INTRODUCTION All living cells have many metabolic reactions that go on inside them Metabolic reactions often produce main products and by-products (waste products) The main products are useful to the cells but the by-products (waste products) are not needed by the cells If these waste products are allowed to stay in the cell, they may poison the cell and so need to be removed from the cell EXCRETION Excretion is the removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials and substances in excess of requirements DIFFERENCE BETWEEN EXCRETION AND EGESTION Excretion is the removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials and substances in excess of requirements Egestion is the passing out of food that has not been digested or absorbed, as faeces, through the anus EXCRETORY PRODUCTS Excretory products are waste products which are made in a cell as a result of a metabolic reaction Toxic materials and substances in excess of requirements excreted in humans include: Waste products of metabolism such as carbon dioxide, excess water, excess salts, bile pigments etc Products of the breakdown of substances in excess of requirements such as urea Spent hormones EXCRETORY ORGANS IN HUMANS There are four main organs responsible for excretion in humans which are: The liver The lungs The kidneys The skin ROLE OF THE LIVER IN THE ASSIMILATION OF AMINO ACIDS The liver removes amino acids from the plasma of the bloodstream and builds them up into proteins including plasma proteins such as fibrinogen which plays a very important role in blood clotting DEAMINATION Deamination is the removal of the nitrogen-containing part of amino acids to form urea The part of the amino acid molecule that contains energy is converted to carbohydrate and stored while urea (formed from the nitrogen-containing part) is excreted by the kidneys THE LIVER AND DEAMINATION The liver is the organ responsible for deamination of excess amino acids in the body It contains enzymes that split up each amino acid molecule FUNCTIONS OF THE LIVER It converts excess amino acids into urea and carbohydrates in the process of deamination It synthesizes plasma proteins such as fibrinogen from amino acids It controls the amount of glucose in the blood with the help of the hormones, insulin and glucagon It stores carbohydrate as the polysaccharide glycogen it breaks down old red blood cells, storing the iron and excreting the remains of the haemoglobin as bile pigments It breaks down harmful substances such as alcohol It stores vitamins A, B, D, E and K It stores potassium It makes cholesterol which is needed to make and repair cell membranes ROLE OF THE LIVER IN EXCRETION It is responsible for the removal of the nitrogen-containing part of amino acids to form urea (i.e. deamination of excess amino acids) It breaks down hormones, toxins, drugs and excess vitamins It breaks down worn out red blood cells and excretes bile THE NEED FOR EXCRETION Carbon dioxide is toxic because it dissolves in fluids to form carbonic acid which affects the action of enzymes and can be fatal if it builds up Ammonia which is made in the liver is very toxic and is therefore converted to urea which is less toxic and must be removed from the body THE EXCRETORY SYSTEM The excretory system comprises two kidneys connected to the urinary bladder by two ureters A tube called the urethra leads from the bladder to the outsides The renal arteries supply blood to the kidneys while the renal veins take blood away from the kidneys The kidneys are at the back of the abdomen, behind the intestines STRUCTURE OF THE KIDNEY A longitudinal section through the kidney show that is has three main regions namely: The dark outer region called the cortex The lighter inner zone called the medulla A space beneath the medulla called the pelvis that joins to the ureter A close look at the kidney under the microscope reveals that the kidney tissue consists of many capillaries and tiny tubes/tubules known as kidney/renal tubules or nephrons, held together by connective tissue STRUCTURE OF THE KIDNEY TUBULE/NEPHRON Each kidney tubule begins with a cup-shaped structure in the cortex called the Bowman’s or Renal capsule The renal capsule leads to the first coiled tubule The renal tubule then loops down into the medulla and back into the cortex to form the loop of Henle The loop of Henle then leads to the second coiled tubule, then to the collecting duct which goes down the medulla to the pelvis, where the tubules join up with the ureter The renal capsule has a coiled network or knot of capillaries inside it called the glomerulus A branch of renal artery leads into the glomerulus and branches out to form the renal vein The kidney tubule is surrounded by a dense network of capillaries FUNCTIONS OF THE KIDNEY To filter the blood by removing metabolic wastes such as urea, excess water, and excess salts as urine To maintain the osmotic balance of the blood by reabsorbing glucose, some water and some salts from the filtrate into the blood URINE FORMATION The removal of urea, excess salts and excess water as urine involves two main process, namely: Filtration/ultrafiltration Reabsorption or selective reabsorption FILTRATION The branch of renal artery bringing blood to the glomerulus is wider than the one taking blood away This creates a very high pressure within the glomerulus The pressure causes small molecules such as glucose, urea, water, and salts to leak out of the plasma in the glomerulus into the renal capsule ROLE OF THE GLOMERULUS IN THE FILTRATION It has tiny gaps in its walls that allow small molecules such as glucose, water, salts and urea to pass out into the renal capsule NOTE Red blood cells and plasma proteins are not filtered out of the blood in the glomerulus because they are too big So they stay in the blood REABSORPTION The solution of glucose, urea, water and salts filtered out travel along the renal tubule As it travels, all of the glucose, most of the water and some of the salts are reabsorbed and pass back into the blood in the capillaries surrounding the renal tubule Reabsorption leads to concentration of urea in the urine as well as loss of excess water and salts SUBSTANCES REMOVED AND REABSORBED INTO THE BLOOD ROLE OF THE TUBULE IN THE REABSORPTION The walls of the renal tubule are and allow glucose, some water, and some salts to be reabsorbed back into the blood URINE FORMATION The remaining fluid passes along the renal tubule into the pelvis, from where it transported by the ureters to the bladder to be stored temporarily Periodically, the sphincter muscles relax to empty the bladder EFFECT OF WATER INTAKE ON THE VOLUME AND CONCENTRATION OF URINE After drinking a lot of fluid/water, a large volume of dilute urine is produced This is because the kidneys adjust the concentration of the blood flowing through them If it is too dilute (i.e. has too much water), less water is reabsorbed, leaving more to enter the bladder and a large amount of dilute urine is produced If it is too concentrated (i.e. has too little water), more water is reabsorbed, leaving less to enter the bladder and a little amount of concentrated urine is produced EFFECT OF TEMPERATURE ON THE VOLUME AND CONCENTRATION OF URINE On a cold day, sweating decreases so more water is removed from the blood by the kidneys, increasing the volume of dilute urine produced On a hot day, sweating increases so less water is removed from the blood by the kidneys, decreasing the volume of concentrated urine produced EFFECT OF EXERCISE ON THE VOLUME AND CONCENTRATION OF URINE During sweating, sweating increases so less water is removed from the blood by the kidneys, decreasing the volume of concentrated urine produced DIFFERENCES IN COMPOSITION BETWEEN THE BLOOD PLASMA AND THE URINE KIDNEY DISEASES One major disease of the kidney is kidney failure Kidney failure may result from an accident involving a drop in blood pressure or from an infection of the kidneys Kidney failure allows wastes (urea etc) to build up in the blood and will cause death if not treated TREATMENT OF KIDNEY FAILURE Kidney failure may be treated by Dialysis or Kidney transplant DIALYSIS Dialysis is a method of regulating the salt balance, maintaining glucose concentration and removing urea from the blood using the process of diffusion THE DIALYSIS MACHINE A dialysis machine consists of a long partially permeable cellulose tube coiled up in a water bath called the dialysis fluid The dialysis fluid contains water, glucose, salts and other substances found in the blood in the correct composition but NOT urea nor uric acid HOW THE DIALYSIS MACHINE WORKS Blood is removed from a vein in the arm, and is kept moving through dialysis tubing in the dialysis machine using a pump The tubing is very long to provide a large surface area The dialysis membrane is partially permeable Urea or uric acid and excess mineral salts are removed from the blood, by diffusion, down their concentration gradients into the dialysis fluid Water moves from the blood by osmosis down a water potential gradient into the dialysate (dialysis fluid) Essential salts and glucose are NOT removed from the blood as their concentration in the dialysis fluid is same to that of the blood Proteins and blood cells are too large to move across the dialysis membrane The bathing solution is changed regularly to maintain a concentration gradient and also kept at body temperature The cleaned blood is then passed through a bubble trap to remove any air bubbles, before being returned to the patient’s vein SUBSTANCES REMOVED FROM THE BLOOD BY DIALYSIS Urea ; Ammonia ; Uric acid ; Creatinine ; (named) salt / ions ; e.g. Na+, Cl–, Mg2+, Ca2+, HCO3 Water ; toxins ; Hormones ; KIDNEY TRANSPLANT This involves removing an old malfunctioning kidney and replacing with a better one by surgery ADVANTAGES OF DIALYSIS OVER KIDNEY TRANSPLANTS The machine will be available to other patients to use The patient can return to a normal lifestyle DISADVANTAGES OF DIALYSIS OVER KIDNEY TRANSPLANTS It may require a lengthy session in hospital, three times a week, leaving the patient very tired after each session Dialysis machines are very expensive to buy and maintain ADVANTAGES OF KIDNEY TRANSPLANTS OVER DIALYSIS There is no need for regular visits to the hospital The patient is less unwell, tired etc There is fewer diet or fluid intake restrictions There are no needles or fistula permanently in the arm DISADVANTAGES OF KIDNEY TRANSPLANTS OVER DIALYSIS Transplants require a compatible donor with a good tissue match There is the risk of death or infection during or after the operation There is a risk of rejection of the donated kidney There is the need to take immuno-suppresant drugs to prevent tissue rejection Transplants are not accepted by some religions END OF CHAPTER
