The mammalian circulatory system and heart, oxygen transport by haemoglobin, and transport of water and assimilates in plants.

What this dot point is asking

WJEC wants you to describe the mammalian double circulatory system and the structure and action of the heart, explain oxygen transport by haemoglobin and the dissociation curve, and describe transport of water and assimilates in plants.

The heart and circulation

The cardiac cycle has three phases. In atrial systole the atria contract and push blood into the ventricles. In ventricular systole the ventricles contract, the rising pressure shuts the atrioventricular valves (the "lub" sound) and opens the semilunar valves, forcing blood into the aorta and pulmonary artery. In diastole the whole heart relaxes, the semilunar valves close (the "dub" sound), and the chambers refill. The sinoatrial node (SAN) sets the rhythm; the impulse spreads across the atria, is delayed at the atrioventricular node, then travels down the bundle of His and Purkyne (Purkinje) fibres so the ventricles contract from the apex upward, squeezing blood out efficiently.

Oxygen transport

The S-shape arises from cooperative binding: the first oxygen binding changes haemoglobin's shape so the next bind more easily. The Bohr effect shifts the curve to the right when carbon dioxide (and acidity) is higher, lowering affinity so haemoglobin unloads more oxygen at active tissues. Foetal haemoglobin has a curve shifted to the left of adult haemoglobin, giving it a higher affinity so it can load oxygen from the mother's blood across the placenta.

Transport in plants

Water enters the roots, crosses to the xylem, and is pulled up in a continuous column held together by cohesion between water molecules and adhesion to the vessel walls; transpiration from the leaves provides the pull (the cohesion-tension theory). Translocation moves sucrose and other assimilates through the phloem sieve tubes from source (such as a leaf) to sink (such as a root or fruit), driven by active loading of sucrose at the source that draws water in and raises pressure.

Examples in context

Example 1. Altitude acclimatisation. At high altitude the partial pressure of oxygen is low, so over weeks the body makes more red blood cells and more 2,3-BPG, which shifts the dissociation curve to favour unloading at tissues. This real physiological adjustment shows how the dissociation curve is tuned to oxygen availability, a favourite WJEC application.

Example 2. Aphids and phloem sap. Aphids feed by inserting their stylets into phloem sieve tubes; the sap is under such high pressure it flows into the insect unaided. Scientists have used aphids cut from their feeding stylets to sample pure phloem contents, providing direct evidence that phloem transports sucrose under pressure from source to sink.

Try this

Q1. Name the structure that acts as the pacemaker of the heart. [1 mark]

• Cue. The sinoatrial node (SAN).

Q2. Explain why a double circulatory system is more efficient than a single one. [2 marks]

• Cue. Blood is repressurised by the heart after the lungs, so it reaches the body at high pressure, giving faster delivery of oxygen.

Q3. A person's heart rate is $80$ beats per minute and stroke volume is $75$ cm cubed. Calculate the cardiac output in dm cubed per minute. [2 marks]

• Cue. $80 \times 75 = 6000 \text{ cm}^3\,\text{min}^{-1} = 6.0 \text{ dm}^3\,\text{min}^{-1}$.