Transport practice questions.

State the primary adaptation that allows arteries to withstand and smooth the high, fluctuating pressure of blood ejected by the ventricles.

• A single layer of endothelial cells for rapid diffusion
• Thick walls of elastic fibres and smooth muscle
• Valves along their length to prevent backflow
• A wide lumen relative to wall thickness

Answer: Thick walls of elastic fibres and smooth muscle

Arteries have thick walls rich in elastic fibres (which stretch and recoil to smooth pressure) and smooth muscle; thin walls and valves are features of capillaries and veins respectively.

Explain why tissue fluid forms at the arterial end of a capillary bed.

• Osmotic pressure exceeds hydrostatic pressure, drawing fluid out
• Hydrostatic pressure exceeds the osmotic pull, forcing fluid out
• Active transport pumps plasma into the tissues
• The capillary wall actively secretes plasma proteins into tissues

Answer: Hydrostatic pressure exceeds the osmotic pull, forcing fluid out

At the arterial end, high hydrostatic pressure exceeds the osmotic pressure due to plasma proteins, so fluid is forced out through the capillary wall to form tissue fluid.

Outline the role of the atrioventricular (AV) node in the cardiac cycle.

• It initiates each heartbeat spontaneously
• It delays the impulse so atria empty first
• It carries blood from atria to ventricles
• It prevents backflow of blood into the atria

Answer: It delays the impulse so atria empty first

The AV node delays transmission of the impulse from the SA node, ensuring the atria finish contracting and emptying before the ventricles contract.

Determine the correct explanation for how water moves up the xylem according to the cohesion-tension theory.

• Root cells actively pump water upward using ATP
• Transpiration pulls cohesive water columns up under tension
• Osmotic pressure in the leaves pushes water down to the roots
• Air pressure at the top of the plant sucks water upward

Answer: Transpiration pulls cohesive water columns up under tension

Evaporation from leaf mesophyll (transpiration) lowers water potential and creates tension; hydrogen bonding gives water cohesion so a continuous column is pulled up the xylem.

Suggest why phloem transport is described as active translocation whereas xylem transport is passive.

• Phloem loading of sucrose at sources requires ATP, creating pressure flow
• Phloem contains no living cells so it needs external energy
• Xylem uses ATP to pump water while phloem relies on gravity
• Both use identical passive mechanisms driven by transpiration

Answer: Phloem loading of sucrose at sources requires ATP, creating pressure flow

Sucrose is actively loaded into sieve tubes at sources, lowering water potential so water enters by osmosis and generates the hydrostatic pressure that drives mass flow; xylem flow needs no metabolic energy.

Which feature keeps blood in the veins of the leg moving towards the heart?

• Valves in the vein, combined with contraction of the surrounding muscles
• A thick layer of elastic fibres that recoils after each ventricular beat
• A narrow lumen that raises the pressure of the blood passing through it
• Smooth muscle in the vein wall that contracts in time with the heartbeat

Answer: Valves in the vein, combined with contraction of the surrounding muscles

Blood in veins is at low pressure, so it needs an external pump: skeletal muscles squeeze the vessel and the pocket-shaped valves prevent backflow. Elastic recoil is what smooths flow in arteries.