Transport in Animals
The mammalian double circulation and its advantages; heart structure, function, and coronary heart disease; the structure–function relationship of arteries, veins, and capillaries; and the four components of blood with their roles — including Extended detail on heart wall thickness, valve action, hepatic portal vein, lymphocytes vs phagocytes, and blood clotting.
Circulatory Systems
CORE EXTENDEDThe circulatory system is a system of blood vessels with a pump (the heart) and valves to ensure one-way flow of blood.
Single vs Double Circulation — Extended
| Feature | Single circulation (fish) | Double circulation (mammals) |
|---|---|---|
| Number of circuits | One — heart → gills → body → heart | Two — pulmonary (heart ↔ lungs) + systemic (heart ↔ body) |
| Number of heart chambers | 2 (one atrium + one ventricle) | 4 (two atria + two ventricles) |
| Blood pressure reaching body | Low — pressure drops after passing through gill capillaries | High — pressure boosted by left ventricle before reaching body |
| Blood oxygenation route | Deoxygenated blood pumped from heart to gills; oxygenated blood flows directly to the body before returning deoxygenated to the heart — no separate re-pressurisation circuit | Oxygenated and deoxygenated blood completely separated by septum; blood re-pressurised by left ventricle after returning from lungs |
1. Higher blood pressure to the body: After blood is oxygenated in the lungs, it returns to the left side of the heart and is pumped again at high pressure to the rest of the body. Fish blood pressure drops after passing through the gill capillaries and reaches the body tissues at low pressure — mammals deliver blood more forcefully.
2. Complete separation of oxygenated and deoxygenated blood: The septum prevents mixing. Tissues always receive fully oxygenated blood, maximising oxygen delivery for aerobic respiration. This supports the high metabolic rate of warm-blooded mammals.
The Heart
CORE EXTENDEDHeart Structures — Core
| Structure | Description and function |
|---|---|
| Left and right atria | Upper chambers; receive blood returning to the heart (from veins); thin walls — only pump blood a short distance into the ventricles below |
| Left and right ventricles | Lower chambers; pump blood out of the heart; thick muscular walls to generate high pressure |
| Septum | Muscular wall separating the left and right sides of the heart; prevents mixing of oxygenated (left) and deoxygenated (right) blood |
| Muscular wall | Cardiac muscle adapted for continuous rhythmic contraction; highly resistant to fatigue; generates the pumping force |
| One-way valves | Prevent backflow of blood; ensure blood flows in one direction only |
| Coronary arteries | Supply oxygen and nutrients to the heart muscle itself; branch off the aorta just above the heart |
Right side (deoxygenated blood): vena cava → right atrium → right ventricle → pulmonary artery → lungs
Left side (oxygenated blood): pulmonary vein → left atrium → left ventricle → aorta → body
Memory: blood always enters atria, exits through ventricles. Right side = pulmonary circuit (to lungs). Left side = systemic circuit (to body).
Valve Types — Extended
| Valve | Location | Opens when… | Closes when… |
|---|---|---|---|
| Atrioventricular (AV) valves | Between each atrium and ventricle | Ventricle relaxes (atrium pressure > ventricle) | Ventricle contracts (prevents backflow into atria) |
| Semilunar (SL) valves | At the base of aorta and pulmonary artery | Ventricle contracts (ventricle pressure > artery) | Ventricle relaxes (prevents backflow from arteries) |
Wall Thickness Explained — Extended
| Comparison | Which is thicker? | Reason |
|---|---|---|
| Left vs right ventricle | Left ventricle is thicker | Left ventricle pumps blood through the entire systemic circulation (all body organs) — a much longer route at higher pressure. Right ventricle only pumps to the nearby lungs — shorter distance, lower pressure needed. |
| Atria vs ventricles | Ventricles are thicker | Atria only push blood into the ventricles directly below — a short, low-resistance path. Ventricles must pump blood out of the heart through arteries against greater resistance — much higher force needed. |
How the Heart Beats — Extended
The heart beat follows a coordinated sequence:
- Both atria contract simultaneously → blood pushed into ventricles; AV valves open
- Both ventricles contract simultaneously → blood pushed into arteries; SL valves open; AV valves close (prevents backflow)
- Both ventricles relax → SL valves close (blood in arteries cannot flow back); atria refill from veins
- Cycle repeats: ~70 times per minute at rest
Monitoring Heart Activity — Core
Records the electrical activity of the heart as a trace. Normal pattern shows regular peaks; abnormal patterns indicate irregular heartbeat or damage.
Each heartbeat produces a pressure wave felt at pulse points (wrist, neck). Pulse rate = heart rate. Measured as beats per minute.
A stethoscope detects the “lub-dub” sounds of the AV and SL valves closing. Abnormal sounds may indicate faulty valves.
Coronary Heart Disease (CHD)
CHD occurs when coronary arteries become blocked by fatty deposits (atherosclerosis), reducing blood flow to the heart muscle. If a coronary artery is completely blocked, the heart muscle it supplies dies — a heart attack (myocardial infarction).
| Risk factor | How it contributes |
|---|---|
| Diet | High saturated fat → increased LDL cholesterol → deposits in artery walls |
| Lack of exercise | Weak heart muscle; high blood pressure; obesity risk; poor cholesterol profile |
| Stress | Raises blood pressure; may lead to unhealthy behaviours (smoking, poor diet) |
| Smoking | Nicotine raises heart rate and blood pressure; carbon monoxide reduces O₂ in blood; chemicals damage artery walls |
| Genetic predisposition | Family history increases risk regardless of lifestyle factors |
| Age | Arteries stiffen and narrow with age; deposits accumulate over time |
| Sex | Males at higher risk, especially below age 50; female hormones (oestrogen) offer some protection pre-menopause |
Diet: Reduce saturated fat (lowers LDL cholesterol); increase fruit, vegetables, fibre; maintain healthy body weight; reduce salt (lowers blood pressure).
Exercise: Strengthens heart muscle; lowers resting heart rate; reduces blood pressure; raises HDL (“good”) cholesterol; reduces obesity risk. Regular aerobic exercise (e.g. 150 min/week moderate intensity) significantly reduces CHD risk.
Why is the left ventricle wall thicker than the right ventricle wall?
- A. The left ventricle receives more blood than the right
- B. The left ventricle pumps blood to the lungs, which are closer
- C. The left ventricle must generate higher pressure to pump blood around the whole body
- D. The right ventricle is thinner because it pumps oxygenated blood
Blood Vessels
CORE EXTENDEDArtery, Vein, Capillary — Comparison
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall thickness | Thick, muscular, elastic | Thinner than artery | One cell thick (endothelium only) |
| Lumen diameter | Narrow relative to wall | Wide relative to wall | Very narrow — barely fits one RBC |
| Valves? | None (except at heart) | ✓ Present throughout — prevent backflow | None |
| Blood pressure | High, pulsatile | Low, steady | Very low |
| Direction of flow | Away from heart | Toward heart | Links arterioles to venules |
Capillaries are the site of exchange between blood and tissues. Their thin walls (one cell thick) allow oxygen, glucose, and other nutrients to diffuse out into tissue fluid, and CO₂ and waste products to diffuse in from cells. Their narrow lumen slows blood flow, maximising exchange time.
Structure–Function Links — Extended
| Vessel | Structural feature | Functional explanation |
|---|---|---|
| Artery | Thick muscular and elastic wall | Muscle resists high-pressure blood flow without bursting; elastic fibres stretch as blood surges with each heartbeat (systole) and recoil to maintain flow during relaxation (diastole) — smoothing out the pulse |
| Vein | Thin wall; wide lumen; valves | Blood in veins is at low pressure — does not need thick walls. Wide lumen reduces resistance to flow. Valves (supported by skeletal muscle contractions) prevent backflow of slow-moving blood returning to the heart |
| Capillary | Wall one cell thick; very small lumen | One cell thick = very short diffusion distance for rapid gas and nutrient exchange. Small lumen slows blood, allowing time for exchange. Huge number of capillaries creates enormous total surface area |
Named Blood Vessels — Core
| Vessel | Connects | Carries |
|---|---|---|
| Vena cava | Body → right atrium | Deoxygenated blood from body |
| Aorta | Left ventricle → body | Oxygenated blood to body |
| Pulmonary artery | Right ventricle → lungs | Deoxygenated blood to lungs |
| Pulmonary vein | Lungs → left atrium | Oxygenated blood from lungs |
| Renal artery | Aorta → kidney | Oxygenated blood to kidney |
| Renal vein | Kidney → vena cava | Deoxygenated blood from kidney |
| Hepatic artery | Aorta → liver | Oxygenated blood to liver |
| Hepatic vein | Liver → vena cava | Deoxygenated blood from liver |
| Hepatic portal vein | Small intestine → liver | Nutrient-rich blood (absorbed from gut) — neither fully oxygenated nor deoxygenated |
The hepatic portal vein is unique: it carries blood rich in absorbed nutrients (glucose, amino acids) directly from the small intestine to the liver before they reach the general circulation. The liver processes these nutrients (e.g. converts excess glucose to glycogen, deaminates excess amino acids). This vessel is very frequently tested in Extended papers.
Which blood vessel carries oxygenated blood away from the heart to the body?
- A. Vena cava
- B. Pulmonary artery
- C. Pulmonary vein
- D. Aorta
Blood
CORE EXTENDEDComponents and Functions
| Component | Description | Function(s) |
|---|---|---|
| Red blood cells (erythrocytes) |
Biconcave disc; no nucleus; packed with haemoglobin; very flexible | Transport oxygen: haemoglobin binds O₂ in lungs (forms oxyhaemoglobin) and releases it in body tissues. No nucleus → more space for haemoglobin. Biconcave shape → larger surface area for O₂ diffusion. |
| White blood cells (leucocytes) |
Larger than RBCs; have a nucleus; fewer in number; various types | Defence against pathogens: phagocytosis and antibody production (see below) |
| Platelets (thrombocytes) |
Tiny cell fragments; no nucleus | Blood clotting — aggregate at wound sites and initiate the clotting cascade (details not required at Core) |
| Plasma | Pale yellow liquid (~55% of blood volume); mainly water | Transports: blood cells, ions, nutrients (glucose, amino acids, fatty acids), urea, hormones, CO₂, and plasma proteins (including antibodies and fibrinogen) |
When a blood vessel is damaged, clotting: (1) prevents blood loss by sealing the wound with a clot, and (2) prevents entry of pathogens through the wound. Together these protect the body from blood loss and infection simultaneously.
White Blood Cells: Phagocytes vs Lymphocytes — Extended
| Cell type | Appearance | Function | How identified in photomicrographs |
|---|---|---|---|
| Phagocytes | Irregular lobed nucleus; large cell | Engulf and digest pathogens by phagocytosis — the cell membrane surrounds and absorbs the pathogen into a vacuole, where it is digested by enzymes | Multi-lobed (irregular) nucleus; granular cytoplasm |
| Lymphocytes | Large, round nucleus almost filling the cell; small amount of cytoplasm | Produce antibodies specific to antigens on pathogens; antibodies bind to pathogens and mark them for destruction or directly neutralise them | Large spherical nucleus; very little cytoplasm visible |
Blood Clotting Mechanism — Extended
When a blood vessel is damaged, platelets aggregate at the wound. A cascade of reactions is triggered, converting the soluble plasma protein fibrinogen into insoluble fibrin threads. These threads form a mesh that traps blood cells and forms a clot (scab), sealing the wound.
Summary: Platelets → clotting cascade → fibrinogen → fibrin mesh → clot
Which component of blood is responsible for transporting urea from the liver to the kidneys?
- A. Red blood cells
- B. White blood cells
- C. Platelets
- D. Plasma
A student observes two types of white blood cell in a photomicrograph: Cell A has a large, round nucleus nearly filling the cell. Cell B has a multi-lobed irregular nucleus.
(a) Identify cells A and B. [2 marks]
(b) Describe how Cell B destroys pathogens. [3 marks]
(c) Describe the process by which Cell A produces a defence response to a specific pathogen. [2 marks]
- (a) Cell A: lymphocyte [1 mark]; Cell B: phagocyte [1 mark]
- (b) Cell B (phagocyte): engulfs the pathogen by phagocytosis [1 mark]; the pathogen is enclosed in a vacuole [1 mark]; digestive enzymes are released into the vacuole and break down the pathogen [1 mark]
- (c) Cell A (lymphocyte): recognises a specific antigen on the pathogen’s surface [1 mark]; produces specific antibodies that bind to those antigens, leading to destruction or marking of the pathogen [1 mark]
Comprehensive Practice Questions
Mixed questions across all of Topic 9.
Which statement correctly compares the structure of arteries and veins?
- A. Veins have thicker walls and a narrower lumen than arteries
- B. Arteries have valves throughout their length; veins do not
- C. Arteries have thicker walls than veins; veins have valves but arteries do not
- D. Arteries and veins have the same wall thickness but different lumen diameters
(a) Name the blood vessel that carries deoxygenated blood from the heart to the lungs. [1 mark]
(b) Describe what is meant by coronary heart disease (CHD). [2 marks]
(c) State two lifestyle risk factors for CHD and, for each, explain how it contributes to the disease. [4 marks]
- (a) Pulmonary artery [1 mark]
- (b) CHD is the blockage of coronary arteries [1 mark] by fatty deposits / atherosclerosis, reducing blood (oxygen) supply to the heart muscle [1 mark]
- (c) Any two lifestyle factors, each with explanation — e.g.:
Smoking: nicotine raises blood pressure / carbon monoxide reduces O₂ in blood / chemicals damage artery walls → increases atherosclerosis risk [2 marks]
Diet high in saturated fat: raises LDL (bad) cholesterol in blood → cholesterol deposits in artery walls → arteries narrow → blockage risk [2 marks]
Lack of exercise: weakens heart; leads to obesity and high blood pressure; poor cholesterol balance [2 marks]
(a) Explain two advantages of the mammalian double circulation over the single circulation of a fish. [4 marks]
(b) After a meal, blood in the hepatic portal vein has a higher concentration of glucose than blood in the hepatic artery. Explain why, and state what happens to this glucose in the liver. [3 marks]
- Advantage 1: Blood is re-pressurised after passing through the lungs [1 mark]; the left ventricle generates high pressure so blood reaches body tissues efficiently with enough force for rapid exchange [1 mark]
- Advantage 2: Oxygenated and deoxygenated blood are completely separated by the septum [1 mark]; body tissues always receive fully oxygenated blood, maximising oxygen delivery and supporting the high metabolic rate of mammals [1 mark]
- The hepatic portal vein carries blood directly from the small intestine to the liver [1 mark]
- After a meal, glucose absorbed from digested food in the small intestine enters this blood, raising its glucose concentration [1 mark]
- In the liver, excess glucose is converted to glycogen for storage (glycogenesis), preventing dangerously high blood glucose levels [1 mark]
High-Frequency Mistakes — Topic 9 Overall
- ❌"Arteries always carry oxygenated blood; veins always carry deoxygenated blood"The pulmonary artery carries deoxygenated blood (right ventricle → lungs). The pulmonary vein carries oxygenated blood (lungs → left atrium). The rule is: arteries carry blood away from the heart; veins carry blood toward the heart — oxygen content varies.
- 💚Confusing atria and ventriclesAtria (singular: atrium) receive blood from veins and push it into ventricles — thin walls. Ventricles pump blood out to arteries under high pressure — thick walls. "A" for Atria = Arrivals (blood arrives); "V" for Ventricles = Very powerful pumps.
- 📈Saying capillaries have no cell wall / no cellsCapillaries are made of a single layer of endothelial cells (one cell thick). They do have a wall — it is just extremely thin, which is what allows rapid diffusion. Saying "no wall" is incorrect.
- 💧Saying red blood cells "carry" oxygen in plasmaOxygen is carried bound to haemoglobin inside red blood cells, not dissolved in plasma. CO₂ is mainly transported dissolved in plasma (and as hydrogen carbonate ions). The distinction is frequently tested.
- ⛺Ext: Forgetting the hepatic portal veinThe liver receives blood from two sources: the hepatic artery (oxygenated blood for the liver’s own metabolism) and the hepatic portal vein (nutrient-rich blood from the small intestine). Many students only remember the hepatic artery and miss the hepatic portal vein entirely.
- 🦠Ext: Confusing phagocytes and lymphocytes in photomicrographsLymphocyte: large spherical nucleus nearly filling the cell, very little cytoplasm. Phagocyte: irregular multi-lobed nucleus, more cytoplasm visible. In photomicrographs, look at the nucleus shape — round and central = lymphocyte; lobed and irregular = phagocyte.
- 💉Ext: Saying fibrin forms the clot directly (without fibrinogen)Blood clotting requires the conversion of the soluble protein fibrinogen (already in plasma) to insoluble fibrin threads. You must name both: fibrinogen → fibrin → mesh → clot. Just saying "fibrin forms" without mentioning fibrinogen is incomplete.
- 🔥Saying the septum stops blood flowing — rather than mixingThe septum separates oxygenated (left) from deoxygenated (right) blood. It does not stop blood from flowing through the heart — it prevents the two streams from mixing. Be precise: "prevents mixing of oxygenated and deoxygenated blood".
Topic 9 is one of the most content-heavy topics and appears across all papers. Highest-yield items: named blood vessels (especially the oxygenation trick — pulmonary artery carries deoxygenated blood); artery/vein/capillary structure comparison table; why left ventricle is thicker; CHD risk factors with mechanism (not just a list); and blood components with their specific functions. For Extended: double circulation advantages, hepatic portal vein, lymphocyte vs phagocyte identification and function, and the fibrinogen → fibrin clotting mechanism are near-certain Paper 4 targets.