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Excretion Topic 11.3
 

Excretion Topic 11.3

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    Excretion Topic 11.3 Excretion Topic 11.3 Presentation Transcript

    • 11.3 Excretion
    • Excretion definition:
      • • The removal from the body of metabolic waste from the cells, tissue fluid and blood
      • • Metabolic waste molecules are the by-products of chemical reactions within the body
      • • These waste molecules are toxic in some form and must not be allowed to accumulate in tissues
      • • The toxic nature might include changes in pH, enzyme inhibition and mutagens along with a wide variety of other effects
    • Role of the kidney in excretion and the maintenance of water balance
      • The blood content of water, urea and salts is
      • determined by the filtering mechanism of the
      • kidney.
      • Blood enters the kidney via the renal artery and
      • its contents are adjusted by removing
      • combinations fob the following molecules:
      • • Water
      • • Urea
      • • Salts
    • Human Kidney
      • There are normally two kidneys commonly referred to as left and right kidney.
      • Each kidney is supplied by branch of the aorta giving the left and right Renal Artery.
      • The filtered blood is returned to the vena cava from each kidney by a Renal Vein.
      • The urine produced by each kidney is transported via a ureter to the bladder.
      • Cortex : Lighter brown color contains the Malpighian bodies which are the capsules that contains Bowman's capsule and a glomerulus at the expanded end of a nephron. There are also the proximal and distal convoluted tubules and the upper sections of collecting ducts.
      • Medulla :The darker, redder region composed of loops of henle and the lower sections of the collecting ducts.
      • Pelvis : Is a cavity which collects the urine emerging from the open ends of the collecting ducts. The nephrons open are found on the margin of the pyramids with the pelvis. The white tissue forms a funnel called the ureter which conducts the urine to the bladder.
      Three regions to the kidney
    • Regions to the Kidney (CMP)
    • Nephron
    • Structure of a glomerulus and associated nephron
      • (a) Afferent arteriole a branch of the renal artery
      • (b)The Malpighian Body
      • The Bowman's capsule (renal capsule) the expanded end of the nephron formed into a cup shape.
      • The glomerulus is a multiple branching of the afferent arteriole before rejoining to the efferent arteriole.
      • Together the Bowman's capsule and the glomerulus are known as the Malpighian body.
    • Structure of a glomerulus and associated nephron
      • (c) Efferent Arteriole (narrower than afferent) join together to form the renal vein
      • (d) Proximal Convoluted Tubule (14mm long / diameter 60um) longest section of the nephron
      • (e) Loop of Henle
      • (f) Distal Convoluted Tubule
      • (g) Collecting Duct which opens into the Pelvic region
    • 3 Parts to the Nephron
      • Malpighian body
      • (Ultrafiltration)
      • B. Proximal Convoluted Tubule
      • ( Selective Reabsorption)
      • C. Loop of Henle
      • (Urine formation)
    • Ultrafiltration, Selective Reabsorption and Urine formation
    • Ultrafiltration : formation of kidney filtrate
    • Ultrafiltration : formation of kidney filtrate
      •  
      • The malpighian body is the location of Ultrafiltration .
      • The glomerulus increases blood pressure by forming narrow branches (also an increase in surface area for filtration).
      • The pressure is maintained by the narrower efferent arteriole which restricts the outflow of blood from the glomerulus.
      • The expanded end of the nephron forms an invaginations to form a cup that accommodates the glomerulus
      • The efferent blood vessel associated itself with the other regions of the nephrons for selective reabsorption.
    • Ultrafiltration : formation of kidney filtrate
      • High Pressure is generated in the glomerula knot.
      • Fenestration's (gaps) between the cells that form the glomerula blood vessel creates a path of low resistance out of the glomerulus.
      • The basement membrane is the effect filtration barrier. Cells and large plasma protein macromolecules cannot pass through this structure.
      • Podocytes for the inner membrane of the Bowman's capsule. The interdigitation of the podocyte extension creates gaps for the filtrate to pass between the cells.
      • Note this means that the filtrate does not pass through the cells of either the glomerulus or the Bowman's capsule
    • Alternative diagram of the podocyte/ arteriole structure:
      • The podocytes of the inner wall of the Bowman's capsule have many fine arm-like projections which wrap around the arterioles. Although the fenestration's of the arteriole allow large molecules to leave the blood vessel, these large molecules are largely prevented from further movement by the small spaces between the podocyte extensions.
      • There is still the fine mesh work of the basement membrane (lamina) that will prevent any large molecules such as proteins from leaving the blood.
    • B. Proximal Convoluted Tubule Selective Reabsorption
      • The process of control and regulation in the kidney begins with a non discriminating filtration ( ultrafiltration ) that removes just as many useful substances as harmful ones from the blood to make filtrate.
      • The kidney then takes back from the filtrate to the blood those substances that it still requires in the blood.( Selective Reabsorption )
      • The beauty of the way the kidney works is that it is able to control how much of a substance it reabsorbs back into the blood ( Regulation )
    • Nephron
    • Proximal Convoluted Tubule
      • 1 . All glucose, all amino acids and 85% of mineral ions are reabsorbed by active transport from the filtrate to the tissue fluid. They then diffuse into the blood capillaries.
      • 2 . Small proteins are reabsorbed by pinocytosis, digested, and the amino acids diffuse into the blood.
      • 3 . 80% of the water is reabsorbed to the blood by osmosis.
      • 4 . As urea molecules are so small and carry no charge that they diffuse passively through the cell membrane. In part this explains why not all urea is excreted as blood passes through the kidney.
      Note that the PCT has a microvilli cell border to increase the SA for absorption from filtrate. There are also a large number of mitochondria which produce the extra ATP required for active transport.
    • C. Loop of Henle and Collecting Duct (Urine Formation)
      • Function : 1. The function of the loop of Henle is to create a salt bath concentration in the surrounding medullary fluid.
      • 2. Later this results in water reabsorption in the collecting duct
      • 3. There is also a reduction in the filtrate volume.
    • Loop of Henle and Collecting Duct in the control of water balance
      • Mechanism: 1. There is a concentrated gradient down through the medullary fluid (a). 2. The descending limb is permeable to water but not to salt. 3. Filtrate enters the loop hypotonic to the medullary fluid so water is lost (b). 4. The concentration difference between medullary fluid and the filtrate is small. 5. The amount of water lost at each stage is small but accumulates on descent. 6. The water is lost but immediately taken up by the blood.  7.. Filtrate volume reduces and filtrate salt concentration increases. 8. The base of the loop is impermeable (c)
      • Fluid turns the impermeable loop.
      • 1. Filtrate moves up the ascending limb. 2. Ascending limb is permeable to salt. 3. Ascending limb is impermeable to water. 4. The filtrate is slightly more concentrated than the surrounding fluid. 5. There is a small but accumulating loss of salt (Na + and Cl - )at each level. 6. The concentration of the filtrate is gradually reduced.
      • 7. The medullary gradient is maintained through exchange with the surrounding blood vessels
      • The concentration gradient of the medullary fluid brings about the removal of water from the collecting duct by osmosis.
      • The permeability of both Distal Convoluted Tubule (DCT) and the Collecting tube(CT) can be increased by the hormone ADH
      • The cell membranes of these tubules do not allow the movement of water by simple diffusion. Rather pores called Aquaporin can be opened the action of ADH.
      • The DCT is involved in other homeostatic functions such as the secretion of H+in pH regulation or K+in salt regulation.
    • Osmoregulation
      • Osmoregulation—the control of the water balance of the blood, tissue or cytoplasm of a living organism.
          • The water content of body fluids has to be controlled such that the movement of water to and from cells can changes be controlled.
          • The body experiences external and internal changes such as drinking water availability, sweating and the accumulation of salts that require adjustments in the water content of blood, tissues and cytoplasm.
          • This is under the control of receptors in the hypothalamus.
          • In responses to changes the hypothalamus controls the sensation of thirst and also the endocrine secretion of anti-diuretic hormone.(ADH).
          • ADH is secreted from the pituitary and causes the opening of cell membrane pores called aquaporins which allows water reabsorption into the blood.
    • Composition of blood and renal fluids
      • The collecting duct is permeable to both water which as the filtrate descends this collecting duct is removed concentrating the filtrate (urine). However the collecting duct also leaks some urea which to the kidney interstitial fluid. Some of this lost urea is reabsorbed by the ascending limb of the loop of henle but not all, hence the 50% reabsorption. This cycling of urea is an important feature of the kidneys ability to produce a concentration gradient through the medulla.
          • Uric acid is a fairly toxic molecule (main nitrogenous excretion in birds) and is largely removed from blood and tissue fluids.
          • Glucose is 100% reclaimed by selective reabsorption. The presence of glucose in the urine would be an indication of diabetes.
          • Amino acids are all selectively reabsorbed in the nephron and then undergo deamination in the liver (urea excretion).
          • Proteins and other macromolecules should not be filtered in the Bowman's capsule and any presence in urine is usually regarded as an indicator of high blood pressure and damage to the basement membrane (nephritis) of the bowman's capsule.
    • Kidney Dialysis (Passive Diffusion)
      • If the kidneys fail artificial regulation of blood composition is required.
      • This diagram shows the key features of this process called dialysis
      • A shunt is created between a deep artery and the surface vein to increase pressure. (The artery itself is too deep and narrow).
      • The blood and dialysis liquid pass in opposite directions (counter current) to maximize diffusion.
      • The dialysis liquid contains appropriate glucose and salt levels but no urea.
      • Blood and Dialysis liquid are separated by a semi-permeable membrane.
      • On completion of a 3-5hour dialysis session the urea has been removed and the concentration of glucose and salt is the same as the fresh dialysis liquid. Note this dialysis liquid is continually refreshed.