Biology • Year 11 • Module 2 • Lesson 14
The Cardiovascular System: Structure and Function
Apply structure-function reasoning to vessel identification, blood composition data, and the double circulation advantage over single circulation.
1. Identify the blood vessel from its description
A biologist examines three cross-sections of blood vessels under a microscope and records the observations below. For each vessel, identify the type (artery, capillary, or vein), then explain how each structural feature mentioned suits its function. 9 marks (3 each)
Vessel X: Thick wall with a prominent smooth muscle layer and abundant elastic fibres. Lumen appears narrow relative to the total vessel diameter. No valves visible.
Vessel type:
Explanation:
Vessel Y: Wall consists of a single layer of flattened cells. Lumen is barely wide enough for one red blood cell to pass through. No muscle layer present.
Vessel type:
Explanation:
Vessel Z: Thin wall with little smooth muscle. Wide lumen relative to wall thickness. Pocket-shaped flaps of tissue project into the lumen at regular intervals.
Vessel type:
Explanation:
2. Interpret blood composition data (IQ3)
Blood samples were taken from the vessels entering and leaving four organs. Analyse the data and answer the questions. 12 marks
| Organ | Sample | O₂ (mL/100 mL) | CO₂ (mL/100 mL) | Glucose (mmol/L) | Urea (mmol/L) |
|---|---|---|---|---|---|
| Liver | Entering (hepatic artery) | 19 | 48 | 4.5 | 4.0 |
| Leaving (hepatic vein) | 14 | 53 | 4.3 | 6.1 | |
| Kidneys | Entering (renal artery) | 19 | 48 | 4.5 | 6.0 |
| Leaving (renal vein) | 15 | 51 | 4.5 | 1.8 | |
| Lungs | Entering (pulmonary artery) | 14 | 53 | 4.3 | 5.2 |
| Leaving (pulmonary vein) | 19 | 48 | 4.3 | 5.2 | |
| Muscle (rest) | Entering (muscle artery) | 19 | 48 | 4.5 | 4.0 |
| Leaving (muscle vein) | 15 | 52 | 4.2 | 4.3 |
2.1 Using the data, identify which organ is the site of urea production. Justify your answer with specific values from the table. 2 marks
2.2 Explain why glucose concentration shows no net change across the kidneys, even though the kidneys filter large volumes of blood. 2 marks
2.3 Describe the pattern of O₂ and CO₂ change across the lungs. Explain the biological process responsible for each change. 4 marks
2.4 Predict how the muscle data would change if the same person were exercising at high intensity instead of resting. Explain the physiological reason for each change you predict. 4 marks
3. Compare single and double circulation
The diagrams below represent simplified single circulation (fish) and double circulation (mammal). Answer the questions using your understanding of these systems. 7 marks
Figure: Simplified single (fish) and double (mammal) circulation. Arrow thickness is not to scale.
3.1 In single circulation (fish), blood loses pressure as it passes through the gill capillaries. What is the consequence of this pressure loss for oxygen delivery to body tissues? 2 marks
3.2 Explain, using the diagram as a reference, how double circulation solves the pressure-loss problem that limits single circulation. 3 marks
3.3 Explain why double circulation is an advantage for endothermic (warm-blooded) animals in particular. 2 marks
Q1.1, Vessel X (3 marks)
Type: Artery.
Thick wall with smooth muscle (1): Arteries carry blood at high pressure (~120 mmHg) directly from the heart. The thick smooth muscle layer and elastic fibres provide structural integrity to prevent rupture and allow elastic recoil during diastole to smooth the pulse into continuous flow. Without this, the artery wall would be damaged by repeated high-pressure surges.
Small lumen (1): A narrow lumen relative to wall thickness helps maintain high blood pressure, ensuring rapid delivery of blood to organs far from the heart.
No valves (1): Arteries carry blood under sustained forward pressure from the heart. This continuous pressure is sufficient to prevent backflow without the need for valves.
Marking criteria: 1 mark per structural feature correctly identified and functionally explained (3 features described in the prompt). Total 3 marks.
Q1.2, Vessel Y (3 marks)
Type: Capillary.
Single-cell wall (1): A wall of only one cell (~0.5–1 μm thick) minimises diffusion distance between blood and tissue cells, maximising the rate of exchange of O₂, CO₂, glucose and metabolic waste by diffusion. Thick walls would impede exchange, not facilitate it.
Barely wide enough for one RBC (1): A lumen of ~5–10 μm forces red blood cells to pass single file, pressing each cell against the thin wall. This maximises the contact surface between haemoglobin and the thin wall, increasing the efficiency of gas exchange.
No muscle layer (1): Blood arrives at capillaries at low pressure after flowing through arterioles. No muscular layer is needed (or energetically justified) because structural strength is unnecessary at low pressure, and a muscle layer would only increase the diffusion distance and impede exchange.
Marking criteria: 1 mark per structural feature correctly identified and functionally explained. Total 3 marks.
Q1.3, Vessel Z (3 marks)
Type: Vein.
Thin wall with little muscle (1): Veins carry blood at low pressure (~5–10 mmHg). Little structural strength is required, so a thinner wall with less smooth muscle is sufficient. A thick muscular wall would be energetically wasteful and provide no functional advantage at low pressure.
Wide lumen (1): A large lumen relative to wall thickness accommodates the slow, low-pressure return flow. Wide bores reduce resistance, allowing blood to drain back to the heart without further significant pressure drive.
Pocket-shaped flaps (valves) (1): These are pocket valves. Veins return blood at low pressure, often against gravity (e.g. from the legs upward to the heart). Valves open when blood is pushed forward by skeletal muscle contractions squeezing the vein walls, then snap shut on reversal to prevent backflow and blood pooling.
Marking criteria: 1 mark per structural feature correctly identified and functionally explained. Total 3 marks.
Q2.1, Urea production organ (2 marks)
The liver is the site of urea production. [1] This is demonstrated by the data: urea concentration rises from 4.0 mmol/L (entering hepatic artery) to 6.1 mmol/L (leaving hepatic vein), a net increase of 2.1 mmol/L across the liver. No other organ in the table shows a net urea increase. [1]
Q2.2, Glucose unchanged across kidneys (2 marks)
The kidneys filter glucose from blood into the renal tubules during the formation of urine [1], but renal tubule cells then completely reabsorb all filtered glucose back into the bloodstream via active transport. Because virtually all filtered glucose is returned, there is no net change in blood glucose concentration leaving the kidneys compared to entering (4.5 mmol/L both sides). [1]
Q2.3, O₂ and CO₂ across lungs (4 marks)
O₂ rises (14 → 19 mL/100 mL): [1] O₂ diffuses from the alveolar air space (where partial pressure is ~100 mmHg) into the blood in the pulmonary capillaries (where partial pressure ~40 mmHg), because diffusion occurs down the concentration gradient. O₂ binds to haemoglobin forming oxyhaemoglobin. [1]
CO₂ falls (53 → 48 mL/100 mL): [1] CO₂ diffuses from the blood (where partial pressure is ~45 mmHg) into the alveolar air space (where partial pressure ~40 mmHg), again down the concentration gradient. CO₂ is then exhaled from the lungs. [1]
Q2.4, Muscle data during intense exercise (4 marks)
O₂ leaving muscle would fall further (below 15 mL/100 mL, possibly to ~10 or lower): [1] During intense exercise, active muscle cells increase their rate of aerobic cellular respiration to produce ATP, consuming much more O₂ per unit time. More O₂ is extracted from the blood, so the blood leaving muscle would contain significantly less O₂ than at rest.
CO₂ leaving muscle would rise further (above 52 mL/100 mL): [1] The increased rate of aerobic respiration produces proportionally more CO₂ as a metabolic waste product. This extra CO₂ diffuses into the blood, raising its concentration in the venous blood draining active muscle.
Glucose leaving muscle would fall further (below 4.2 mmol/L): [1] Glucose is the primary substrate for aerobic respiration in active muscle. Intense exercise greatly increases the glucose demand, so more glucose is extracted from the blood at the capillaries.
Urea would increase slightly more (above 4.3 mmol/L): [1] Protein catabolism may increase slightly during sustained intense exercise as amino acids contribute to energy production, generating a small increase in urea production by active muscle cells.
Marking criteria: 1 mark per correctly predicted change with correct physiological reasoning. Total 4 marks. Accept urea answer as optional; if not mentioned award from remaining three categories.
Q3.1, Consequence of pressure loss at gills (2 marks)
After blood loses pressure in the gill capillaries [1], it arrives at body tissues at significantly reduced pressure. This means blood flows more slowly through the systemic capillaries, limiting the rate at which O₂ can be delivered to actively respiring cells. Fish therefore have a lower ceiling on their aerobic capacity compared to mammals, constraining their maximum sustained activity level. [1]
Q3.2, How double circulation solves the pressure problem (3 marks)
In double circulation, blood that has lost pressure after passing through the lung capillaries returns to the heart (the left atrium and then left ventricle). [1] The left ventricle then pumps this blood at full force, generating approximately 120 mmHg systolic pressure, into the aorta and the systemic circuit. [1] This re-pressurisation step means that body tissues always receive blood at the maximum possible pressure regardless of how much pressure was lost in the pulmonary capillaries. In single circulation there is no second pump to restore pressure between the gas-exchange organ and the body, so body tissues receive blood at only residual pressure. [1]
Q3.3, Advantage for endothermic animals (2 marks)
Endothermic (warm-blooded) animals maintain a constant high body temperature, requiring a continuously high rate of aerobic cellular respiration to generate metabolic heat. [1] This demands a rapid and sustained supply of O₂ and glucose to all body tissues. Double circulation provides the high systemic blood pressure needed to deliver O₂ quickly enough to all capillary beds to support this elevated metabolic rate, a rate that single circulation with its reduced systemic pressure cannot reliably sustain. [1]