Excretion

• Substances accumulated by animals

  • Ammonia
  • Urea
  • Uric acid
  • Carbon dioxide
  • Water
  • Ions like Na+, K+, Cl–, phosphate, sulphate, etc.
• Mechanisms of elimination of substances


  Special emphasis on common nitrogenous wastes
• Major forms of nitrogenous wastes excreted by animals

  • Ammonia
  • Urea
  • Uric acid
• Ammonotelism


  The process of excreting ammonia
• Ureotelic animals

  • Mammals
  • Many terrestrial amphibians
  • Marine fishes
• Uricotelic animals


  • Reptiles
  • Birds
  • Land snails
  • Insects
• Humans have a pair of kidneys, one pair of ureters, a urinary bladder, and a urethra
• Kidneys are reddish-brown, bean-shaped structures situated between the levels of the last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity
• Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 g
• The inner concave surface of the kidney has a notch called hilum through which ureter, blood vessels, and nerves enter
• The outer layer of the kidney is a tough capsule
• Inside the kidney, there are two zones, an outer cortex and an inner medulla
• The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces
• The cortex extends in between the medullary pyramids as renal
• Processes involved in urine formation

  • Glomerular filtration
  • Reabsorption
  • Secretion
• First step in urine formation

  Filtration of blood by the glomerulus called glomerular filtration
• On average, 1100-1200 ml of blood is filtered by the kidneys per minute, roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute
• Glomerular filtration

  Process where blood is filtered through 3 layers: endothelium of glomerular blood vessels, epithelium of Bowman’s capsule, and a basement membrane between these two layers
• Podocytes

  Epithelial cells of Bowman’s capsule arranged intricately to leave minute spaces called filtration slits or slit pores
• Almost all constituents of the plasma except proteins pass onto the lumen of Bowman’s capsule through the filtration membranes, considered ultrafiltration
• Amount of filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR), approximately 125 ml/minute in a healthy individual, i.e., 180 litres per day
• Juxta glomerular apparatus (JGA)

  Efficient mechanism for regulating glomerular filtration rate, activated by a fall in GFR releasing renin to stimulate glomerular blood flow
• Juxtaglomerular apparatus (JGA)


  A special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. A fall in GFR can activate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR back to normal
• A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres) suggests that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules
• Reabsorption


  The process where tubular epithelial cells in different segments of the nephron absorb substances actively or passively, such as glucose, amino acids, Na+, etc., actively and nitrogenous wastes passively. Reabsorption of water also occurs passively in the initial segments of the nephron
• Tubular cells during urine formation

  Secrete substances like H+, K+, and ammonia into the filtrate. Tubular secretion is important in urine formation as it helps in the maintenance of ionic and acid-base balance of body fluids
• Proximal Convoluted Tubule (PCT) is lined by simple cuboidal brush border epithelium which increases the surface area for reabsorption. Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and water are reabsorbed by this segment. PCT also helps to maintain the pH and ionic balance of the body fluids by selective secretion of hydrogen ions, ammonia and potassium ions into the filtrate and by absorption of HCO3– from it
• Reabsorption is minimum in the ascending limb of Henle’s Loop. The descending limb is permeable to water but almost impermeable to electrolytes, concentrating the filtrate as it moves down. The ascending limb allows transport of electrolytes actively or passively, diluting the concentrated filtrate as it moves upward
• Distal Convoluted Tubule (DCT) allows conditional reabsorption of Na+ and water. It is also capable of reabsorption of HCO3– and selective secretion of hydrogen and potassium ions and NH3 to maintain the pH and sodium-potassium balance in blood
• Collecting Duct extends from the cortex of the kidney to the inner parts of the medulla. Large amounts of water could be reabsorbed from this region to produce a concentrated urine. This segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity. It also plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of H+ and K+ ions
• Mechanism of concentration of the filtrate

  Mammals have the ability to produce a concentrated urine. The Henle’s loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern. The proximity between the Henle’s loop and vasa recta, as well as the counter current in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium
• Transport of NaCl and urea facilitated by the special arrangement of Henle’s loop and vasa recta

  NaCl is transported by the ascending limb of Henle’s loop, exchanged with the descending limb of vasa recta, and returned to the interstitium by the ascending portion of vasa recta. Small amounts of urea enter the thin segment of the ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule. This transport is called the counter current mechanism
• The counter current mechanism helps to maintain a concentration gradient in the medullary interstitium
• Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed
• Regulation of kidney function


  The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA, and to a certain extent, the heart. Osmoreceptors in the body are activated by changes in blood volume, body fluid volume, and ionic concentration. An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback. ADH can also affect the kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR. The JGA plays a complex regulatory role. A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR. Angiotensin II also activates the adrenal cortex to release Aldosterone. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR. An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure. ANF mechanism acts as a check on the renin-angiotensin mechanism
• Micturition

  Urine formed by the nephrons is carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS). The signal is initiated by the stretching of the urinary bladder as it gets filled with urine. In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine. The process of release of urine is called micturition and the neural mechanisms causing it is called the micturition reflex. An adult human excretes, on average, 1 to 1.5 litres of urine per day. The urine formed is a light yellow coloured watery fluid which is slightly acidic (pH-6.0) and has a characteristic odour. On average, 25-30 gm of urea is excreted per day. Various conditions can affect the characteristics of urine. Analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney. For example, the presence of glucose (Glycosuria) and ketone bodies (Ketonuria) in urine are indicative of diabetes mellitus
• Analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney
• Indications of diabetes mellitus in urine

  • Presence of glucose (Glycosuria)
  • Presence of ketone bodies (Ketonuria)
• Organs involved in excretion

  • Kidneys
  • Lungs
  • Liver
  • Skin
• Lungs remove large amounts of CO2 and water daily