Blood transports molecules throughout the body. State where the blood absorbs hormones.

Blood transports molecules throughout the body. State where the blood absorbs carbon dioxide.

Describe three features of alveoli that adapt them to gas exchange.

Explain how the structure of capillaries relates to their functions.

endocrine glands / named endocrine gland (e.g. pancreas/hypothalamus/pituitary/ovary/testes)
Do not accept gland alone.

cells / tissues / named cells/tissue (e.g. muscles / muscle tissue / muscle cells / epithelial tissue / other reasonable example)
Do not accept alveolus or named organ.

high density of capillaries surrounding alveoli;
large surface area due to shape / large number of alveoli;
thin walls / walls one cell thick; (do not accept membranes)
moist layer covering the (inner) surface of the alveoli;
Award [2 max] for a list of features.

capillaries' walls thin/one cell thick for better diffusion; (do not accept membranes)
small diameter/narrow lumen to fit into small places/between cells;
small diameter for greater surface area for molecular exchange;
pores between cells of the walls so plasma can leak out;
pores between cells of the walls allow phagocytes/immune components to enter tissues;
only one red blood cell allowed to pass at a time for efficient oxygen uptake;

3 a (i) and (ii) were well answered by many though a significant minority wrote only "gland‟ in 3 a (i).

A surprising number of candidates incorrectly wrote the alveoli as the source where carbon dioxide is brought into the blood.

Candidates did well on this question. Many mistakenly wrote that mucus was secreted by alveoli which would impact gas exchange negatively and many failed to appreciate the collective impact of many alveoli giving rise to a larger surface area.  In both b and c, many students referred to membranes as being one cell thick.  The term alveolar membrane is ambiguous as many students do not sufficiently differentiate between cell membranes and the wall of the alveolus. The term "wall‟ is preferable to membrane. Again students must not confuse this with cell walls.

Candidates did reasonably well here though answers were poorly articulated when describing pores and many referred to leakage only rather than indicating the materials including plasma and phagocytes that would leave through the pores. Many did not specifically link the structures of capillaries to their functions. Students need to recognize that an explanation requires reasons and mechanisms so the demands of question 3c were greater than those of 3b where only a description was required. In both 3b and 3c many students needed to more careful with word choice as they referred to the alveolus and capillary as being one cell thick when both structures have thin walls that are one cell thick.