Adaptations for transport: the mammalian heart and the cardiac cycle; blood vessels and tissue fluid; haemoglobin and the oxygen dissociation curve; and transport in plants (xylem and phloem).

What this dot point is asking

Eduqas wants you to describe the mammalian heart and the cardiac cycle, describe blood vessels and tissue fluid, explain haemoglobin and the oxygen dissociation curve (including the Bohr effect), and describe transport in plants by xylem and phloem. This is the transport core of Component 3.

The mammalian heart and the cardiac cycle

The heart is a double circulatory pump: the right ventricle pumps deoxygenated blood to the lungs (pulmonary circuit) and the left ventricle (thicker walled) pumps oxygenated blood to the body (systemic circuit). The cardiac cycle has three phases:

1. Atrial systole: the atria contract, pushing blood into the ventricles through the open atrioventricular valves.
2. Ventricular systole: the ventricles contract; rising pressure closes the atrioventricular valves (preventing backflow to the atria) and opens the semilunar valves, forcing blood into the aorta and pulmonary artery.
3. Diastole: the heart relaxes; pressure falls, closing the semilunar valves (preventing backflow to the ventricles), and the atria refill.

The valves open and close because of pressure differences, opening only when the pressure behind them is higher, ensuring one-way flow.

Blood vessels and tissue fluid

• Arteries: thick, elastic, muscular walls to withstand and smooth high pressure; narrow lumen.
• Capillaries: walls one cell thick and a large total surface area for exchange.
• Veins: thinner walls, wide lumen and valves to return low-pressure blood to the heart.

Haemoglobin and the oxygen dissociation curve

Haemoglobin carries oxygen as oxyhaemoglobin. Its binding is cooperative (binding one oxygen makes the next easier), giving an S-shaped (sigmoid) oxygen dissociation curve: at high partial pressure of oxygen (lungs) haemoglobin is nearly saturated (loads oxygen); at low partial pressure (tissues) it releases oxygen.

Transport in plants

• Xylem carries water and minerals up from the roots. The cohesion-tension theory: water evaporates from the leaves (transpiration), creating tension that pulls a continuous column of water up the xylem, held together by cohesion (hydrogen bonding) and adhesion to the walls.
• Phloem translocates sugars (mainly sucrose) from source (leaves) to sink (growing or storage regions), by active loading at the source that draws water in and creates a pressure gradient (the mass-flow hypothesis).

Examples in context

Example 1. Why the left ventricle wall is thicker. It must generate enough pressure to pump blood all around the body, whereas the right ventricle only pumps to the nearby lungs, a direct structure-to-function point examiners reward.

Example 2. High-altitude adaptation. People at altitude make more red blood cells (and some animals have haemoglobin with a curve shifted left) to load oxygen from thin air, applying the dissociation curve to a real environment.

Try this

Q1. Explain why the atrioventricular valves close during ventricular systole. [2 marks]

• Cue. The ventricles contract, raising ventricular pressure above atrial pressure, which forces the valves shut and prevents backflow into the atria.

Q2. State what the Bohr effect does to the oxygen dissociation curve and why it is useful. [2 marks]

• Cue. Shifts it right (lowers affinity) when carbon dioxide is high, so more oxygen is unloaded at respiring tissues.

Q3. Name the process by which sugars are transported in the phloem. [1 mark]

• Cue. Translocation (by mass flow from source to sink).