Multicellular Organisms

Higher Multicellular Organisms

Stem Cells and Differentiation

Stem cells are undifferentiated cells that can divide and differentiate into specialised cell Types.

Types:

Type	Source	Potential
Totipotent	Early embryo	Can become any cell type (including placental)
Pluripotent	Blastocyst	Can become any cell type (not placental)
Multipotent	Adult tissues (e.g., bone marrow)	Can become limited range of cell types
Unipotent	Adult tissues	Can become only one cell type

Cell differentiation: The process by which cells become specialised. All body cells have the Same genes, but different genes are switched on (expressed) or off in different cell types.

Transcription factors and epigenetic modifications (e.g., DNA methylation, histone Modification) control which genes are expressed.

Worked Example: Stem cell therapy for Parkinson’s disease.

Parkinson’s disease is caused by the death of dopamine-producing neurones in the brain. Stem cell Therapy involves:

1. Obtaining pluripotent stem cells (from embryos or by reprogramming adult cells).
2. Differentiating the stem cells into dopamine-producing neurones in the laboratory.
3. Transplanting these neurones into the patient’s brain.
4. The transplanted neurones produce dopamine, reducing the symptoms of Parkinson’s disease.

Ethical considerations: The use of embryonic stem cells is controversial because it involves the Destruction of embryos. Adult stem cells and induced pluripotent stem cells (iPSCs) avoid this issue But may be less versatile.

Worked Example: Induced pluripotent stem cells (iPSCs).

In 2006, Shinya Yamanaka discovered that adult somatic cells could be reprogrammed to become Pluripotent by introducing four transcription factors (Oct4, Sox2, Klf4, c-Myc) — the “Yamanaka Factors.” iPSCs behave like embryonic stem cells and can differentiate into any cell type. This has Enormous potential for regenerative medicine because it avoids the ethical issues associated with Embryonic stem cells and allows patient-specific therapies (using the patient’s own cells).

Tissues, Organs, and Systems

Tissue: A group of similar cells performing a specific function.

Organ: A group of tissues working together.

Organ system: A group of organs working together.

Major organ systems in mammals:

• Nervous system
• Circulatory system
• Respiratory system
• Digestive system
• Excretory system
• Reproductive system
• Immune system
• Endocrine system

The Nervous System

Neurones: Cells that transmit electrical impulses.

Types:

• Sensory neurones: Transmit impulses from receptors to the CNS
• Motor neurones: Transmit impulses from the CNS to effectors
• Relay neurones: Connect sensory and motor neurones within the CNS

Structure of a motor neurone:

• Cell body (with nucleus)
• Dendrites (receive signals)
• Axon (long fibre, transmits impulses)
• Myelin sheath (insulating layer; speeds up transmission by saltatory conduction)
• Nodes of Ranvier (gaps in myelin sheath where action potentials jump)
• Synaptic terminals (release neurotransmitters)

Resting potential: Approximately -70 \mathrm{ mV inside the axon relative to outside. Maintained by the sodium-potassium pump (3 \mathrm{ Na^+ out, 2 \mathrm{ K^+ in) and selective Permeability of the membrane.

Action potential:

1. Stimulus causes voltage-gated \mathrm{Na^+ channels to open
2. \mathrm{Na^+ rushes in, depolarising the membrane (to about +40 \mathrm{ mV)
3. Voltage-gated \mathrm{K^+ channels open
4. \mathrm{K^+ rushes out, repolarising the membrane
5. The membrane briefly becomes hyperpolarised before the resting potential is restored

Refractory period: A brief period after an action potential during which the neurone cannot be Stimulated again. This ensures one-way transmission.

Worked Example: Saltatory conduction.

In a myelinated axon, the action potential “jumps” from one node of Ranvier to the next, rather than Propagating along the entire length of the axon membrane. This speeds up transmission because:

1. Less membrane needs to be depolarised (only at the nodes, not along the entire internode).
2. The local currents between nodes are larger because the myelin insulates the internode, preventing current leakage.

The speed of conduction in a myelinated axon can be up to 100 m/s, compared to about 1 m/s in an Unmyelinated axon of the same diameter.

Synapses

A synapse is the junction between two neurones or between a neurone and an effector.

Process:

1. Action potential arrives at the presynaptic terminal
2. Voltage-gated \mathrm{Ca^{2+} channels open; calcium ions enter
3. Synaptic vesicles fuse with the presynaptic membrane
4. Neurotransmitter is released into the synaptic cleft
5. Neurotransmitter binds to receptors on the postsynaptic membrane
6. Ion channels open, causing depolarisation (excitatory) or hyperpolarisation (inhibitory)
7. Neurotransmitter is broken down by enzymes or reabsorbed (recycled)

Example: Acetylcholine is a common excitatory neurotransmitter. It is broken down by Acetylcholinesterase in the synaptic cleft.

Worked Example: Spatial and temporal summation.

A single excitatory postsynaptic potential (EPSP) is too small to reach the threshold for an Action potential ( about $-55$ mV from a resting potential of $-70$ mV). However, if Multiple EPSPs arrive at the same synapse in rapid succession (temporal summation) or at different Synapses on the same postsynaptic neurone simultaneously (spatial summation), their effects can add Up. If the combined depolarisation reaches threshold, an action potential is generated in the Postsynaptic neurone.

The Endocrine System

The endocrine system uses chemical messengers (hormones) secreted by endocrine glands into the Bloodstream.

Key differences from the nervous system:

Feature	Nervous	Endocrine
Signal	Electrical impulse	Chemical (hormone)
Speed	Fast	Slower
Duration	Short-lived	Longer-lasting
Target	Specific	Widespread (cells with receptors)
Pathway	Neurones	Bloodstream

Examples of hormones:

Hormone	Gland	Function
Adrenaline	Adrenal medulla	Fight or flight response
Insulin	Pancreas (beta cells)	Lowers blood glucose
Glucagon	Pancreas (alpha cells)	Raises blood glucose
Thyroxine	Thyroid	Regulates metabolism
ADH	Posterior pituitary	Water reabsorption in kidneys
FSH/LH	Anterior pituitary	Reproductive cycle

Blood Glucose Regulation

Normal blood glucose: Approximately 4-6 \mathrm{ mmol/L.

After a meal (blood glucose rises):

1. Beta cells in the pancreas detect increased glucose
2. Beta cells secrete insulin
3. Insulin stimulates:

• Glycogenesis (glucose to glycogen in liver and muscle)
• Increased glucose uptake by cells
• Increased respiration

4. Blood glucose falls to normal

Between meals / during exercise (blood glucose falls):

1. Alpha cells in the pancreas detect decreased glucose
2. Alpha cells secrete glucagon
3. Glucagon stimulates:

• Glycogenolysis (glycogen to glucose in liver)
• Gluconeogenesis (amino acids/lactate to glucose)

4. Blood glucose rises to normal

Diabetes:

• Type 1: Autoimmune destruction of beta cells; no insulin produced. Treated with insulin injections.
• Type 2: Cells become resistant to insulin; reduced insulin sensitivity. Treated with diet, exercise, and medication.

The Circulatory System

Blood vessels:

Feature	Artery	Vein	Capillary
Wall thickness	Thick	Thin	One cell thick
Lumen	Narrow	Wide	Very narrow
Valves	No	Yes	No
Blood pressure	High	Low	Low
Blood flow	Fast	Slow	Very slow

Double circulatory system:

1. Pulmonary circulation: Heart to lungs (deoxygenated) and lungs to heart (oxygenated)
2. Systemic circulation: Heart to body (oxygenated) and body to heart (deoxygenated)

Cardiac cycle:

1. Atrial systole: Atria contract, pushing blood into ventricles
2. Ventricular systole: Ventricles contract, pushing blood into aorta and pulmonary artery
3. Diastole: Heart muscle relaxes; blood flows into atria from veins

Cardiac output:

\mathrm{Cardiac output = \mathrm{stroke volume \times \mathrm{heart rate

Example: If stroke volume is 70 \mathrm{ mL and heart rate is 72 \mathrm{ bpm:

\mathrm{Cardiac output = 70 \times 72 = 5040 \mathrm{ mL/min \approx 5.0 \mathrm{ L/min

The Respiratory System

Gas exchange occurs in the alveoli.

Features of alveoli for efficient gas exchange:

• Large surface area (millions of alveoli)
• Thin walls (one cell thick)
• Dense capillary network
• Moist surface for gas dissolution
• Maintained concentration gradient by blood flow and ventilation

Fick’s Law of Diffusion:

\mathrm{Rate of diffusion \propto \frac{\mathrm{Surface area \times \mathrm{Concentration difference}{\mathrm{Diffusion distance}

The Digestive System

Macromolecule digestion:

Macromolecule	Enzyme	Products	Site
Starch	Amylase	Maltose	Mouth, small intestine
Maltose	Maltase	Glucose	Small intestine
Proteins	Protease (pepsin, trypsin)	Amino acids	Stomach, small intestine
Lipids	Lipase	Fatty acids + glycerol	Small intestine
DNA/RNA	Nuclease	Nucleotides	Small intestine

Absorption: The small intestine is adapted for absorption with villi and microvilli, which Increase surface area. Each villus has a dense capillary network (for amino acids and glucose) and a Lacteal (for fatty acids and glycerol).

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Common Pitfalls

1. Action potential direction: Depolarisation is caused by \mathrm{Na^+ influx; repolarisation by \mathrm{K^+ efflux.

2. Synapse transmission: Neurotransmitters are released into the synaptic cleft, not directly into the next neurone.

3. Insulin vs. Glucagon: Insulin lowers blood glucose; glucagon raises it.

4. Arteries vs. Veins: Arteries carry blood away from the heart; veins carry blood towards the heart. Pulmonary arteries carry deoxygenated blood.

5. Cardiac cycle timing: The atrioventricular valves close during ventricular systole (producing the first heart sound, “lub”).

6. Confusing the roles of the SA node and AV node. The SA node (sinoatrial node) is the pacemaker that initiates each heartbeat. The AV node (atrioventricular node) relays the impulse from the atria to the ventricles, introducing a delay that allows the ventricles to fill.

7. Forgetting that the heart is myogenic. Cardiac muscle generates its own electrical impulses without nervous stimulation, although the autonomic nervous system can modify the heart rate.

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Practice Questions

1. Describe the sequence of events at a cholinergic synapse.

2. Explain how the structure of a motor neurone is adapted to its function.

3. Compare the nervous and endocrine systems as communication systems.

4. A person has a stroke volume of 80 \mathrm{ mL and a heart rate of 65 \mathrm{ bpm. Calculate the cardiac output.

5. Describe how insulin and glucagon work together to maintain blood glucose homeostasis.

6. Explain how alveoli are adapted for efficient gas exchange, referring to Fick’s Law.

7. Describe the role of the pancreas as both an endocrine and exocrine gland.

8. Explain the importance of the refractory period in the transmission of action potentials.

9. Explain why the myelin sheath increases the speed of nerve impulse transmission.

10. Describe how the structure of a red blood cell is adapted for oxygen transport.

11. Explain the role of ADH in water balance regulation, including the mechanism of action in the kidney.

12. Describe the path of a molecule of oxygen from the alveolus to a muscle cell, naming all the structures it passes through.

13. Explain why adrenaline increases heart rate and breathing rate during the “fight or flight” response.

14. Compare the structure and function of arteries, veins, and capillaries.

15. Explain how the small intestine is adapted for absorption, including the roles of villi, microvilli, and the lacteal.

16. Describe the role of the liver in the digestion and absorption of lipids.

17. Explain why a person with type 1 diabetes must monitor their blood glucose levels carefully and inject insulin at appropriate times.

18. Describe the pathway of blood through the human heart, naming all four chambers, the four valves, and the major blood vessels connected to each chamber.

19. Explain the difference between an excitatory and an inhibitory neurotransmitter, and describe the effect of each on the postsynaptic membrane.

20. Explain the concept of negative feedback, using blood glucose regulation as an example. Include the roles of the pancreas, liver, and the hormones insulin and glucagon.

21. Describe the role of the loop of Henle in producing concentrated urine, explaining the countercurrent multiplier mechanism.

22. Explain how the structure of an artery is adapted for carrying blood at high pressure.

23. Describe the process of lipid digestion and absorption, explaining the role of bile.

24. Explain the difference between spatial summation and temporal summation at synapses.

25. Describe how a vaccination provides protection against a specific disease, including the roles of B cells, T cells, antibodies, and memory cells.

Review: The Immune System in Detail

The immune system has two main components: the innate immune system (non-specific, immediate) and The adaptive immune system (specific, slower but long-lasting).

Innate immune system:

• Physical barriers: Skin, mucous membranes, stomach acid, cilia in the respiratory tract.
• Phagocytes: Neutrophils (short-lived, abundant, first responders) and macrophages (long-lived, tissue-resident, also present antigens to T cells). They engulf and digest pathogens by phagocytosis.
• Complement system: A group of proteins that enhance phagocytosis (opsonisation), attract phagocytes (chemotaxis), and directly lyse pathogens by forming membrane attack complexes.
• Inflammation: Redness, heat, swelling, and pain caused by increased blood flow, capillary permeability, and migration of phagocytes to the site of infection. Histamine released by mast cells increases capillary permeability.

Adaptive immune system:

• Cell-mediated immunity: T cells (cytotoxic T cells kill virus-infected cells; helper T cells coordinate the immune response by releasing cytokines; memory T cells provide long-term immunity).
• Humoral immunity: B cells produce antibodies (plasma cells are antibody-secreting B cells; memory B cells provide long-term immunity).
• Antibodies: Y-shaped proteins that bind to specific antigens on pathogens. They can neutralise toxins, agglutinate (clump) pathogens, opsonise pathogens for phagocytosis, and activate the complement system.

Worked Example: The immune response to a bacterial infection.

1. Innate response: Bacteria enter through a cut. Skin is breached, but mast cells detect the bacteria and release histamine, causing vasodilation and increased capillary permeability (inflammation). Phagocytes (neutrophils, then macrophages) migrate to the site and engulf the bacteria.
2. Antigen presentation: Macrophages digest the bacteria and display bacterial antigens on their surface (MHC class II molecules).
3. Adaptive response: Helper T cells bind to the antigen-MHC complex on macrophages and become activated. They release cytokines that activate B cells and cytotoxic T cells.
4. B cell activation: B cells produce antibodies specific to the bacterial antigens. The antibodies neutralise toxins, agglutinate bacteria, and opsonise them for phagocytosis.
5. Memory formation: Some B cells and T cells differentiate into memory cells, providing long-term immunity. If the same bacteria enter the body again, the secondary immune response is faster, stronger, and more specific.

Primary vs secondary immune response:

Feature	Primary Response	Secondary Response
Speed	Slower (5—10 days to peak)	Faster (2—3 days to peak)
Antibody level	Lower	Much higher
Antibody class	Mainly IgM initially	Mainly IgG
Memory cells	Formed during response	Already present
Duration	Short-lived	Longer-lasting

Review: Gas Exchange in the Lungs

Structure of the alveoli:

• Extremely thin walls (one cell thick) — short diffusion distance.
• Large total surface area (approximately 70 $m^2$ in adult lungs).
• Surrounded by dense capillary network — maintains concentration gradient.
• Surfactant (a mixture of phospholipids and proteins) lines the alveoli, reducing surface tension and preventing collapse.

Mechanism of ventilation:

• Inspiration (breathing in): The intercostal muscles and diaphragm contract. The rib cage moves upwards and outwards, and the diaphragm flattens. The volume of the thorax increases, decreasing the pressure inside the lungs below atmospheric pressure. Air flows in.
• Expiration (breathing out): The intercostal muscles and diaphragm relax. The rib cage moves downwards and inwards, and the diaphragm resumes its domed shape. The volume of the thorax decreases, increasing the pressure inside the lungs above atmospheric pressure. Air flows out.

Worked Example: Calculating breathing rate and minute ventilation.

A person breathes 15 times per minute. Each breath has a tidal volume of 500 mL.

Minute ventilation $= 15 \times 500 = 7500$ mL/min $= 7.5$ L/min.

The alveolar ventilation is lower because some air (approximately 150 mL) remains in the dead space (airways where no gas exchange occurs):

Alveolar ventilation $= 15 \times (500 - 150) = 15 \times 350 = 5250$ mL/min $= 5.25$ L/min.

This shows that a significant proportion of each breath does not reach the alveoli and therefore Does not contribute to gas exchange.

Review: Osmoregulation and the Kidney

The kidney plays a central role in maintaining water and ion balance in the body.

Ultrafiltration: In the glomerulus, high blood pressure forces water, glucose, amino acids, Urea, and ions out of the blood and into the Bowman’s capsule. Large proteins and blood cells are Too large to pass through the basement membrane and remain in the blood.

Selective reabsorption: As the filtrate passes through the proximal convoluted tubule, all Glucose, all amino acids, and most water and ions are reabsorbed into the blood by active transport And diffusion.

The loop of Henle: Creates a concentration gradient in the medulla of the kidney. The descending Limb is permeable to water but not ions; water leaves by osmosis into the increasingly concentrated Medulla. The ascending limb is permeable to ions but not water; \mathrm{Na^+ and \mathrm{Cl^- are Actively pumped out, making the medulla increasingly concentrated. This countercurrent multiplier Mechanism allows the kidney to produce concentrated urine.

ADH (antidiuretic hormone): Released by the posterior pituitary gland in response to increased Blood osmolarity (detected by osmoreceptors in the hypothalamus). ADH makes the collecting duct more Permeable to water by inserting aquaporin channels into the membrane. More water is reabsorbed, and More concentrated urine is produced. When water intake is high, ADH secretion is reduced, less water Is reabsorbed, and dilute urine is produced.

Worked Example: Why glucose is not normally found in urine.

In a healthy person, all glucose is reabsorbed in the proximal convoluted tubule by active Transport. The transporters have a maximum capacity (transport maximum, Tm) of approximately 375 Mg/min. If blood glucose levels are normal (approximately 5 mmol/L), the filtered load of glucose is Well below Tm, and all glucose is reabsorbed. In a person with uncontrolled diabetes, blood glucose Levels can exceed 10 mmol/L, and the filtered load exceeds Tm. The excess glucose cannot be Reabsorbed and appears in the urine (glycosuria).

Review: Summary Table of Hormones

Hormone	Source	Target	Function
Insulin	Pancreas (beta)	Liver, cells	Lowers blood glucose; promotes glucose uptake
Glucagon	Pancreas (alpha)	Liver	Raises blood glucose; stimulates glycogenolysis
ADH	Pituitary	Kidney	Increases water reabsorption
Adrenaline	Adrenal medulla	Heart, lungs	Increases heart rate, breathing rate
Thyroxine	Thyroid	All cells	Increases metabolic rate
FSH	Pituitary	Ovaries	Stimulates follicle development
LH	Pituitary	Ovaries	Triggers ovulation

Review: The Reproductive System

Male Reproductive System

The male reproductive system produces and delivers sperm.

Key structures:

• Testes: Produce sperm (in the seminiferous tubules) and testosterone (by Leydig cells). Sperm production occurs at a temperature slightly below body temperature, which is why the testes are located outside the body in the scrotum.
• Epididymis: Sperm mature and are stored here.
• Vas deferens: Transports sperm from the epididymis to the urethra during ejaculation.
• Seminal vesicles: Produce seminal fluid containing fructose (energy source for sperm) and prostaglandins (stimulate uterine contractions).
• Prostate gland: Produces alkaline fluid that neutralises vaginal acidity, protecting sperm.
• Penis: Delivers sperm to the female reproductive tract.

Spermatogenesis: The process of sperm production in the seminiferous tubules. Spermatogonia (diploid) undergo mitosis to produce more spermatogonia and primary spermatocytes. Primary Spermatocytes undergo meiosis I to produce secondary spermatocytes, which undergo meiosis II to Produce spermatids. Spermatids mature into spermatozoa (sperm) through a process called Spermiogenesis.

Female Reproductive System

The female reproductive system produces eggs, provides a site for fertilisation, and supports fetal Development.

Key structures:

• Ovaries: Produce eggs (oocytes) and the hormones oestrogen and progesterone. Females are born with all the eggs they will ever have (approximately 1-2 million at birth, declining to approximately 400,000 at puberty).
• Fallopian tubes (oviducts): Site of fertilisation. Cilia waft the egg towards the uterus.
• Uterus: Site of implantation and fetal development. The endometrium (uterine lining) thickens in response to oestrogen and progesterone.
• Cervix: The opening of the uterus into the vagina. Produces mucus that changes in consistency during the menstrual cycle.
• Vagina: Receives the penis during intercourse; birth canal.

The menstrual cycle:

1. Follicular phase (days 1-14): FSH stimulates follicle development in the ovary. The developing follicle secretes oestrogen, which stimulates the endometrium to thicken. A surge in LH triggers ovulation (release of the egg from the ovary) around day 14.
2. Luteal phase (days 14-28): The remaining follicle becomes the corpus luteum, which secretes progesterone. Progesterone maintains the thickened endometrium. If fertilisation does not occur, the corpus luteum degenerates, progesterone levels drop, and the endometrium is shed (menstruation).

Fertilisation and Implantation

Fertilisation occurs in the fallopian tube when a sperm penetrates the egg. The sperm releases Enzymes from its acrosome to digest the outer layers of the egg. When one sperm enters, the egg Undergoes a cortical reaction that prevents polyspermy (entry of additional sperm). The nuclei of The sperm and egg fuse, forming a diploid zygote.

The zygote divides by mitosis as it travels along the fallopian tube, forming a ball of cells called A blastocyst. The blastocyst implants in the endometrium approximately 6-7 days after fertilisation.

Worked Example: Hormonal control of the menstrual cycle.

• FSH stimulates follicle development and oestrogen production.
• Oestrogen stimulates endometrial thickening and inhibits FSH (negative feedback). At high levels near ovulation, oestrogen stimulates LH release (positive feedback).
• LH triggers ovulation and maintains the corpus luteum.
• Progesterone maintains the endometrium and inhibits FSH and LH (negative feedback).
• If fertilisation occurs, the embryo produces hCG (human chorionic gonadotropin), which maintains the corpus luteum and therefore progesterone production, preventing menstruation.

Review: The Structure and Function of Blood

Plasma

The liquid component of blood, making up approximately 55% of blood volume. It contains:

• Water: Solvent for dissolved substances.
• Dissolved substances: Glucose, amino acids, urea, hormones, ions.
• Plasma proteins: Albumin (maintains osmotic pressure), globulins (antibodies and transport), fibrinogen (blood clotting).

Red Blood Cells (Erythrocytes)

• Biconcave disc shape increases surface area to volume ratio for gas exchange.
• No nucleus, allowing more space for haemoglobin.
• Haemoglobin binds reversibly to oxygen: \mathrm{Hb + 4\mathrm{O_2 \rightleftharpoons \mathrm{HbO_8.
• In tissues, oxygen dissociates from haemoglobin due to lower oxygen partial pressure and lower pH (Bohr effect).
• Red blood cells have a lifespan of approximately 120 days and are destroyed in the spleen and liver.

White Blood Cells (Leucocytes)

• Neutrophils: The most abundant phagocyte. Engulf and digest bacteria. Short-lived (hours to days).
• Lymphocytes: Produce antibodies (B lymphocytes) and coordinate the immune response (T lymphocytes). Long-lived; some become memory cells.
• Monocytes: Become macrophages when they migrate into tissues. Long-lived phagocytes that also present antigens to T cells.

Platelets (Thrombocytes)

Small cell fragments involved in blood clotting. When a blood vessel is damaged, platelets Accumulate at the site and release clotting factors. This triggers the clotting cascade:

1. Thromboplastin is released from damaged tissue and platelets.
2. Thromboplastin converts prothrombin to thrombin (with calcium ions).
3. Thrombin converts fibrinogen to fibrin.
4. Fibrin forms a mesh that traps red blood cells, forming a clot.

Worked Example: The importance of calcium ions in blood clotting.

Calcium ions (\mathrm{Ca^{2+}) are essential cofactors at multiple steps in the clotting cascade. Without calcium ions, thromboplastin cannot convert prothrombin to thrombin, and thrombin cannot Convert fibrinogen to fibrin. This is why calcium chelators (substances that bind and remove calcium Ions) such as EDTA are used as anticoagulants in blood collection tubes. Similarly, the Anticoagulant drug heparin works by enhancing the activity of antithrombin, which inactivates Thrombin, preventing fibrin formation.

Review: Lymphatic System

The lymphatic system is a network of vessels, nodes, and organs that complements the circulatory System.

Functions:

1. Fluid balance: Lymphatic vessels collect excess interstitial fluid (tissue fluid) and return it to the bloodstream. Approximately 3 litres of fluid leak out of capillaries each day and must be returned via the lymphatic system.
2. Fat absorption: Lacteals (lymphatic capillaries in the villi of the small intestine) absorb fatty acids and glycerol, which are transported as chylomicrons in the lymph before entering the bloodstream via the thoracic duct.
3. Immune defence: Lymph nodes filter lymph, trapping pathogens and foreign particles. Lymphocytes in the lymph nodes multiply and mount an immune response. Swollen lymph glands indicate an active immune response.

Lymph circulation: Lymph moves through lymphatic vessels by contraction of surrounding skeletal Muscles and by breathing. Valves in the lymphatic vessels prevent backflow. Lymph is returned to the Bloodstream via the thoracic duct (drains the left side of the body and both legs) and the right Lymphatic duct (drains the right side of the body) into the subclavian veins.

Worked Example: Oedema and the lymphatic system.

Oedema (swelling) occurs when excess fluid accumulates in the tissues. This can happen if the Lymphatic system is blocked (e.g., by a parasitic infection such as elephantiasis, which is caused By filarial worms blocking lymphatic vessels). If lymphatic drainage is impaired, the excess Interstitial fluid cannot be returned to the bloodstream, and the tissue swells. This demonstrates The critical role of the lymphatic system in maintaining fluid balance.

Review: Types of Muscle Tissue

The human body contains three types of muscle tissue:

Skeletal muscle:

• Attached to bones; responsible for voluntary movement.
• Striated (striped appearance) due to the regular arrangement of actin and myosin filaments.
• Multinucleated (many nuclei per cell fibre).
• Contracts rapidly and powerfully but fatigues quickly.
• Controlled by the somatic nervous system.

Smooth muscle:

• Found in the walls of hollow organs (intestines, blood vessels, bladder, uterus).
• Non-striated (smooth appearance).
• Uninucleated (one nucleus per cell).
• Contracts slowly and rhythmically; does not fatigue .
• Controlled by the autonomic nervous system.

Cardiac muscle:

• Found only in the heart.
• Striated (like skeletal muscle).
• Uninucleated (one nucleus per cell, sometimes two).
• Cells are connected by intercalated discs (containing gap junctions for rapid electrical coupling).
• Contracts rhythmically and continuously; does not fatigue.
• Myogenic (generates its own electrical impulses via the SA node).

Worked Example: The sliding filament mechanism of muscle contraction.

When a muscle contracts, the actin (thin) and myosin (thick) filaments slide past each other, Shortening the sarcomere. The process:

1. An action potential arrives at the neuromuscular junction, releasing acetylcholine.
2. The action potential spreads along the muscle fibre membrane and into T-tubules.
3. Calcium ions are released from the sarcoplasmic reticulum.
4. Calcium binds to troponin, which moves tropomyosin away from the myosin-binding sites on actin.
5. Myosin heads bind to actin, forming cross-bridges.
6. The myosin heads undergo a power stroke, pulling the actin filaments towards the centre of the sarcomere.
7. ATP binds to the myosin heads, causing them to detach from actin.
8. ATP is hydrolysed, re-cocking the myosin heads for another cycle.
9. The cycle repeats as long as calcium and ATP are present.

When the action potential ends, calcium is actively pumped back into the sarcoplasmic reticulum. Troponin returns to its original shape, tropomyosin covers the binding sites, and the muscle Relaxes.

Energy sources for muscle contraction:

Muscles use three sources of ATP during contraction:

1. ATP from creatine phosphate: Creatine phosphate transfers a phosphate group to ADP, regenerating ATP very rapidly. This system provides ATP for approximately 5—10 seconds of intense activity.
2. Anaerobic glycolysis: Glucose is broken down to pyruvate, then to lactate, yielding 2 ATP per glucose. This provides ATP for approximately 30—60 seconds of vigorous activity.
3. Aerobic respiration: Pyruvate enters the mitochondria and is fully oxidised via the Krebs cycle and electron transport chain, yielding approximately 30-32 ATP per glucose. This provides ATP for prolonged, moderate activity.

Muscle fibre types:

Feature	Type I (slow-twitch)	Type IIa (fast-twitch oxidative)	Type IIb (fast-twitch glycolytic)
Contraction speed	Slow	Fast	Very fast
Fatigue resistance	High	Moderate	Low
Colour	Red (rich in myoglobin)	Pink	White
Mitochondria	Many	Moderate	Few
Energy source	Aerobic	Aerobic and anaerobic	Anaerobic (glycolysis)
Primary fuel	Fatty acids, glucose	Glucose, glycogen	Glycogen, phosphocreatine
Example activity	Marathon running, posture	Sprinting, swimming	Weightlifting, jumping

Worked Examples

Example 1: Comparing mitosis and meiosis

Compare mitosis and meiosis in terms of the number of divisions, daughter cells produced, and genetic variation.

Solution:

Feature	Mitosis	Meiosis
Divisions	1	2
Daughter cells	2	4
Chromosome number	Same as parent	Half of parent
Genetic variation	No	Yes (crossing over, independent assortment)
Function	Growth, repair, asexual reproduction	Production of gametes