15.1         Excretion

Notes: This allows a return to the diversity of topic 13 and also the Theme of 'Universality versus Diversity'. The relationship here is between the function and the habitat (in terms of available water. (Note the difference between marine and freshwater fish.)

 15.1.1     Outline the need for excretion in all living organisms.

15.1.2    State that excretory products in plants include oxygen, and in animals include carbon dioxide and nitrogenous compounds.

15.1.3     Discuss the relationship between the different nitrogenous waste products and habitat in mammals, birds, amphibians and fish.

Surplus amino acids must be degraded to relatively harmless nitrogen containing compounds. Freshwater fish and amphibian larvae can get rid of ammonia although highly toxic (due to its basicity) because it can be diluted by the readily available water. Marine species need to conserve water (saline environment is equivalent to physiological drought) so they convert nitrogenous wastes to the less toxic and less soluble urea [(NH2),CO] and trimethylamine oxide [(CH,)3NO]. Likewise adult amphibians need to conserve water so they also produce urea. Birds are unable to carry too much water so they excrete uric acid (formula not required) which is insoluble and expelled as a paste (most of the water is removed before excretion). (Note that many insects and some reptiles also excrete uric acid, an end product of purine metabolism - but this is not required.) Mammals excrete the urea produced in the ornithine cycle. Some desert mammals produce very concentrated urine (having a long loop of Henle) - see 15.2-5.

15.2.1     Draw the structure of the kidney including cortex, medulla, pelvis, ureter and renal blood vessels.

15.2.2     Draw the structure of a glomerulus and associated nephron.

15.2.3     Explain the process of ultrafiltration including blood pressure, fenestrated blood capillaries, and basement membrane.

Utrafiltration is filtration at the molecular level - holding back plasma proteins. The capsule epithelial cells have an irregular network of filtration slits that hold back only the blood cells. The blood capillary walls are fenestrated (perforated) and allow blood plasma through. The basement membrane acts as the dialysis membrane where ultrafiltration occurs. Further details of podocyte structure is not required.

15.2.4     Explain the process of selective reabsorption in the proximal convoluted tubule including the roles of microvilli, pinocytosis and active transport.

15.2.5     Explain the production of hypertonic urine including the roles of the loop of Henle, medulla, collecting duct, ADH and water potential gradients (cross reference 16.2.2).

15.2.6     Compare the composition of blood in the renal artery and renal vein, and glomerular filtrate and urine.

15.2.7     Outline the structure and action of kidney dialysis machines.

Renal dialysis by kidney machine depends only on passive diffusion. The blood of the patient taken from a vein, passes through tubes or between membranes of partially permeable material (polymers, cellulose esters, visking, etc.). Emphasise the fact that an enormous surface area is required and that the process takes 4-6 hours repeated three times a week A dialysis fluid on the other side has the same water potential and similar solute concentrations, to the blood plasma except that there is no urea. Urea therefore diffuses from the blood and is washed away with the dialysis fluid. There should be no other net change, but if necessary water.- and other solutes can be replenished before the dialysed blood is returned to the patient.