lesson 5

• Osmoregulation


  Regulation of water and ion balance
• Excretion


  1. Elimination of metabolic wastes including nitrogenous wastes produced from the breakdown of proteins

  2. Helps in the regulation of water and ion balance
• Ammonia


  • The primary nitrogenous waste for aquatic invertebrates, teleost, and larval amphibians
  • Readily soluble in water but also highly toxic
  • Can be excreted from the body only in dilute solutions
• Urea


  • Produced by mammals, most amphibians, some reptiles, some marine fishes, and some terrestrial invertebrates
  • Formed by combining ammonia with bicarbonate ion (HCO3-) and converting the product into urea
  • About 100,000x less toxic than ammonia
  • Its excretion requires only about 10% as much water compared to ammonia
• Uric acid


  • Excreted by birds, insects, and terrestrial reptiles
  • Relatively nontoxic but more energetically expensive to produce than urea
  • Largely insoluble in water and it is excreted as a semisolid paste or precipitate with very little water loss
• Nitrogenous Waste Products


  • Amino Acids
  • Nitrogenous Bases
  • Amine groups
• Cell surface or cell membrane


  • Allows passage of wastes in unicellular organisms
• Contractile vacuole


  • A specialized cytoplasmic organelle in many freshwater protists (e.g. Paramecium) that expels excess water out of the cell to prevent lysis
• Protonephridia or Flame Bulb System


  • Network of tubules that lack internal openings but have external openings at the body surface called nephridiopores
  • The smallest branches of the tubule network end with a large cell called a flame bulb or cell
  • Water and solutes in body fluids enter the flame cell and get filtered
  • Specific molecules and ions are removed by reabsorption while other ions and nitrogenous wastes are released into the tubule network and excreted via the nephridiopore
• Metanephridia


  • The excretory tubule of most annelids and adult mollusks
  • The tubular network has a funnel-like internal opening called a nephrostome that collects body fluids
  • As the body fluids move through the network, some molecules and ions are reabsorbed while other ions and nitrogenous wastes are secreted into the tubule
  • The bladder stores the nitrogenous wastes as urine and later on excreted from the body surface via the nephridiopore
• Malpighian Tubules

  • The excretory tubules of insects and other terrestrial arthropods
  • The tubules have ends that are immersed in the hemolymph (circulatory fluid) while the distal ends empty into the gut
  • Malpighian tubules do not filter body fluids; instead they employ secretion to generate the fluid for release from the body
  • They help actively secrete uric acid and ions like Na+ and K+ into the tubules, allowing the water to move osmotically from the hemolymph into the tubule
  • The fluid then passes into the hindgut (intestine and rectum) of the insect as dilute urine
  • Reabsorption of ions and water occurs in the hindgut wall, causing the formation of uric acid crystals that are released with the feces
• Mammalian urinary system


  Consists of two kidneys, each with a ureter, a tube leading to a urinary bladder (for storage), with an open channel called urethra leading to the body surface
• Kidneys


  • Serve as specialized organs for osmoregulation and excretion
• Parts of the kidney


  • Renal capsule -the outer coat of connective tissue
  • Cortex -the zone near the capsule consisting of blood vessels and nephrons
  • Medulla -inner zone also consisting of blood vessels and nephrons
  • Nephrons -the functional units of the kidney where urine is formed
  • Renal pelvis -central cavity in the kidney where urine coming from the nephrons is channeled before going to the ureter
• Each kidney contains about 1.3 M nephrons, approximately 80 km long if connected end to end
• About 1,600 liters of blood pass through the kidneys each day (300x the blood volume); approximately 180 liters become filtrate but only about 1.5 liters of urine get excreted
• More than 99% of the water and almost all sugar, vitamins and other organic nutrients are reabsorbed across the tubule epithelium
• Nephrons


  • Functional units of kidney
• Nephrons


  1. Bowman's capsule

  2. Glomerulus

  3. Renal tubules

  4. Peritubular capillaries

  5. Collecting duct
• Bowman's capsule


  An in-folded region that encloses a network of blood capillaries called the glomerulus where initial filtration of the blood plasma occurs
• Renal tubules

  Receive and modify the glomerular filtrate, consisting of a proximal convoluted tubule, followed by a U-shaped loop of Henle, and a distal convoluted tubule
• Peritubular capillaries

  Bring substances to and take substances away from the renal tubules
• Collecting duct

  Receives the urine from the renal tubule leading to the renal pelvis
• Nephrons filter and retain water and solutes, leaving concentrated urine to be collected in the central renal pelvis
• The glomerulus serves as the initial site for filtration and the glomerular filtrate produced is directed into the Bowman's capsule
• Filtration and reabsorption in nephrons

  1. Bowman's capsule collects the filtrate

  2. Filtrate directed through the continuous renal tubules: proximal tubule, loop of Henle, distal tubule, collecting duct, renal pelvis

  3. Peritubular capillaries exit the glomerulus, converge, then branch again around the nephron tubules where they participate in reclaiming water and essential solutes
• The Mechanism of Urine Formation

  1. Filtration

  2. Tubular reabsorption

  3. Tubular secretion
• Filtration

  • Blood pressure forces filtrate (water and small solutes) out of the glomerular capillaries
  • Blood cells, proteins, and other large solutes cannot pass the capilary wall and they remain in the blood
  • Filtrate is collected by the Bowman's capsule and funneled into the proximal tubule
• Tubular reabsorption


  • Useful materials such as salts, water, glucose, and amino acids move out from the renal tubules and into adjacent peritubular capillaries
• Tubular secretion


  • Surplus hydrogen and potassium ions, uric acid, toxins and other drugs move from the blood into the renal tubules
• Factors influencing filtration


  • Blood enters the glomerulus under high pressure in order to facilitate filtration
  • Glomerular capillaries are highly "leaky" to water and small solutes
  • The volume of blood flow affects the rate of filtration
• Regulation of mammalian kidney function

  1. Receptors in the juxtaglomerular apparatus function in the kidney's autoregulation system

  2. Receptors trigger constriction or dilation of the afferent arteriole to keep blood flow and filtration constant during small variations in blood pressure

  3. Antidiuretic hormone (ADH) promotes water conservation

  4. ADH is secreted from the hypothalamus via the pituitary when osmoreceptors detect an increase in the osmolarity of body fluids

  5. ADH makes the walls of distal tubules and collecting ducts more permeable to water, and thus the urine becomes more concentrated

  6. Aldosterone enhances sodium reabsorption

  7. When too much sodium is lost, extracellular fluid volume is reduced, and pressure receptors detect corresponding drop in blood pressure

  8. The kidney secretes an enzyme, renin, which indirectly stimulates the adrenal cortex to secrete aldosterone, which in turn stimulates reabsorption in the distal tubule and collecting ducts

  9. Cells in the hypothalamus thirst center inhibit saliva production and stimulate the urge to drink when there is an increase in the solute concentration in extracellular fluid
• Acid-base balance


  Maintained by controlling hydrogen ions through buffer systems, respiration, and excretion by the kidneys
• Only the urinary system can eliminate excess hydrogen ions, permanently, and restore the bicarbonate buffering ions to the blood
• Receptors in the juxtaglomerular apparatus

  • Function in the kidney's autoregulation system
  • Trigger constriction or dilation of the afferent arteriole to keep blood flow and filtration constant during small variations in blood pressure
• Antidiuretic hormone (ADH)


  Promotes water conservation
• ADH secretion


  1. Secreted from the hypothalamus via the pituitary

  2. When osmoreceptors detect an increase in the osmolarity of body fluids
• ADH effect


  • Makes the walls of distal tubules and collecting ducts more permeable to water
  • Urine becomes more concentrated
• Aldosterone


  Enhances sodium reabsorption
• Aldosterone secretion


  1. When too much sodium is lost, extracellular fluid volume is reduced, and pressure receptors detect corresponding drop in blood pressure

  2. Kidney secretes renin, which indirectly stimulates the adrenal cortex to secrete aldosterone

  3. Aldosterone stimulates reabsorption in the distal tubule and collecting ducts