Human Physiology (SL) — IB Biology SL SL Study Guide

For: IB Biology SL candidates sitting IB Biology SL.

Covers: Digestion and absorption, the blood system (heart and blood vessels), defence against infectious disease, gas exchange and ventilation, and neurons and synapses, aligned with the IB DP Biology SL core syllabus.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Biology SL style for educational use. They are not reproductions of past IBO papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official IBO mark schemes for grading conventions.

1. What Is Human Physiology (SL)?

Human Physiology is Core Topic 6 of the IB Biology SL syllabus, focused on the structure and function of key human organ systems that maintain homeostasis, support survival, and enable response to external and internal stimuli. This topic makes up ~20% of your final SL exam score, with questions appearing in both Paper 1 (multiple choice) and Paper 2 (structured short answer), including 3-6 mark extended response items focused on mechanism explanations. It builds directly on IGCSE Biology knowledge of organ systems and core IB SL topics of cell membrane transport and enzyme function.

2. Digestion and absorption

Digestion is the breakdown of large, insoluble biological molecules into small, soluble monomers that can cross cell membranes to be absorbed into the bloodstream. The process relies on specific enzymes that catalyse hydrolysis reactions, each targeting a unique substrate:

• Amylase (secreted by salivary glands and the pancreas): Breaks starch into the disaccharide maltose
• Endopeptidases (pepsin in the stomach, trypsin from the pancreas): Breaks long polypeptide chains into shorter peptide fragments
• Lipase (secreted by the pancreas): Breaks triglycerides (fats) into fatty acids and glycerol
• Brush border enzymes (produced by small intestine epithelial cells, e.g. maltase, peptidases): Break disaccharides and peptides into absorbable monomers (glucose, amino acids)

The small intestine is adapted for maximum absorption via three key structural features: villi (finger-like projections on the intestinal wall), microvilli (tiny projections on individual epithelial cells) that increase surface area by ~600x, a single-cell thick epithelium to shorten diffusion distance, and a dense network of capillaries and a lacteal (lymph vessel) in each villus to maintain a steep concentration gradient for diffusion.

Worked Example: If the basic surface area of a 6m long human small intestine is 0.6 m², and villi increase surface area by a factor of 10 while microvilli increase it by a further factor of 60, what is the total functional surface area for absorption? Calculation: $0.6{\text{m}}^2\times 10\times 60=360{\text{m}}^2$, roughly the area of a tennis court.

Examiners frequently ask you to distinguish between diffusion (passive uptake of fatty acids) and active transport (energy-dependent uptake of glucose and amino acids to ensure full absorption even when lumen concentrations are lower than blood concentrations).

3. The blood system — heart, blood vessels

The mammalian circulatory system is a closed double circuit: the pulmonary circuit carries deoxygenated blood from the right side of the heart to the lungs to pick up oxygen, and the systemic circuit carries oxygenated blood from the left side of the heart to all body tissues.

Blood vessel structure and function

The heart’s left ventricle has the thickest muscular wall, as it must generate enough pressure to pump blood around the entire systemic circuit. The cardiac cycle follows three sequential stages:

1. Atrial systole: Atria contract, atrioventricular (AV) valves open, blood flows into relaxed ventricles
2. Ventricular systole: Ventricles contract, AV valves close (producing the "lub" heart sound), semi-lunar valves open, blood is ejected into the aorta and pulmonary artery
3. Diastole: All chambers relax, semi-lunar valves close (producing the "dub" heart sound), blood fills the atria from the vena cava and pulmonary vein

Worked Example: Calculate the cardiac output of a patient with a resting heart rate of 68 beats per minute (bpm) and a stroke volume of 75 cm³ per beat, using the formula: $$\text{Cardiac Output (CO)}=\text{Heart Rate}\times \text{Stroke Volume}$$ Calculation: $CO=68\times 75=5100{\text{cm}}^3/\text{min}=5.1\text{L}/\text{min}$

4. Defence against infectious disease

The body has three lines of defence against pathogens (disease-causing organisms like bacteria, viruses and fungi):

1. First line (non-specific physical/chemical barriers): Skin (waterproof physical barrier, sebum lowers pH to kill pathogens), mucus membranes (mucus traps pathogens in the respiratory tract, cilia sweep mucus out of the lungs), stomach acid (kills ingested pathogens)
2. Second line (non-specific immune response): Phagocytic white blood cells (macrophages, neutrophils) engulf and destroy pathogens via phagocytosis, using lysosomal enzymes to break down foreign material
3. Third line (specific adaptive immune response): B lymphocytes produce antibodies, Y-shaped proteins that bind to unique antigens (marker molecules) on pathogens to mark them for destruction. T lymphocytes activate B cells and kill host cells infected with viruses. Memory B and T cells remain in the bloodstream long after infection to provide long-term immunity.

Vaccination works by exposing the body to weakened, dead pathogens or isolated antigens, triggering a primary immune response and memory cell production without causing disease. On subsequent exposure to the live pathogen, memory cells initiate a faster, stronger secondary immune response that eliminates the pathogen before symptoms develop. Antibiotics only kill bacteria, as they target prokaryote-specific structures (cell walls, 70S ribosomes) and metabolic pathways that do not exist in viruses, which use host eukaryotic cell machinery to replicate.

Worked Example: Explain why antibiotics are prescribed for a bacterial throat infection but not for influenza (a viral infection): Antibiotics inhibit bacterial cell wall synthesis and prokaryotic protein production, processes that do not occur in viruses. Influenza viruses replicate inside human host cells, using human ribosomes and metabolic pathways, so antibiotics have no target to act on and will not reduce viral load or symptoms.

5. Gas exchange and ventilation

Three distinct processes support oxygen use and carbon dioxide removal in the body, and examiners frequently penalise students for using these terms interchangeably:

• Ventilation (breathing): Bulk movement of air into and out of the lungs to maintain a concentration gradient between alveoli and the blood
• Gas exchange: Passive diffusion of oxygen from alveoli into the blood, and carbon dioxide from the blood into alveoli, across the alveolar-capillary membrane
• Cell respiration: Metabolic production of ATP in cells, which consumes oxygen and produces carbon dioxide as a waste product

Alveoli (tiny air sacs in the lungs) are adapted for efficient gas exchange via four key features: large total surface area (~70 m²), 1-cell thick shared wall with capillaries to shorten diffusion distance, a moist lining to dissolve gases before diffusion, and a dense surrounding capillary network to maintain a steep concentration gradient.

Ventilation relies on pressure changes in the thoracic cavity:

• Inhalation: External intercostal muscles contract, diaphragm contracts and flattens, thoracic cavity volume increases, internal pressure drops below atmospheric pressure, air flows into the lungs
• Exhalation: Internal intercostal muscles contract, diaphragm relaxes and domes upwards, thoracic cavity volume decreases, internal pressure rises above atmospheric pressure, air flows out of the lungs

Worked Example: Calculate the minute ventilation of a person with a tidal volume (volume of air per normal breath) of 0.5 L and a breathing rate of 14 breaths per minute, using the formula: $$\text{Minute Ventilation}=\text{Tidal Volume}\times \text{Breathing Rate}$$ Calculation: $\text{Minute Ventilation}=0.5\text{L}\times 14=7\text{L}/\text{min}$

6. Neurons and synapses

Neurons are specialised cells that transmit electrical impulses around the body. Their structure includes: dendrites that receive signals from other neurons, a cell body containing the nucleus, a long axon insulated by a myelin sheath to speed up impulse transmission via saltatory conduction, and axon terminals that connect to other neurons or effector cells (muscles, glands).

Neurons maintain a resting potential of -70 mV, where the inside of the cell is more negative than the outside, maintained by a sodium-potassium pump that actively transports 3 Na⁺ ions out of the cell for every 2 K⁺ ions pumped in, plus K⁺ leak channels that allow positive K⁺ ions to diffuse out of the cell.

An action potential (nerve impulse) follows two key stages:

1. Depolarization: A stimulus opens Na⁺ channels in the axon membrane, Na⁺ ions flow into the cell, raising the membrane potential to +30 mV
2. Repolarization: Na⁺ channels close, K⁺ channels open, K⁺ ions flow out of the cell, returning the membrane potential to -70 mV, followed by a short refractory period where the neuron cannot fire another impulse to ensure signals travel in one direction only.

Synapses are junctions between two neurons, separated by a 20 nm wide synaptic cleft. Impulse transmission across synapses is chemical:

1. An action potential reaches the pre-synaptic axon terminal, opening Ca²⁺ channels
2. Ca²⁺ ions flow into the pre-synaptic cell, triggering synaptic vesicles containing neurotransmitter (e.g. acetylcholine) to fuse with the pre-synaptic membrane
3. Neurotransmitter diffuses across the cleft and binds to receptors on the post-synaptic membrane, opening Na⁺ channels to trigger a new action potential in the post-synaptic neuron
4. Neurotransmitter is broken down by enzymes (e.g. acetylcholinesterase) to prevent continuous stimulation of the post-synaptic cell.

Worked Example: Nerve gases block the action of acetylcholinesterase. Explain the effect on synapse function: Acetylcholinesterase normally breaks down acetylcholine in the synaptic cleft after transmission. If it is blocked, acetylcholine remains bound to post-synaptic receptors, causing continuous, repeated action potentials in the post-synaptic neuron, leading to uncontrolled muscle spasms and eventual paralysis.

7. Common Pitfalls (and how to avoid them)

• Wrong move: Labeling the right side of the heart as carrying oxygenated blood. Why students do it: Anterior heart diagrams show the heart as if you are looking at another person, so the left side of the heart appears on the right side of the page, causing orientation confusion. Correct move: Always identify the left ventricle first, which has the thickest muscular wall, as it pumps blood around the systemic circuit and carries oxygenated blood.
• Wrong move: Stating that antibiotics kill viruses. Why students do it: Students lump all pathogens into a single category and forget that antibiotics only target prokaryote-specific structures. Correct move: Explicitly state that antibiotics have no effect on viruses, as viruses use host eukaryotic cell machinery to replicate, so there are no prokaryotic targets for antibiotics to act on.
• Wrong move: Using ventilation, gas exchange and cell respiration interchangeably. Why students do it: All three processes are linked to oxygen use, so students mix up their definitions. Correct move: Define each term explicitly in answers: ventilation = breathing (bulk air movement), gas exchange = diffusion across the alveolar-capillary membrane, cell respiration = ATP production in mitochondria.
• Wrong move: Only describing depolarization when explaining action potentials, with no mention of repolarization or the refractory period. Why students do it: Depolarization is the most memorable step of the process, so students forget the full cycle. Correct move: Structure action potential answers into three clear parts: resting potential, depolarization, repolarization + refractory period, to get full marks for 3+ mark questions.
• Wrong move: Stating that the small intestine secretes amylase, lipase and trypsin. Why students do it: Students confuse brush border enzymes produced by the intestinal epithelium with pancreatic secretions released into the small intestine. Correct move: Clarify that most digestive enzymes (amylase, lipase, trypsin) are secreted by the pancreas, and only disaccharidases and peptidases are produced by the small intestine itself.

8. Practice Questions (IB Biology SL Style)

Question 1 (3 marks)

List three structural adaptations of the small intestine and explain how each supports efficient absorption of digested nutrients.

Worked Solution (aligned to IB mark scheme):

1. Villi and microvilli on the epithelial lining (1 mark): Increase total surface area by ~600x, maximizing contact between digested food and absorption surfaces to increase absorption rate.
2. Single-cell thick epithelial layer (1 mark): Shortens the diffusion distance for monomers to cross into the bloodstream, speeding up passive and active transport.
3. Dense capillary network and lacteal in each villus (1 mark): Rapidly transports absorbed nutrients away from the intestine, maintaining a steep concentration gradient to enable continuous diffusion.

Question 2 (4 marks)

A student has a resting heart rate of 62 bpm and a stroke volume of 78 cm³ per beat. a) Calculate their resting cardiac output, showing your working and including units. [2 marks] b) During a football match, their cardiac output increases to 16.8 L per minute. State two mechanisms the heart uses to achieve this increase. [2 marks]

Worked Solution: a) $\text{Cardiac Output}=\text{Heart Rate}\times \text{Stroke Volume}$ (1 mark for correct formula) $CO=62\times 78=4836{\text{cm}}^3/\text{min}=4.84\text{L}/\text{min}$ (1 mark for correct calculation and units) b) Any two valid answers, 1 mark each: Increased heart rate, increased stroke volume, increased force of ventricular contraction.

Question 3 (3 marks)

Explain how vaccination provides long-term protection against a specific viral disease.

Worked Solution:

1. The vaccine contains inactivated viral antigens or weakened virus that cannot cause disease, triggering an immune response (1 mark).
2. Specific B and T lymphocytes are activated, producing antibodies against the antigen, and long-lived memory B and T cells remain in the bloodstream after the initial response (1 mark).
3. On exposure to the live virus, memory cells initiate a faster, stronger secondary immune response, producing large amounts of specific antibodies to eliminate the virus before symptoms develop (1 mark).

9. Quick Reference Cheatsheet

10. What's Next

This core Human Physiology topic is the foundation for two optional IB Biology topics: Neurobiology and Behaviour, and the Human Physiology Option D, if you choose to study them for Paper 3. It also connects directly to core topics of Cell Biology (membrane transport, enzyme function) and Metabolism, so reviewing those topics will strengthen your understanding of physiological mechanisms and help you answer cross-topic exam questions, which make up ~15% of Paper 2 marks. Exam questions on this topic are most often structured short answer items, so practising 3-6 mark response questions using the mark scheme key terms will help you maximize your score.

If you have any questions about specific concepts, past paper questions, or exam strategy for IB Biology SL, you can ask Ollie, our AI tutor, at any time for personalized explanations and practice drills. You can also find more IB Biology SL study resources, including topic quizzes and full mock exams, on the homepage.