Home / A level Biology 8.1 The circulatory system – Exam style question – Paper 2

A level Biology 8.1 The circulatory system – Exam style question – Paper 2

A level Biology Paper 2 - All Chapters
Question

(a) In the mammalian circulatory system, red blood cells travel through different types of blood vessel as they pass from the heart to respiring tissues and back to the heart.

Fig. 2.1 shows the types of blood vessels through which red blood cells travel in the circulatory system.

 

Complete Fig. 2.1 by writing the names of the missing types of blood vessels through which red blood cells travel.

(b) Water is the main component of blood. It has an important role in the transport of substances around the body.

Fig. 2.2 shows the ionic compound sodium chloride dissolving in water.

 

With reference to Fig. 2.2, explain how water acts as a solvent for sodium chloride.

(c) Fig. 2.3 shows a Galapagos penguin, Spheniscus mendiculus, swimming in the water.

 

Penguins are birds that live on land but spend a lot of time swimming underwater hunting for food. Penguins can remain underwater for up to twenty minutes. During this time they do not breathe but their tissues continue to respire.

Haemoglobin in the red blood cells of penguins has a higher affinity for oxygen than haemoglobin in other birds that do not swim underwater.

Fig. 2.4 shows the oxygen dissociation curve for a bird that does not swim underwater.

 

(i) Draw a line on Fig. 2.4 to suggest the position of the oxygen dissociation curve for penguin haemoglobin.

(ii) Penguin haemoglobin is very sensitive to a decrease in pH caused by an increase in the carbon dioxide concentration in the blood.

Explain how a decrease in pH affects penguin haemoglobin, and suggest how this helps the penguin to swim underwater for a long time.

(d) The heart rate of a penguin decreases while it is swimming underwater.

Heart rate is regulated by a group of specialised cells in the wall of the right atrium. The activity of these cells is modified by nerve impulses.

Name the group of specialised cells in the wall of the right atrium that regulates heart rate.

▶️ Answer/Explanation
Solution

(a) The missing blood vessels are arterioles (between arteries and capillaries) and venules (between capillaries and veins).

Explanation: The complete pathway is: heart → arteries → arterioles → capillaries → venules → veins → heart. Arterioles are smaller branches of arteries that lead to capillaries, while venules are small vessels that collect blood from capillaries and join to form veins.

(b) Water acts as a solvent for sodium chloride because:

  • Water is a polar molecule with δ+ hydrogen atoms and δ- oxygen atoms
  • The δ+ hydrogen atoms attract and surround the chloride ions (Cl⁻)
  • The δ- oxygen atoms attract and surround the sodium ions (Na⁺)
  • This breaks the ionic bonds between Na⁺ and Cl⁻, separating the ions
  • The separated ions become evenly distributed throughout the water

Explanation: Water’s polarity enables it to dissolve ionic compounds like NaCl by surrounding and separating the ions. The partial charges on water molecules interact strongly with the charged ions, overcoming the ionic bonds and creating a hydration shell around each ion.

(c)(i) The penguin’s oxygen dissociation curve would be shifted to the left of the normal bird’s curve, maintaining the same sigmoid shape starting at (0,0).

Explanation: The left shift indicates higher oxygen affinity, meaning penguin haemoglobin binds oxygen more readily at any given partial pressure. This adaptation allows penguins to extract more oxygen from their lungs before diving and release it more slowly to tissues during dives.

(c)(ii) A decrease in pH (Bohr effect):

  • Causes haemoglobin to release oxygen more readily (reduced affinity)
  • H⁺ ions bind to haemoglobin, changing its shape and promoting oxygen release
  • This is beneficial as active muscles produce CO₂, lowering pH
  • More oxygen is delivered to respiring tissues when needed most
  • Allows penguins to maintain aerobic respiration longer during dives

Explanation: The Bohr effect ensures oxygen is released precisely where it’s needed most – in active muscles during diving. The pH sensitivity is particularly important for penguins as it allows efficient oxygen unloading during prolonged dives when breathing isn’t possible.

(d) The sinoatrial node (SAN).

Explanation: The sinoatrial node is the heart’s natural pacemaker, located in the right atrium wall. It initiates electrical impulses that regulate heart rate. In penguins, nerve impulses can modify its activity to slow the heart during dives (dive reflex), conserving oxygen for vital organs.

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