Excretion as an Example of Homeostatic Control

• Excretion: the removal of metabolic waste from the body
• Metabolic waste: a substance that is produced in excess by the metabolic processes in cells; may become toxic
• Almost all products formed in excess by chemical processes occurring in cells must be removed from body so they don't build up and inhibit enzyme activity or become toxic
• Main excretory products:

  • carbon dioxide from respiration
  • nitrogen-containing compounds e.g urea (i.e nitrogenous waste)
  • other compounds e.g bile pigments found in faeces
• The lungs:

  • Carbon dioxide passed from cells of respiring tissues into bloodstream where it is transported to lungs, mostly in form of hydrogencarbonate ions
  • In lungs, carbon dioxide diffuses into alveoli where it is excreted as you breathe out.
• The liver:

  • Directly involved in excretion
  • Many metabolic roles and some substances produced will pass into bile for excretion with faces e.g pigment bilirubin
  • Involved in converting excess amino acids to urea
  • Amino acids broken down by process of deamination
  • Nitrogen-containing part of molecule is combined with carbon dioxide to make urea
• The kidneys:

  • Urea passed into bloodstream to be transported to kidneys
  • Urea also transported in solution - dissolved in plasma
  • Urea is removed from blood to become a part of urine in the kidneys
  • Urine is stored in bladder before excretion from urethra
• The skin:

  • Excretion is not primary function
  • Sweat contains a range of substances e.g salts, urea, water, uric acid and ammonia
  • Urea, uric acid and ammonia are all excretory products
  • Loss of water and salts may be important part of homeostasis - maintaining body temperature and water potential of blood
• Importance of excretion:

  • Build up of products of metabolism could be fatal
  • Some metabolic products like carbon dioxide and ammonia are toxic
  • Interfere with cell process by altering pH so normal metabolism prevented
  • Some metabolic products can act as inhibitors and reduce essential enzyme activity
• Carbon dioxide transported in blood as hydrogencarbonate ions, the formation of which forms hydrogen ions. Carbonic acid also dissociates to release hydrogen ions. Occurs in red blood cells catalysed by enzyme carbonic anhydrase but can also occur in blood plasma.
• Hydrogen ions affect pH of cytoplasm in red blood cells , changing 3D shape and altering affinity of haemoglobin for oxygen, hence altering oxygen transport. Hydrogen ions then combine with haemoglobin to form haemoglobonic acid. Carbon dioxide that isn't converted to hydrogencarbonate ions combines directly with haemoglobin to produce carbaminohaemoglobin. Both haemoglobinic acid and carbaminohaemoglobin are unable to combine with oxygen as normal which further reduces oxygen transport.
• Blood plasma:

  • Excess oxygen ions can decrease pH of plasma.
  • Maintaining pH is essential as changes can alter structure of many proteins in blood helping to transport wide range of substances around body.
  • Proteins in blood act as buffers to resist change in pH
  • If change in pH is small then extra ions detected by respiratory center in medulla oblongata of brain, causing increase in breathing rate to help remove excess carbon dioxide
• If blood pH drops below 7.35, headaches, drowsiness, tremors, confusion and restlessness can be caused, along with rapid heart rate and changes in blood pressure. Known as respiratory acidosis. Caused by diseases or conditions affecting lungs e.g emphysema, chronic bronchitis, asthma or severe pneumonia. Blockage of airway due to swelling, foreign object, or vomit can also induce acute respiratory acidosis.
• Nitrogenous compounds:

  • Wasteful to excrete amino acids as, whilst they cannot be stored, they contain almost as much energy as carbohydrates
  • Transported to liver where potentially toxic amino acid group is removed through deamination
  • Amino group initially forms very soluble and highly toxic ammonia which is converted to a less soluble and less toxic compound called urea which is transported to kidneys for excretion
  • Remaining keto acid can be used directly in respiration to release energy or may be converted to carbohydrate of fat for storage
• Deamination: amino acid + oxygen -> keto acid + ammonia
• Formation of urea: ammonia + carbon dioxide -> urea + water
• Liver cells are called hepatocytes
• Essential that the liver has a good supply of blood as it has an important role in homeostasis
• Liver supplied with blood from two sources - the hepatic artery and hepatic portal vein
• Hepatic artery:

  • Oxygenated blood from heart travels from aorta via hepatic artery into liver which supplies oxygen essential for aerobic respiration
  • Liver cells are very active as they carry out many metabolic processes, many of which require energy in the form of ATP so it is important that liver has good supply of oxygen for aerobic respiration.
• Hepatic portal vein:

  • Deoxygenated blood rich in the products of digestion enters liver through this vein from the digestive system
  • Concentrations of various substances will be uncontrolled as they have just entered body from products of digestion in intestines
  • Blood may also contain toxic compounds absorbed from intestine and it is important that substances like that do not circulate around body before concentrations have been adjusted
• Blood leaves liver via the hepatic vein which rejoins the vena cava and blood returns to body's normal circulation
• Fourth vessel is connected to liver - bile duct. Bile is a secretion from liver which functions in digestion and excretion. Bile duct carries bile from liver to gall bladder. Stored in gall bladder until it is required to help the digestion of fats in small intestine. Also contains some excretory pigments such as bilirubin (excreted from body with faeces).
• Liver divided into lobes which are then divided into cylindrical lobules. Cells, blood vessels and chambers in liver are arranged so there is greatest possible contact between blood and liver cells.
• Inter-lobular vessels: hepatic artery and portal vein enter liver and split into smaller and smaller vessels which run parallel and between lobules
• Branches from hepatic artery and portal can enter lobules at intervals. Blood from two vessels is mixed and passed along sinusoid lined with liver cells. Cells are able to remove substances from blood (which is in close contact with liver cells)and return other substances to the blood.
• Kupffer cells = specialized macrophages that move about within the sinusoid and are responsible for the breakdown and recycling of old red blood cells. One of products of haemoglobin breakdown is bilirubin which is one of bile pigments excreted as part of bile.
• Bile made in liver cells and released into bile canaliculi which join together to form bile duct which transports bile to gall bladder
• Intra-lobular vessel: branch of hepatic vein at the center of each lobule where sinusoids empty into.
• Branches of hepatic vein from different lobules join together and form hepatic vein which drains blood from liver
• Hepatocytes are relatively unspecialised, with simple cuboidal shape and many microvilli on surface. Dense cytoplasm as they have many metabolic functions, and is specialised in numbers of certain organelles it contains.
• Ornithine cycle: series of biochemical reaction converting ammonia to urea
• Liver is metabolically active and has wide range of functions including:

  • control of blood glucose, amino acid and lipid levels
  • synthesis of bile, plasma proteins and cholesterol
  • synthesis of red blood cells in fetus
  • storage of vitamin A, D and B12, iron, glycogen
  • detoxification of alcohol and drugs
  • breakdown of hormones
  • destruction of red blood cells
• Liver stores sugars in form of glycogen and is able to store approximately 100 - 120g of glycogen (8% of fresh weight of liver). Glycogen forms granules in cytoplasm of hepatocytes which can be broken down to release glucose into blood as necessary.
• Some compounds the liver detoxifies such as hydrogen peroxide are produced in the body and others such as alcohol may be consumed as part of a diet or may be taken for health or recreational reasons such as medicines and recreation
• Toxins rendered harmless by oxidation, reduction, methylation or by combination with another molecule. Liver cells contain many enzymes that render toxic molecules less toxic.
• Catalase: converts hydrogen peroxide to oxygen and water. Catalase has a particularly high turnover number (number molecules of hydrogen peroxide that one molecule of catalase can render harmless in one second) of 5 mn.
• Cytochrome P450: involved in breakdown of drugs including cocaine and also used in other metabolic reactions e.g electron transport in respiration. Their role in metabolising drugs can interfere with other metabolic roles and cause the unwanted side effects of some medicinal drugs.
• Alcohol/ethanol = drug that depresses nerve activity and contains chemical potential energy which can be used for respiration.
• Alcohol broken down by hepatocytes by action of enzyme ethanol dehydrogenase which results in product ethanal.