B3.2 TransportIB Biology HL: Revision notes

Capillaries: branched and narrow (large surface area), walls one cell thick, fenestrations where exchange is rapid. Arteries: thick smooth muscle and elastic fibres withstand high pressure and recoil to maintain it; thick wall relative to lumen. Veins: valves and thin, flexible walls compressed by skeletal muscle; thin wall, wide lumen. Heart rate is measured at the carotid or radial pulse or with a digital monitor.

Plaque in coronary arteries narrows the lumen and may trigger a clot; heart muscle is starved of oxygen, causing angina or a heart attack. A correlation coefficient measures how strong a correlation is, but even a strong correlation (e.g. saturated fat and CHD) does not prove causation.

Transpiration from leaf cell walls creates tension in the xylem; cohesion keeps a continuous water column. Xylem vessels: no contents, no end walls, lignified walls, pits.

Dicot stem: epidermis, cortex, ring of vascular bundles (phloem outside, xylem inside). Dicot root: epidermis with root hairs, wide cortex, central xylem star with phloem between the arms.

At the arteriole end of a capillary, high blood pressure forces plasma out by pressure filtration, forming tissue fluid. At the venule end, lower pressure lets fluid drain back in, drawn by the osmotic effect of plasma proteins. Tissue fluid resembles plasma but lacks red blood cells and most plasma proteins; it delivers O₂, glucose and ions to cells and takes up CO₂ and wastes.

Excess tissue fluid enters lymph ducts, which have thin walls with gaps and valves; lymph returns to the blood in veins near the heart.

Bony fish have a single circulation: heart → gills → body → heart. Pressure falls in the gill capillaries, so blood reaches the body slowly. Mammals have a double circulation: right side → lungs → left side → body. Blood is re-pressurised after the lungs, so high pressure reaches the body while lung capillaries receive lower pressure.

Heart adaptations: cardiac muscle contracts without fatigue; pacemaker (sinoatrial node) sets the rhythm; thin-walled atria collect blood; thick-walled ventricles (left thickest) pump it; atrioventricular and semilunar valves prevent backflow; the septum separates oxygenated and deoxygenated blood; coronary vessels supply the heart muscle. Flow: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.

The sinoatrial node initiates each beat. Atrial systole pushes blood through the open atrioventricular valve. In ventricular systole, ventricular pressure exceeds atrial pressure (AV valve closes) and then aortic pressure (semilunar valve opens). In diastole, the semilunar valve closes as ventricular pressure falls below aortic, then the AV valve opens and the ventricle fills.

Blood pressure is written systolic/diastolic, e.g. 120/80 mm Hg: systolic = peak arterial pressure during ventricular contraction; diastolic = minimum during relaxation.

Root pressure is a positive pressure potential generated when root cells actively transport mineral ions into the xylem; water follows by osmosis. It moves water when transpiration is insufficient, e.g. in high humidity or in spring before deciduous leaves open.

Phloem sieve tubes carry sap by translocation from sources to sinks. Sieve tube elements have sieve plates, reduced cytoplasm and no nucleus, easing flow. Companion cells have many mitochondria to supply ATP for loading sugars, and are linked to sieve tubes by plasmodesmata.