Transport Systems, Excretion and Homeostasis
The human circulatory system
Humans have double circulation: the right side of the heart pumps blood to the lungs and the left side pumps it around the body. Arteries carry blood away from the heart and have thick elastic walls; veins carry blood back and contain valves; capillaries are one cell thick for exchange. Blood contains plasma, red cells with haemoglobin, white cells and platelets.
Transport in plants
Xylem carries water and mineral salts upward from the roots, driven by transpiration pull and cohesion between water molecules. Phloem carries sucrose and amino acids upward and downward to growing regions and storage organs. Transpiration is faster in warm, dry, windy and bright conditions.
Excretion and the kidney
Excretion removes waste made by metabolism. The kidneys filter blood in the glomerulus, then reabsorb glucose, useful salts and most water in the tubules. Urea is made in the liver from excess amino acids. Urine contains water, urea and excess salts.
Homeostasis
Homeostasis keeps internal conditions steady using negative feedback. Temperature control involves sweating, vasodilation and vasoconstriction; blood glucose is controlled by insulin and glucagon; water balance is controlled by ADH acting on the kidney tubules.
Worked example
On a hot day, sweating increases and skin arterioles widen (vasodilation). More heat is lost by evaporation and radiation, so body temperature stays near 37 °C. If temperature rises, these responses increase; if it falls, shivering and vasoconstriction reverse the change.
Traps that cost marks
Do not confuse arteries with veins or xylem with phloem. Glucose should not normally appear in urine because it is reabsorbed. Urea is made in the liver, not the kidney, and ADH increases water reabsorption rather than urine volume.