium	—

Blood Groups (ABO and Rhesus):

Blood Group	Antigens	Can Donate To	Can Receive From
A	A antigen	A, AB	A, O
B	B antigen	B, AB	B, O
AB	A + B antigens	AB only	All groups
O	No antigens	All groups	O only

Rhesus Factor:

• Rh+ has D antigen on RBC surface
• Rh- lacks D antigen
• Erythroblastosis foetalis: Rh- mother carrying Rh+ baby can develop antibodies (sensitisation), causing haemolysis in subsequent pregnancies

Blood Vessels:

Vessel	Function	Structure
Arteries	Carry blood away from heart (high pressure)	Thick muscular walls, narrow lumen
Veins	Return blood to heart (low pressure)	Thin walls, valves present
Capillaries	Exchange site (thin, permeable)	Single cell layer

⚡ A/L Important: Blood pressure is highest in arteries, lowest in veins. Mean arterial pressure (MAP) = diastolic + ⅓(pulse pressure).

Respiratory System

Gas Exchange (Alveoli):

O₂: Alveolar air → Pulmonary capillary blood (diffusion)
CO₂: Pulmonary capillary blood → Alveolar air (diffusion)

Partial pressures:
- Alveolar O₂: ~100 mmHg (higher than blood ~40 mmHg)
- Alveolar CO₂: ~40 mmHg (lower than blood ~46 mmHg)

Lung Volumes and Capacities:

Measurement	Approx. Value	Description
Tidal volume	500 mL	Normal breath in/out
Vital capacity	~4800 mL	Max forced exhalation after max inhalation
Residual volume	~1200 mL	Air remaining after max exhalation

Ventilation Mechanism:

• Inspiration: Diaphragm contracts and flattens → intercostal muscles raise ribs → thoracic cavity expands → intrapleural pressure drops → air rushes in
• Expiration: Diaphragm relaxes → thoracic cavity shrinks → air expelled passively

⚡ A/L Common Mistake: Students confuse diffusion and ventilation. Ventilation is bulk air movement in and out of lungs; diffusion is gas movement across alveolar membranes.

Digestive System

Digestion Overview:

Region	Enzymes/Action	Substrates → Products
Mouth	Salivary amylase (ptyalin)	Starch → maltose
Stomach	Pepsin + HCl	Proteins → polypeptides
Duodenum	Pancreatic amylase, lipase, trypsin	Starch, fats, proteins
Ileum	Brush border enzymes (maltase, peptidases)	Disaccharides → monosaccharides; peptides → amino acids

Absorption:

Nutrient	Absorption Site	Mechanism
Glucose	Ileum	Active transport (SGLT1) + facilitated diffusion
Amino acids	Ileum	Active transport
Fatty acids + glycerol	Ileum	Micelles → diffusion into epithelial cells
Water	All intestine	Osmosis

⚡ A/L Key Point: Villi and microvilli massively increase surface area of the small intestine (folded → finger-like → brush border), optimising nutrient absorption.

Excretory System

Kidney Structure:

Part	Function
Cortex	Contains glomerulus and convoluted tubules
Medulla	Contains loops of Henle and collecting ducts (pyramids)
Pelvis	Collects urine, drains to ureter

Nephron Function:

Process	Location	Mechanism
Glomerular filtration	Bowman’s capsule	Pressure-driven ultrafiltration (blood → nephron)
Reabsorption	Proximal tubule	Selective reabsorption (glucose, amino acids, water)
Secretion	Distal tubule	Active secretion of wastes (H⁺, K⁺, drugs)
Concentration	Loop of Henle	Counter-current multiplier (gradient creation)

Urine Formation:

• Volume: ~1-2 L/day (adult)
• ADH (vasopressin): Increases water reabsorption in collecting duct when dehydrated
• Anti-diuretic hormone: Target is kidney collecting ducts → aquaporins inserted → more water reabsorbed → concentrated urine

⚡ A/L Important: The counter-current multiplier in the loop of Henle creates a concentration gradient in the medulla (300 mOsm → 1200 mOsm), enabling production of concentrated urine up to 4x plasma osmolality.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Human Physiology — Complete Notes for A/L Sri Lanka

Nervous System

Neuron Structure:

Part	Function
Cell body (soma)	Contains nucleus, metabolic centre
Dendrites	Receive signals from other neurons
Axon	Conducts nerve impulse away from cell body
Myelin sheath	Insulates axon, speeds up conduction
Nodes of Ranvier	Gaps in myelin — saltatory conduction
Synaptic terminals	Release neurotransmitter

Resting Potential:

• Inside of neuron: -70 mV (negative relative to outside)
• Maintained by Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in per ATP)
• High K⁺ inside, high Na⁺ outside

Action Potential:

1. Resting: Na⁺/K⁺ pump maintains gradient
2. Depolarisation: Na⁺ channels open → Na⁺ rushes in → +30 mV
3. Repolarisation: K⁺ channels open → K⁺ rushes out → -70 mV
4. Refractory period: Na⁺ channels inactivated, cannot fire again immediately

Synaptic Transmission:

Axon terminal → Ca²⁺ influx → Vesicle fusion → Neurotransmitter release → Receptor binding → EPSP/IPSP

• EPSP (Excitatory): Na⁺ channels open, depolarisation
• IPSP (Inhibitory): Cl⁻ influx or K⁺ efflux, hyperpolarisation

⚡ A/L Important: Summation — multiple subthreshold EPSPs can combine (spatial or temporal summation) to reach threshold and trigger an action potential.

Endocrine System

Key Human Hormones:

Gland	Hormone	Function
Pituitary (anterior)	Growth hormone (GH)	Growth, metabolism
Pituitary (anterior)	TSH	Stimulates thyroid
Pituitary (anterior)	FSH, LH	Reproductive functions
Thyroid	Thyroxine (T₄), T₃	Metabolic rate, development
Parathyroid	PTH	Increases blood Ca²⁺
Pancreas	Insulin	Lowers blood glucose
Pancreas	Glucagon	Raises blood glucose
Adrenal cortex	Cortisol	Stress response
Adrenal medulla	Adrenaline (epinephrine)	Fight-or-flight
Ovaries	Oestrogen, progesterone	Female reproduction
Testes	Testosterone	Male reproduction

Blood Glucose Regulation:

Condition	Insulin	Glucagon	Effect
High blood glucose (fed state)	↑	↓	Glucose taken up by liver, muscle, adipose
Low blood glucose (fasting)	↓	↑	Glycogenolysis, gluconeogenesis

⚡ A/L Common Mistake: Students confuse the roles of type 1 and type 2 diabetes. Type 1: autoimmune destruction of beta cells → no insulin production (requires insulin injection). Type 2: insulin resistance → relative insulin deficiency (managed with diet, exercise, drugs).

Homeostasis and Feedback

Principles of Homeostasis:

• Set point: Normal value the body maintains
• Receptor: Detects deviation from set point
• Integrating centre: Compares signal to set point (hypothalamus, medulla)
• Effector: Responds to correct the deviation
• Feedback: Response feeds back to influence the original stimulus

Negative Feedback Loop:

Stimulus → Receptor → Integrating centre → Effector → Response → Feedback reduces stimulus

Examples:

System	Stimulus	Response
Thermoregulation	Core temp rises	Sweating, vasodilation
Thermoregulation	Core temp falls	Shivering, vasoconstriction
Blood glucose	Glucose rises	Insulin release
Blood glucose	Glucose falls	Glucagon release

⚡ A/L Key Point: Positive feedback amplifies the stimulus — useful in limited contexts (childbirth — oxytocin, blood clotting — thrombin). Negative feedback maintains stability.

GCE A/L Sri Lanka Past Paper Tips

Common Structured Questions:

1. “Describe the path of blood through the heart, naming all chambers and valves” (10 marks)
2. “Explain how an action potential is generated and propagated along a neuron” (12 marks)
3. “Describe the role of ADH in osmoregulation” (8 marks)
4. “Explain the factors that affect the rate of breathing” (8 marks)
5. “Compare the roles of insulin and glucagon in blood glucose regulation” (10 marks)

Diagram Questions:

1. Draw and label a transverse section of the heart (8 marks)
2. Draw and label a nephron (8 marks)
3. Draw and label a synapse (6 marks)
4. Draw and label the gas exchange system (lungs + airways) (8 marks)

Practical Questions:

• Record pulse rate and blood pressure
• Examine heart structure from a sheep heart practical
• Detect glucose in urine (Benedict’s test — clinical application)

⚡ A/L Strategy: For human physiology, always connect structure to function — why is the left ventricle wall thicker than the right? Because it pumps blood to the entire body (systemic), while the right ventricle only pumps to the lungs (pulmonary).

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