Describe the properties of water that make it a useful component of blood.

Explain the relationship between structure and function of arteries, capillaries and veins.

Outline how leucocytes defend the body against pathogens.

a. water is a polar molecule / hydrogen bonding;
b. (makes it) (versatile) solvent;
c. example of dissolved substance (eg salts/proteins or other example);
d. (water is) fluid/liquid at body temperature;
e. example of transported material (eg nutrients/metabolic wastes/gases/hormones/blood cells or other example);
f. high heat capacity/specific heat allows water to carry heat without warming up;
g. (allows) blood to move heat (for warming/cooling/homeostasis);

Arteries: [3 max]
a. thick walls to withstand high pressure/maintain blood flow/pressure;
b. collagen fibres/elastic fibres/connective tissue (in outer layer) give wall strength/flexibility/ability to stretch and recoil;
c. (smooth) muscle layer (contracts) to maintain pressure;
d. narrow lumen maintains high pressure;
e. smooth endothelium for efficient transport/reduced friction;

Capillaries: [3 max]
f. wall has one layer of cells allowing (fast) diffusion of substances;
g. pores to allow lymphocytes/plasma to exit / to increase permeability;
h. extensive branching increases surface area for exchange of materials;
i. small diameter allows them to fit between cells/perfuse tissue;
j. narrow diameter increases oxygen diffusion from RBC;

Veins: [3 max]
k. thin walls allow (skeletal) muscles to exert pressure on veins;
l. thin outer layer of collagen/elastic/muscle fibres provide structural support;
m. wide lumen allows great volume of blood to pass;
n. valves prevent backflow;
NB Every structure requires a function for the mark.

a. leucocytes/phagocytes/macrophages can recognize pathogens/foreign matter;
b. (phagocytes) engulf pathogens by endocytosis/phagocytosis;
c. migration to tissues/squeezing out of capillaries;
d. each pathogen has specific antigens;
e. leukocytes/lymphocytes produce antibodies by reacting to specific antigen/ pathogens;
f. antibody joins to (specific) antigen inactivating/destroying them;
g. lymphocyte makes a clone/copies itself;
h. thus increasing the total number of (specific) antibodies;

This question troubled the rote learner who was unable to apply a general idea to a specific case. Candidates knew key properties of water but could not specifically relate them to blood. Most candidates correctly answered that the polarity of water molecules makes water a good solvent but forgot to give examples of dissolved substances in blood or materials that blood transports. High specific of water was cited but not how blood temperature can remain steady because of it.

Many candidates only wrote about the direction of blood flow through arteries, the heart veins. They completely missed out on the link between structure and function. Other candidates who did write about structural features of blood vessels failed to relate the features to function. Many confused the size of lumen with the degree of pressure in the vessels. Understanding of capillary structure and function appeared to be less than that of arteries or veins. Pores to increase permeability and allow lymphocytes to escape, extensive branching to increase surface area for exchange, and small diameters to allow capillaries to penetrate spaces between cells are examples of ideas often missed.

Many candidates knew that leucocytes can recognize pathogens and engulf them by phagocytosis/endocytosis. More knowledgeable candidates mentioned production of antibodies with specificity to antigens on pathogens. Further details about antigen inactivation and lymphocyte cloning to amplify antibody production were seen only in the very best answers.