Excretion as an example of homeostatic control

• Define the term "excretion".

  The removal of the waste products of metabolism from the body
• Explain the importance of excretion in maintaining metabolism and homeostasis

  Excretion removes excess to maintain a stable environment. Build up of toxic waste products could do serious harm.
• Name the 3 main metabolic waste products in mammals, describe where they come from and where they are excreted

  CO2 - waste product of cellular respiration, excreted from the lungsBile pigments - formed from breakdown of haemoglobin from old red blood cells in liver, excreted in bile from liver into small intestine via gall bladder and bile duct.
• Name the nitrogenous waste products in mammals, fish, birds and insects

  Mammals - ureaFish - ammoniaBirds and insects - uric acid
• Describe the location of the liver in mammals and draw and label a diagram to show the blood vessels going to and from the liver.

  Liver lies just below the diaphragm.

  
• Describe the function of the hepatic portal vein

  Hepatic portal vein supplies nutrient rich blood to the liver from gastrointestinal tract and spleen. It provides up to 75% of the blood supply to the liver.
• Name the type of cell which makes up the liver and describe the adaptations they have for their functions.

  Liver cells are called hepatocytes. They have large nuclei, prominent Golgi apparatus and lots of mitochondria as they are metabolically active.
• Draw and label a diagram to show the arrangement of tissues within the liver

  Bile canaliculus - the hepatocytes secrete bile into these canaliculi and from there it drains into the gall bladder to be stored.Blood from the venule (of the hepatic portal vein) and arteriole (of the hepatic artery) mixes in the sinusoids and the venule of the hepatic vein carries it away

  
• Label and annotate a photomicrograph of liver tissue at low and high power to show key histological features

  
• Describe the role of Kupffer cells in the liver.

  Macrophages of the liver. Ingesting foreign particles and protecting against disease.
• Describe how bile is formed and how it travels to where it is stored


  Bile is formed by hepatocytes from the breakdown products of old red blood cells, particularly haemoglobin. It is secreted by hepatocytes into canaliculi from where it drains into the gall bladder where it is stored until it passes to the small intestine via the bile duct.
• Describe 6 roles of the liver.

  Carbohydrate metabolism* - control of blood glucose concentration.Bile production - bile pigments and bile saltsHeat production - thyroxine secreted by the thyroid gland when the hypothalamus detects the body temperature is below the optimum causes the liver to do more metabolic processes in order to generate more heat and warm up the blood passing through itBreakdown of blood proteins e.g. hormonesProduction of plasma proteinsDetoxification* - ethanol (in blood) and hydrogen peroxide (made within hepatocytes) - hepatocytes contain the enzyme alcohol dehydrogenase which converts ethanol to ethanal which is then converted to ethanoate and used to build fatty acids or in aerobic respiration.Transamination - conversion of one amino acid into anotherDeamination* of excess amino acids and production of urea
• Describe the role hepatocytes play in the control of blood glucose concentration.

  Hepatocytes respond to insulin by absorbing more glucose and doing glycogenesis.Hepatocytes respond to glucagon by doing glycogenolysis.
• Define the terms "transamination" and "deamination".

  Transamination - conversion of one amino acid into anotherDeamination - the removal of the amino group from amino acids
• Summarise the process of deamination.

  Amino acids absorbed from ileum. Used for building new proteins. Excess to liver. Amino acid molecules split. Amino group combines with carbon dioxide to from urea. Sent to kidneys to be excreted. Rest of molecule used in respiration.
• Draw a diagram to show how urea is produced from excess amino acids via the ornithine cycle

  Ornithine + CO2 + NH3 -> citruline +H2OCitruline + NH3 -> arginine + H2OArginine + H2O -> ornithine + ureaOverall:CO2 + 2NH3 -> H2O + urea

  
• Define the term "detoxification" and describe two examples of detoxification by the liver.

  Detoxification - removal or breakdown of toxinsHydrogen peroxide - hepatocytes contain enzyme catalase which splits hydrogen peroxide into oxygen and waterEthanol - hepatocytes contain the enzyme alcohol dehydrogenase which converts ethanol to ethanal which is then converted to ethanoate and used to build fatty acids or in aerobic respiration.
• Describe the location of the kidneys in humans and draw and label a diagram to show the human urinary system

  
• Name the 3 main areas of the kidney and describe the role of each area.

  Cortex - Filtering the bloodMedulla - water potential gradient to reabsorb water from urinePelvis - where urine collects before passing out down ureter
• Draw and label a diagram to show the internal structure of a kidney.

  
• Name the functional unit of the kidney.

  Nephron
• Draw and label a diagram of a nephron.

  
• Define the terms "Bowman's capsule", "glomerulus", "proximal convoluted tubule", "loop of Henle", "distal convoluted tubule", and "collecting duct".

  Bowman's capsule - cup-shaped structure that contains the glomerulus and is the site of ultrafiltration in the kidneyGlomerulus - cluster of capillaries in the Bowman's capsule where small molecules leave the blood due to ultrafiltrationProximal convoluted tubule - the first twisted section of the nephron after the Bowman's capsule where many substances are reabsorbed into the bloodLoop of Henle - a long loop of nephron that creates a steep concentration gradient across the medullaDistal convoluted tubule - the second twisted section of the nephron where the permeability of the walls varies in response to ADH levels in the bloodCollecting duct - final part of the tubule that passes through the renal medulla and the place where hypertonic urine is produced if needed. The permeability of the walls is affected by ADH and it is the main site of water balancing.
• Describe the functions of the kidney.

  The kidneys produce urine. They filter the blood and reabsorb molecules in the concentrations they are needed. They maintain the correct water balance within the body as well as pH and ion concentration etc.
• Define the term "osmoregulation"

  The balancing and control of the water potential of the blood
• Label and annotate a photomicrograph of kidney tissue at low and high power to show key histological features

  
• Describe the role of the glomerulus and Bowman's capsule

  Filter all the small molecules out of the blood.The afferent arteriole to the glomerulus is much larger than the efferent arteriole meaning blood is under high pressure and small molecules are forced out (limit for ultrafiltration is 69,000).
• Define the terms "ultrafiltration", "ultrafiltrate" and "podocyte"

  Ultrafiltration - the process by which blood plasma is filtered through the walls of the Bowman's capsule under pressureUltrafiltrate - glomerular filtrate - the molecules that are filtered out of the blood - water, glucose etc. that then pass down the nephronPodocyte - cells in the Bowman's capsule that wrap around capillaries. They have extensions called pedicels that act as further sieves to catch any large molecules such as proteins that have made it through the squamous endothelium and basement membrane before they enter the tubule.
• Draw and label a diagram to show the microstructure of the interface between the glomerulus and the Bowman's capsule.

  
• Compare the composition of ultrafiltrate as it enters the nephron and blood plasma

  Filtrate has the same concentration of water, glucose, amino acids, urea and inorganic ions as blood plasma but it hasn't got any proteins in it whereas blood plasma has.
• Describe the role of the proximal convoluted tubule and explain how the cells of the proximal convoluted tubule are adapted for their function

  Selective reabsorption. All of the glucose, amino acids, vitamins and hormones are actively transported back into the blood. Some (85%) of the sodium ions are also actively transported and chloride ions follow passively, as does water.The cells have many microvilli to increase surface area and lots of mitochondria to provide ATP for active transport.
• Describe the role of the loop of Henle and explain how it acts as a counter current multiplier to achieve its role.

  The loop of Henle creates a water potential gradient through the medulla.The descending limb is permeable to water which leaves by osmosis.The ascending limb is impermeable to water and sodium ions are actively transported out, chloride ions follow passively.Due to the loop, there is a counter current flow.This leaves a high water potential in the tissue fluid nearer the cortex and a low water potential towards the pelvis.
• Describe the roles of the distal convoluted tubule and explain how it can perform these roles

  Reabsorption of salt. Na+ actively Cl- follows passively.Control of blood pH.Absorption of water according to osmoregulation - permeability of walls is affected by ADH.Cells have many mitochondria to provide ATP for active transport.
• Describe the role of the collecting duct and explain how it can perform this role.

  Main site for determining concentration and volume of urine.Water leaves collecting duct by osmosis into the tissue fluid. As water potential of tissue fluid decreases (concentration of sodium ions increases) towards the pelvis, water can be reabsorbed along the whole length as water potential of surrounding solution is always lower. The permeability of the walls is affected by ADH, determining how much water leaves.
• State 3 ways in which the body gains water and 4 ways in which water is lost from the body.

  Drinking, Food, RespirationSweating, Breathing out water vapour, Faeces, Urine
• State the main way (from the ways water can be gained by, or lost from, the body) the body can adjust its water balance.

  Concentration of urine
• State the full name and the abbreviation of the hormone that controls the amount of water lost in urine, where it is produced and where it is stored

  ADHAntidiuretic Hormone
• Describe what causes ADH to be released from the posterior pituitary gland.

  A fall in the water potential of the blood, detected by the hypothalamus
• Describe how ADH affects the kidney and explain how this allows osmoregulation.

  It acts on the distal convoluted tubule and collecting duct making cells more permeable to water by adding aquaporins to the surface membrane. This means more water is absorbed back into the blood to raise it's water potential back to the optimum.
• Draw a diagram to show the negative feedback loop that controls the water potential of the blood