CDS Biology · Free notes
Human Physiology: CDS Biology
Human Physiology has 47 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II): 25 EASY, 21 MODERATE and 1 HARD. They come from 17 different papers, and 2022 alone asked nine. Digestion and nutrition lead with fourteen, then the blood and the heart with seven each. Nearly every item is one fact about one organ: which enzyme acts first, which vitamin prevents rickets, which chamber receives blood from the lungs. Learn each organ system as a short chain of parts and jobs, starting with the tissues they are made of.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Tissues and Muscles
6 PYQsThe tissues every organ is built from: the epithelium that lines it, the connective tissues that store fat and hold it together, and the three kinds of muscle.
Digestion and Nutrition
14 PYQsHow enzymes break food down along the gut, what bile, villi and tooth enamel do, and which disease comes from missing which nutrient.
Blood and Breathing
7 PYQsWhat blood is made of, how haemoglobin carries the gases of breathing, why high altitude makes you breathless, what lymph is, the ABO blood groups, and how a vaccine prepares the body.
Heart and Circulation
7 PYQsThe four chambers of the heart, the double circulation through the lungs and the body, the main blood vessels, and what blood pressure and the pulse measure.
Excretion and Reproductive Organs
6 PYQsWhere urea comes from, how the kidneys, ureters, bladder and urethra get rid of it, and what each organ of the male reproductive system does.
Nerves, Senses and Hormones
7 PYQsThe parts of the brain and what each controls, the reflex arc, the blind spot of the eye, and the main endocrine glands with their hormones.
Formula & revision sheet
0 formulas · 13 reference tables · 28 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
0 formulas · 13 reference tables · 28 gotchas across all subtopics — the exam-eve cheat-sheet
Tissues and Muscles
Reference tables (2)
Epithelial and connective tissue6 rows
| Tissue | Where it is found | Its cell or key part |
|---|---|---|
| Squamous epithelium | Lining of the mouth and oesophagus; skin | Thin, flat cellsQ CDS 2022 (I): an injury to the delicate lining of the mouth and oesophagus damages squamous epithelium. |
| Cuboidal epithelium | Kidney tubules, glands | Cube-shaped cells |
| Adipose tissue | Below the skin, around organs | Adipocytes, which store fatQ Asked twice, CDS 2018 (I) and 2022 (II): fat is stored in adipose tissue, in adipocytes. |
| Cartilage | Nose, ear, ends of bones | Chondrocytes, with collagen fibresQ CDS 2026 (II): collagen matters for cartilage and is not a globin. |
| Bone | Skeleton | Osteocytes |
| Blood | Inside the blood vessels | A fluid connective tissue |
The three kinds of muscle5 rows
| Feature | Skeletal | Smooth | Cardiac |
|---|---|---|---|
| Control | Voluntary | Involuntary | Involuntary |
| Stripes | Striated | Unstriated | Striated |
| Cell shape | Cylindrical, unbranched | Spindle-shaped | Cylindrical, branchedQ CDS 2021 (I): a cardiac muscle cell is cylindrical, branched and uninucleate. |
| Nuclei per cell | Many | One | One |
| Found in | Biceps and other limb muscles | Ureters, iris, bronchi, gut | HeartQ CDS 2019 (II): the biceps has no smooth muscle; the ureters, iris and bronchi do. |
Watch out for (4)
- Adipocyte stores fat; the other '-cytes' do not→ Epithelial and connective tissue
- Collagen is not a globin→ Epithelial and connective tissue
- The biceps is skeletal muscle→ The three kinds of muscle
- Branched means cardiac→ The three kinds of muscle
Digestion and Nutrition
Reference tables (3)
Digestive enzymes, from the mouth to the intestine6 rows
| Enzyme | Where it acts | What it digests | Medium |
|---|---|---|---|
| Salivary amylase | Mouth | Starch | About neutral |
| Lysozyme | Saliva and tears | Cell walls of bacteria | About neutralQ CDS 2024 (I): the antibacterial enzyme in saliva and tears is lysozyme. |
| Pepsin | Stomach | Proteins | AcidicQ CDS 2022 (II): trypsin and amylase are the pair that do not work in acid. |
| Trypsin | Small intestine (from the pancreas) | Proteins | Alkaline |
| Lipase | Small intestine | Fats | Alkaline |
| Lactase | Small intestine | Lactose (milk sugar) | AlkalineQ CDS 2026 (I): an elderly person who can still digest milk is making enough lactase. |
The gut: bile, villi and teeth5 rows
| Part | Key fact |
|---|---|
| Small intestine | The longest part of the gutQ CDS 2022 (II): of the stomach, small intestine, large intestine and rectum, the small intestine is longest. |
| Villi | Finger-like projections that absorb digested foodQ CDS 2025 (II): the finger-like projections of the small intestine are villi. |
| Bile | No enzymes; emulsifies fat and makes the medium alkaline |
| Gall bladder | Stores bile |
| Tooth enamel | Calcium phosphate; the hardest substance in the bodyQ CDS 2025 (II): enamel is made of calcium phosphate. |
Deficiency diseases7 rows
| Missing nutrient | Disease or effect |
|---|---|
| Vitamin A | Night blindness |
| Vitamin C | Scurvy |
| Vitamin D | Rickets in childrenQ CDS 2022 (I): rickets comes from a lack of vitamin D. |
| Iron | AnaemiaQ CDS 2022 (II): iron deficiency causes anaemia, not scurvy, rickets or cholera. |
| Iodine | Goitre |
| Protein | KwashiorkorQ CDS 2022 (II): kwashiorkor is caused by a lack of protein. |
| Fluoride | Tooth decay (dental caries)Q CDS 2016 (II): too little fluoride gives caries; mottled teeth and bent bones come from too much. |
Watch out for (7)
- Amylase acts first, in the mouth→ Digestive enzymes, from the mouth to the intestine
- Only pepsin needs acid→ Digestive enzymes, from the mouth to the intestine
- Bile has no enzymes→ The gut: bile, villi and teeth
- Enamel is calcium phosphate→ The gut: bile, villi and teeth
- Fluoride: too little against too much→ Deficiency diseases
- Kwashiorkor is protein, marasmus is protein plus energy→ Deficiency diseases
- Rickets is D, scurvy is C→ Deficiency diseases
Blood and Breathing
Reference tables (2)
Blood, haemoglobin and lymph5 rows
| Part or situation | What to remember |
|---|---|
| Haemoglobin | Carries both oxygen and carbon dioxideQ CDS 2019 (I): haemoglobin carries both gases, not oxygen alone. |
| Mature red blood cell | No nucleus: cannot make protein or divideQ CDS 2026 (II): protein synthesis and cell division are what an erythrocyte cannot do. |
| Platelets | Release clotting factorsQ CDS 2024 (I): clotting factors come from platelets, not RBCs, eosinophils or monocytes. |
| Lymph | Fluid leaked from capillaries; no red cellsQ CDS 2018 (I): lymph forms from leakage of blood through capillaries. |
| High altitude | Low partial pressure of oxygenQ CDS 2016 (II): altitude sickness is caused by low partial pressure of oxygen. |
ABO blood groups and vaccination4 rows
| Blood group | Antigen on red cells | Antibody in plasma | Can receive from |
|---|---|---|---|
| A | A | Anti-B | A and OQ CDS 2025 (II): the four ABO types are A, B, AB and O. |
| B | B | Anti-A | B and O |
| AB | A and B | Neither | All groups (universal recipient) |
| O | Neither | Anti-A and anti-B | O only (universal donor) |
Watch out for (4)
- The percentage of oxygen does not fall→ Blood, haemoglobin and lymph
- Lymph is not pumped by the heart→ Blood, haemoglobin and lymph
- AO and BO are not blood groups→ ABO blood groups and vaccination
- Donor O, recipient AB→ ABO blood groups and vaccination
Heart and Circulation
Reference tables (2)
Double circulation and the blood vessels6 rows
| Vessel | Carries blood from | To |
|---|---|---|
| Pulmonary artery | Right ventricle | Lungs (oxygen-poor blood) |
| Pulmonary vein | Lungs | Left atrium (oxygen-rich blood)Q Asked twice, CDS 2021 (II) and 2026 (I): oxygen-rich blood from the lungs enters the left atrium first. |
| Aorta | Left ventricle | The whole body |
| Vena cava | The whole body | Right atrium |
| Coronary artery | Aorta | The heart muscle |
| Capillaries | Arteries | Veins |
Blood pressure and the pulse5 rows
| Term | Meaning |
|---|---|
| Systolic pressure | About 120 mm Hg, while the ventricles contractQ CDS 2025 (I): the normal systolic pressure is 120 mm Hg. |
| Diastolic pressure | About 80 mm Hg, while the ventricles relax |
| Sphygmomanometer | The instrument that measures blood pressureQ CDS 2025 (I): blood pressure is measured manually with a sphygmomanometer. |
| Pulse | Stretch and recoil of the aorta and main arteriesQ CDS 2024 (I): the pulse comes from the aorta and main arteries, not the heart chambers or valves. |
| Resting pulse rate | About 72 per minute |
Watch out for (4)
- Artery means away, not oxygen-rich→ Double circulation and the blood vessels
- Left is oxygen-rich→ Double circulation and the blood vessels
- Both numbers are about the ventricles→ Blood pressure and the pulse
- The pulse is in the arteries→ Blood pressure and the pulse
Excretion and Reproductive Organs
Reference tables (2)
Urea, the kidneys and the path of urine6 rows
| Organ | Its part in removing urea |
|---|---|
| Liver | Makes urea from the ammonia of broken-down proteinsQ Asked twice: CDS 2019 (I) (urea is produced in the liver) and 2022 (II) (from proteins). |
| Kidney | Filters urea and water from the blood to make urine |
| Ureter | Carries urine from the kidney to the bladder |
| Urinary bladder | Stores urine |
| Urethra | Carries urine out of the body |
| Bacteria on the floor | Break urea into ammonia, the smell of stale urineQ CDS 2017 (II): the smell is from bacteria turning urea into ammonia. |
The male reproductive organs5 rows
| Male organ | What it does |
|---|---|
| Testis | Makes sperm and testosterone |
| Scrotum | Keeps the testes a little cooler than the body |
| Epididymis | Stores sperm while they matureQ CDS 2018 (II): epididymis, vas deferens, seminal vesicles and prostate together serve maturation and motility, not spermatogenesis. |
| Vas deferens | Carries sperm towards the urethra |
| Seminal vesicles and prostate | Add fluid that feeds the sperm and lets them swim |
Watch out for (4)
- Urea is made in the liver, not the kidney→ Urea, the kidneys and the path of urine
- Ureter before bladder, urethra after→ Urea, the kidneys and the path of urine
- Sperm are made only in the testis→ The male reproductive organs
- The cervix is female→ The male reproductive organs
Nerves, Senses and Hormones
Reference tables (2)
The brain, reflexes and the eye7 rows
| Part | Job or fact |
|---|---|
| Cerebrum (forebrain) | Thinking, memory, the senses, voluntary actions |
| Cerebellum (hindbrain) | Balance and posture |
| Medulla (hindbrain) | Heartbeat, breathing, blood pressure |
| Spinal cord | Carries messages to and from the brain; centre of many reflexes |
| Blind spot | Where the optic nerve leaves; no rods or conesQ CDS 2025 (I): the point of the eye with no photoreceptors is the blind spot, not the fovea. |
| Fovea | Most cones; sharpest vision |
| Schizophrenia | Hallucinations and delusionsQ CDS 2026 (II): schizophrenia is associated with hallucination, not multiple personalities. |
Endocrine glands and their hormones6 rows
| Gland | Hormone | What it does |
|---|---|---|
| Pituitary (single) | Growth hormone, TSH and others | Controls growth and other glandsQ CDS 2020 (I): the pituitary is the one gland that is not paired; adrenals, testes and ovaries are. |
| Thyroid | Thyroxine | Controls metabolism; needs iodineQ CDS 2023 (I): iodine is what the thyroid needs to make thyroxin. |
| Pancreas | Insulin | Lets cells take up glucose; lowers blood sugarQ CDS 2016 (II): insulin, not GH, TSH or cortisol, is essential for glucose uptake by cells. |
| Adrenals (pair) | Adrenaline | Prepares the body for an emergency |
| Testes (pair) | Testosterone | Male characters |
| Ovaries (pair) | Oestrogen | Female characters |
Watch out for (5)
- Blind spot, not fovea→ The brain, reflexes and the eye
- Schizophrenia is not split personality→ The brain, reflexes and the eye
- Sensory in, motor out→ The brain, reflexes and the eye
- The pituitary is single→ Endocrine glands and their hormones
- Iodine, not common salt→ Endocrine glands and their hormones
PYQ weightage by concept
13 concepts · 47 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
13 concepts · 47 PYQs — where the marks actually sit, so you know what to drill first
Tissues and Muscles6 PYQs · 13%
| Concept | PYQs | Share |
|---|---|---|
| Epithelial and connective tissue | 4 | 9% |
| The three kinds of muscle | 2 | 4% |
Digestion and Nutrition14 PYQs · 30%
| Concept | PYQs | Share |
|---|---|---|
| Digestive enzymes, from the mouth to the intestine | 5 | 11% |
| Deficiency diseases | 5 | 11% |
| The gut: bile, villi and teeth | 4 | 9% |
Blood and Breathing7 PYQs · 15%
| Concept | PYQs | Share |
|---|---|---|
| Blood, haemoglobin and lymph | 5 | 11% |
| ABO blood groups and vaccination | 2 | 4% |
Heart and Circulation7 PYQs · 15%
| Concept | PYQs | Share |
|---|---|---|
| Blood pressure and the pulse | 4 | 9% |
| Double circulation and the blood vessels | 3 | 6% |
Excretion and Reproductive Organs6 PYQs · 13%
| Concept | PYQs | Share |
|---|---|---|
| Urea, the kidneys and the path of urine | 4 | 9% |
| The male reproductive organs | 2 | 4% |
Nerves, Senses and Hormones7 PYQs · 15%
| Concept | PYQs | Share |
|---|---|---|
| The brain, reflexes and the eye | 4 | 9% |
| Endocrine glands and their hormones | 3 | 6% |
Test yourself on Human Physiology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.