Excretion as an example of homeostatic control — OCR A-Level Biology

Excretion is the removal of metabolic waste products from the body. These wastes are produced by reactions inside cells, for example carbon dioxide from respiration and urea from the breakdown of excess amino acids in the liver (deamination). Excretion matters because waste products can be toxic or disturb homeostasis: carbon dioxide dissolves to form acidic ions that lower blood pH, and urea raises the concentration of solutes in body fluids. Removing these wastes keeps the internal environment within limits so enzymes and cells function, and it helps maintain water and solute balance. Excretion differs from egestion, which is the removal of undigested food as faeces from the gut, not a product of metabolism.

The liver is a large lobed organ supplied by the hepatic artery (oxygenated blood) and hepatic portal vein (nutrient-rich blood from the gut), drained by the hepatic vein. It is built from hepatocytes arranged around a central vein, with sinusoids and Kupffer cells. Its functions include: formation of urea from excess amino acids by deamination; detoxification of substances such as alcohol and drugs; storage of glycogen, vitamins and iron; breakdown of hormones; production of bile for fat emulsification; and control of blood glucose by glycogenesis and glycogenolysis. A method is to link each function to the structure that performs it, for example hepatocytes for deamination and glycogen storage.

This practical skill requires learners to examine a prepared stained section of liver tissue under a light microscope and produce an accurate, labelled biological drawing. Learners should first view at low power to locate the tissue and orientate the field, then increase magnification to identify features such as hepatocytes, sinusoids, central veins and connective tissue septa. When drawing, use a sharp pencil, unbroken clear lines, no shading, and a title stating the tissue and stain. Labels should be straight lines touching the structure, with annotations naming hepatocytes, sinusoids, central vein and lobule boundaries. Magnification should be recorded. The drawing is assessed on accuracy of proportions, clarity of lines, correct labelling and appropriate scale or magnification.

The mammalian kidney is a paired excretory organ built from an outer cortex, an inner medulla and a renal pelvis that drains into the ureter. Each kidney contains about a million nephrons: a Bowman's capsule enclosing a glomerulus, a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule and a collecting duct. Blood arrives under pressure in the afferent arteriole, is filtered at the glomerulus, and the filtrate is modified along the nephron. Ultrafiltration separates small molecules by size and charge; selective reabsorption returns glucose, amino acids and most water and ions; the loop of Henle generates a medullary solute gradient; the collecting duct adjusts water reabsorption under ADH control. The kidney therefore removes nitrogenous waste such as urea, osmoregulates body fluids and regulates blood pH and ion balance.

This practical criterion requires you to carry out a kidney dissection safely, examine the specimen and produce an accurate drawing of both its external and internal structure. Working on a dissecting board with a scalpel and scissors, you first observe the external surface, noting the convex shape, the renal artery and vein and the ureter entering at the hilum. You then make a longitudinal cut to expose the internal structure, identifying the darker outer cortex, the striated medulla and the pale renal pelvis. Your drawing must be made with a sharp pencil, show correct proportions and clear outlines, and be labelled with the structures you can actually see. Magnification and a title should be included where appropriate. Assessment focuses on safe technique, accurate observation and a clear, correctly labelled biological drawing rather than on artistic quality.

This practical skill has two linked parts: examining a prepared stained section of kidney and producing a labelled drawing that shows nephron histology. A stain such as haematoxylin and eosin colours nuclei and cytoplasm differently, so tissue layers become distinguishable. Under low power, locate the cortex and medulla, then identify a renal corpuscle with its glomerulus inside Bowman's capsule, and a tubule cut in cross-section with a ring of cuboidal epithelium around a lumen. Switch to high power to resolve cell detail. When drawing, use a sharp pencil, unbroken lines, no shading, and keep proportions accurate. Label only structures you can actually see, using straight label lines that touch the structure. Add a title and state the magnification. The drawing is assessed on accuracy, proportion, line quality and correct labelling, not on artistic finish.

Blood water potential is controlled by negative feedback centred on osmoreceptors in the hypothalamus. If water potential falls (blood too concentrated), osmoreceptors shrink and stimulate the posterior pituitary to release more ADH. ADH increases the permeability of the collecting duct to water by inserting aquaporins, so more water is reabsorbed by osmosis and a smaller volume of more concentrated urine is produced; water potential rises back towards normal. If water potential rises (blood too dilute), less ADH is released, the collecting duct is less permeable, less water is reabsorbed and a larger volume of dilute urine is produced. The loop of Henle and the countercurrent multiplier create the medullary solute gradient that allows this osmotic reabsorption.

Kidney failure means the nephrons can no longer filter blood and reabsorb useful solutes effectively, so the composition of body fluids drifts from the norm. Learners should link reduced filtration to accumulation of urea and other nitrogenous waste, and reduced reabsorption to loss of glucose, amino acids and useful ions in urine. Loss of water regulation disturbs water potential and blood volume, and loss of osmoregulation plus acid–base balance changes blood pH. Treatments must be matched to these effects: dialysis removes waste by diffusion down a concentration gradient across a partially permeable membrane, while a kidney transplant replaces filtration and reabsorption and needs immunosuppression. Diet and fluid restriction reduce the waste and water load.

Excretory products carry information about what the body is doing, so measuring them helps diagnose disease. Learners should connect specific substances in urine or blood to a condition. Glucose in urine can indicate diabetes mellitus because the reabsorption of glucose in the proximal convoluted tubule is overwhelmed when blood glucose is high. Protein in urine can indicate damage to the glomerulus, since a healthy filter holds large molecules back. Urea and creatinine concentrations in blood indicate how well the kidneys are filtering. Hormones and their breakdown products, such as hCG in pregnancy testing, can be detected in urine. The key skill is linking a measured excretory product to the organ or process that has failed.