Option D: Human physiology (15/25 hours)

D.1 Human nutrition

Falsification of theories with one theory being superseded by another—scurvy was thought to be specific to humans, because attempts to induce the symptoms in laboratory rats and mice were entirely unsuccessful. (1.9)

Theory of Knowledge

There are positive effects of exposure to sun such as the production of Vitamin D as well as health risks associated with exposure to UV rays. How can conflicting knowledge claims be balanced?

International Mindedness

The Vitamin and Mineral Nutrition Information System (VMNIS), formerly known as the Micronutrient Deficiency Information System (MDIS), was established in 1991 following a request by the World Health Assembly to strengthen surveillance of micronutrient deficiencies at the global level.


• Essential nutrients cannot be synthesized by the body, therefore they have to
be included in the diet.
• Dietary minerals are essential chemical elements.
• Vitamins are chemically diverse carbon compounds that cannot be
synthesized by the body.
• Some fatty acids and some amino acids are essential.
• Lack of essential amino acids affects the production of proteins.
• Malnutrition may be caused by a deficiency, imbalance or excess of nutrients
in the diet.
• Appetite is controlled by a centre in the hypothalamus.
• Overweight individuals are more likely to suffer hypertension and type II
• Starvation can lead to breakdown of body tissue.

Applications and skills

• Application: Production of ascorbic acid by some mammals, but not others that need a dietary supply.
• Application: Cause and treatment of phenylketonuria (PKU).

• Application: Lack of Vitamin D or calcium can affect bone mineralization and cause rickets or osteomalacia.
• Application: Breakdown of heart muscle due to anorexia.
• Application: Cholesterol in blood as an indicator of the risk of coronary heart disease.
• Skill: Determination of the energy content of food by combustion.
• Skill: Use of databases of nutritional content of foods and software to
calculate intakes of essential nutrients from a daily diet.

D.2 Digestion

Serendipity and scientific discoveries—the role of gastric acid in digestion was established by William Beaumont while observing the process of digestion in an open wound caused by gunshot. (1.4)


• Nervous and hormonal mechanisms control the secretion of digestive juices.
• Exocrine glands secrete to the surface of the body or the lumen of the gut.
• The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms.
• Acid conditions in the stomach favour some hydrolysis reactions and help to control pathogens in ingested food.
• The structure of cells of the epithelium of the villi is adapted to the absorption of food.
• The rate of transit of materials through the large intestine is positively correlated with their fibre content.
• Materials not absorbed are egested.

Applications and skills

• Application: The reduction of stomach acid secretion by proton pump inhibitor drugs.
• Application: Dehydration due to cholera toxin.
• Application: Helicobacter pylori infection as a cause of stomach ulcers.
• Skill: Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs.

D.3 Functions of the liver

Educating the public on scientific claims—scientific studies have shown that high-density lipoprotein could be considered “good” cholesterol. (5.2)

Theory of Knowledge

Excessive alcohol consumption may cause liver cirrhosis. Are attitudes to drugs and alcohol an example of something that is relative to culture? Is all knowledge dependent on culture?


• The liver removes toxins from the blood and detoxifies them.
• Components of red blood cells are recycled by the liver.
• The breakdown of erythrocytes starts with phagocytosis of red blood cells by Kupffer cells.
• Iron is carried to the bone marrow to produce hemoglobin in new red blood cells.
• Surplus cholesterol is converted to bile salts.
• Endoplasmic reticulum and Golgi apparatus in hepatocytes produce plasma proteins.
• The liver intercepts blood from the gut to regulate nutrient levels.
• Some nutrients in excess can be stored in the liver.

Applications and skills

• Application: Causes and consequences of jaundice.
• Application: Dual blood supply to the liver and differences between sinusoids and capillaries.


Aim 6: Temporary mounts of hepatocytes can be prepared from fresh liver.

Aim 8: Given the pressure on health resources, especially the availability of organs for transplant, should an alcoholic be allowed a liver transplants.

D.4 The heart

Developments in scientific research followed improvements in apparatus or instrumentation—the invention of the stethoscope led to improved knowledge of the workings of the heart. (1.8)


• Structure of cardiac muscle cells allows propagation of stimuli through the heart wall.
• Signals from the sinoatrial node that cause contraction cannot pass directly from atria to ventricles.
• There is a delay between the arrival and passing on of a stimulus at the atrioventricular node.
• This delay allows time for atrial systole before the atrioventricular valves close.
• Conducting fibres ensure coordinated contraction of the entire ventricle wall.
• Normal heart sounds are caused by the atrioventricular valves and semilunar valves closing causing changes in blood flow.

Applications and Skills

• Application: Use of artificial pacemakers to regulate the heart rate.
• Application: Use of defibrillation to treat life-threatening cardiac conditions.
• Application: Causes and consequences of hypertension and thrombosis.
• Skill: Measurement and interpretation of the heart rate under differen conditions.
• Skill: Interpretation of systolic and diastolic blood pressure measurements.
• Skill: Mapping of the cardiac cycle to a normal ECG trace.
• Skill: Analysis of epidemiological data relating to the incidence of coronary heart disease.


CardioLab virtual by Sciencecourseware


Theory of knowledge

Symbols are used as a form of non-verbal communication. Why is the heart used as a symbol for love? What is the importance of symbols in different areas of knowledge?

D.5 Hormones and metabolism (AHL)

Cooperation and collaboration between groups of scientists—the International Council for the Control of Iodine Deficiency Disorders includes a number of scientists who work to eliminate the harm done by iodine deficiency. (4.3)


• Endocrine glands secrete hormones directly into the bloodstream.
• Steroid hormones bind to receptor proteins in the cytoplasm of the target cell to form a receptor–hormone complex.
• The receptor–hormone complex promotes the transcription of specific genes.
• Peptide hormones bind to receptors in the plasma membrane of the target cell.
• Binding of hormones to membrane receptors activates a cascade mediated by a second messenger inside the cell.
• The hypothalamus controls hormone secretion by the anterior and posterior lobes of the pituitary gland.
• Hormones secreted by the pituitary control growth, developmental changes, reproduction and homeostasis.

Applications and Skills

• Application: Some athletes take growth hormones to build muscles.
• Application: Control of milk secretion by oxytocin and prolactin.


Aim 8: There are numerous drugs that can enhance performance. Is the use of these drugs acceptable in terms of conducting a fair test as long as all athletes have equal access to them?

D.6 Transport of respiratory gases (AHL)

Scientists have a role in informing the public—scientific research has led to a change in public perception of smoking. (5.1)


Aim 8: Some sports, such as high-altitude mountain climbing or scuba diving, may push the limits of the human body beyond endurance and cause damage. Should they be controlled or banned?


• Oxygen dissociation curves show the affinity of hemoglobin for oxygen.
• Carbon dioxide is carried in solution and bound to hemoglobin in the blood.
• Carbon dioxide is transformed in red blood cells into hydrogencarbonate ions.
• The Bohr shift explains the increased release of oxygen by hemoglobin in respiring tissues.
• Chemoreceptors are sensitive to changes in blood pH.
• The rate of ventilation is controlled by the respiratory control centre in the medulla oblongata.
• During exercise the rate of ventilation changes in response to the amount of CO2 in the blood.
• Fetal hemoglobin is different from adult hemoglobin allowing the transfer of oxygen in the placenta onto the fetal hemoglobin.

Applications and Skills

• Application: Consequences of high altitude for gas exchange.
• Application: pH of blood is regulated to stay within the narrow range of 7.35 to 7.45.
• Application: Causes and treatments of emphysema.
• Skill: Analysis of dissociation curves for hemoglobin and myoglobin.
• Skill: Identification of pneumocytes, capillary endothelium cells and blood
cells in light micrographs and electron micrographs of lung tissue.


Training camps for athletes are frequently located at high altitude to increase the hemoglobin content of the blood. This puts the athlete at an advantage when they return to lower ground for competition.

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