IB Biology/Excretion

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Topic 12: Excretion[edit]


Outline the need for excretion in all living organisms.

  • The idea behind excretion holds true for all living organisms. If excess toxins, salts, and water builds up it may cause serious bodily harm to that organism. The kidneys make sure that this doesn’t happen and allow for normal bodily functions to continue. If not for this system the organism may also end up needing to spend excessive amounts of energy to filter out the toxins that could kill it.

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

Excretory products in plants include oxygen, and in animals they include nitrogenous compounds and carbon dioxide.

Discuss the relationship between the different nitrogenous waste products and habitat in mammals, birds, and reptiles

  • Any metabolism involving proteins and nucleic acids produces a nitrogen containing waste with the main waste product being Ammonia. The direct excretion of Ammonia, in one form or another, is the most biologically efficient way of disposing of waste. In fish ammonia can simply be excreted purely as it is extremely water soluble. Fishes mainly lose Ammonia through the epithelium of their gills as NH4+. Fresh water fishes also take up sodium as an exchange for the Ammonia. Animals, however, can not excrete pure Ammonia since it is too toxic to be excreted without first being diluted and terrestrial animals can’t dispose of it quickly enough. Thus mammals excrete urea, a product which is much less toxic than Ammonia and requires less water to be excreted which is important to terrestrial creatures where water is more scarce. Finally birds excrete Uric Acid as their form of Ammonia removal. It is much less soluble in water than Ammonia is, thus leaving much more water for the bird which is needed during long migration patterns. Uric acid can also precipitate, allowing for shelled offspring who are permeable to gas but not permeable to liquid, as Ammonia or Urea would be.

The Human Kidney[edit]

11.3.1 - Define Excretion

  • Excretion is the removal of waste products of metabolic pathways from the body

11.3.2 - Draw (and label) the structure of the kidney

  • Things to include - Cortex, Medula, Ureter, Renal Blood Vessels, and Pelvis

11.3.3 - Draw the structure of the glomerulus and associated nephron to show the function of each part

11.3.4 - Explain the process of ultra-filtration

  • Blood pressure forces fluid from the capillaries of the Glomerulus across the Epithelium of the Bowman’s Capsule, and into the lumen of Nephron tubule. The porous capillaries act as a filter as they have membranes which are permeable to water and small solutes but not to blood cells or larger molecules. The efferent arteriole reabsorbs amino acids, glucose, water, and salts and NH2, excess water, and sodium chloride descend into the loop of Henle. The more water and sodium is absorbed and tubular excretion begins in distal convoluted tube. Large molecules which weren’t absorbed enter for excretion and the collecting duct takes it to the renal pyramid.
  • 15-20% of blood plasma is filtered into glomerulus, that exit through fenestrated blood capillaries (porous). The basement membrane is a protein layer between the glomerulus and Bowman's Capsule that prevents blood cells and large proteins from entering the Bowman's Capsule.

11.3.5 - Define osmoregulation

  • Osmoregulation - the control of water and solute levels

11.3.6 - Explain the reabsorption of glucose, water, and salts in the proximal convoluted tube

  • In the Proximal Convoluted Tubule there is selective re-absorption of water and salt, depending on how dehydrated the body is. In the presence of ADH, much more water is reabsorbed than when ADH isn’t present. Microvilli also helps to expand surface area to allow for more absorption. Filtration, on the other hand, is not selective at all as it is important that essential nutrients return to the body so sugar, vitamins, and other organic nutrients are reabsorbed into the filtrate. Some are actively transported (glucose, amino acids, sodium ions) and some are passively transported (H2O, potassium ions, chloride ions).

11.3.7 - Explain the roles of the Loop of Henle, medulla, collecting duct and ADH in maintaining the water balance of the blood

  • Water leaves the Nephron via Osmosis while in the descending limb of the Henle’s loop due to the increasing salt concentration of the medulla in the kidney. Blood will pass into the capillaries where it is removed and salt will diffuse into the filtrate. The ascending limb is impermeable to water, allowing various salts to leave. Fluid that leaves the loop is less concentrated than the tissue around it, and blood entering the medulla will lose water due to osmosis and pick up salt and urea through diffusion. The exact opposite happens in the ascending capillary. When a lack of water is detected in the kidney, ADH is released by the pituitary gland which increases permeability of the walls in the distal convoluted tube and the collecting duct. When ADH isn’t present, walls are impermeable and water is used to dilute urine.

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

  • Renal Artery – Large molecules, More Toxins, Oxygenated, More Salt/Ions, More H2O, Less CO2, More Nutrients, Supplies Kidneys with O2
  • Renal Vein - Less Toxins, Deoxygenated, Less Salt, Less H2O, More CO2, Less Nutrients, Returns blood from Kidneys to heart, blood contains no wastes, with less O2, urea, salt/ions, and more CO2
  • Glomerular Filtrate: Small molecules, Amino acids, glucose, Na+ Cl, NH2, Urea, Nutrient excess, Nitrogenous waste, H+, H20, hormones, vitamins
  • Urine: Wastes: Urea, Na+, Cl, NH2, H2O, large molecules

Outline the structure and action of kidney dialysis machines

  • Blood enters machine from patient’s vein and runs through partially permeable tubules and into the dialysis chamber. The tube allows nitrogenous waste to diffuse from the blood and into the dialysis fluid. Urea diffuses through the membrane, as dialysis fluid contains none, removing it from the body. Water and solutes will then be added to the blood if necessary by diffusion from the dialysis fluid which runs across the semipermeable tubules containing the circulating blood. The blood is then run through an air bubble trap before it is reintroduced to the vein.