Category Archives

6 Articles
Role of Hormones in Osmoregulation

Role of Hormones in Osmoregulation

Role of Hormones in Osmoregulation

  • Osmoregulation is the maintenance of water potentials within the blood.
  • The amount of water potential found within the blood is detected by osmoreceptors located in the hypothalamus.

Water potential is reduced by:

  1. Sweating
  2. Limited Consumption of water
  3. Breathing
  4. Urinating
  5. A change in water potential is detected by osmoreceptors in the hypothalamus
  • When water potential is low, there is a high-water potential in the osmoreceptors relative to outside (the blood) therefore, water will move out of the osmoreceptors through osmosis and therefore reduce in size.
  1. The osmoreceptors triggers the hypothalamus to release Antidiuretic Hormone (ADH)
  2. ADH moves to the posterior pituitary gland before being secreted into the capillaries to travel to the kidneys
  • ADH causes an increase the permeability to water of the cell surface membrane of the cells lining the distal convoluted tubules and collecting duct by doing the following:
  1. Specific protein receptors on the cell surface membrane of these cells bind to the ADH leading to activation of phosphorylase within the cell
  2. This results in the vesicles within the cell to move to and fuse with the cell surface membrane
  3. Vesicles contain pieces of plasma membrane which have numerous water channel proteins, called aquaporin, so by fusing with the plasma membrane the number of water channels increase which thus increases the cell surface membranes permeability
  4. ADH increases the permeability of the collecting duct to urea which therefore passes out, further lowering the water potential of the fluid around the collecting duct.
  5. Overall, more water leaves the collecting duct by osmosis back into the blood
  • b. water potential of the blood does not, and cannot increase however can be maintained to reduce further water loss.

Antidiuretic Hormone (ADH)

  • Water potential is measured by osmoreceptors in the hypothalamus
  • A decrease in water potential results in:
    1. water will move out of the osmoreceptor cells by osmosis
    2. Causes the cells to decrease in volume
    3. Sends a signal to other cells in the hypothalamus
    4. Hypothalamus sends signal to posterior pituitary gland
    5. Posterior pituitary gland releases antidiuretic hormone into blood
    6. AHD binds to receptors on plasma membrane of cells in the Distal convoluted tubules and collecting duct
    7. Aquaporin (protein channels) are inserted into the plasma membrane which allow water to pass (osmosis) making the walls of the DCT and collecting duct more permeable to water
    8. More water is reabsorbed from these tubules into the medulla and into the blood by osmosis
    9. A small amount of concentrated urine is produced

Dehydration

  • Water content of the blood drops (water potential decreases)
  • Detected by osmoreceptors in hypothalamus
  • Posterior pituitary gland is stimulated to release more ADH into blood
  • More ADH means the DCT and collecting duct are more permeable to water
  • More water is reabsorbed
  • Small amount of concentrated urine

Hydration

  • Water content of the blood increases (water potential increases)
  • Detected by osmoreceptors in hypothalamus
  • Posterior pituitary gland is stimulated to release less ADH into blood
  • Less ADH means the DCT and collecting duct are less permeable to water
  • Less water is reabsorbed
  • large amount of dilute urine
Regulation of Body Temperature

Regulation of Body Temperature

  • Mechanisms for Heat loss

    • Evaporation of water
      • (i.e. sweating)
    • Loss of heat to the environment
      • Through conduction (e.g. from the ground)
      • Through convection (e.g. to the surrounding air or water)
      • Radiation
    • Vasodilation
      • Diameter of arterioles near surface are made larger
      • Increases volume of blood reaching the skin surface through the capillaries
    • Increased Sweating
      • Evaporate more water from the skin surface requires (heat) energy
      • Occurs on skin in skin only animals and on paws/tongue on fur animals
    • Lowering of body hair
      • Hair erector muscles in skin relax and elasticity of the skin so hair lowers
      • Reduces thickness of layer of still air
    • Behavioural Mechanism:
      • Avoiding the heat of the day by sheltering
      • Seeking out shade

    Mechanisms for Heat gain

    • Production of heat
      • Through metabolism of food during respiration
    • Gain of heat from the environment
      • Through conduction (e.g. from the ground)
      • Through convection (e.g. to the surrounding air or water)
      • Radiation
    • Vasoconstriction
      • Diameter of arterioles near surface are made smaller
      • Reduces volume of blood reaching the skin surface through the capillaries
      • Most of the blood entering the skin therefore passes beneath the insulating layer of fat and so loses less heat
    • Shivering
      • Muscles of the body undergo involuntary rhythmic contractions which produces metabolic heat
    • Raising of hair
      • Hair erector muscles in skin contract and so hair raises
      • Enables a thicker layer of still air which forms a layer of insulation
    • Increased metabolic rate
      • More of the hormones that increase metabolic rate are produced
      • As a result, metabolic activity, including respiration is increased and so more heat is generated
    • Decrease in Sweating
      • In cold conditions sweating ceases
    • Behavioural Mechanisms
      • Sheltering from the wind
      • basking in the sun
      • huddling together

    Endo/ectotherms

    • Birds and mammals are endotherms as they derive most of their heat from metabolic activity
    • All other animals are ectotherms as they obtain their heat from sources outside their body

    Control of Body Temperature

    • The change in temperature is detected by thermoreceptors
    • The receptors pass to the hypothalamus (coordinator) which is located in the brain
    • It results in the effectors in the skin to yield a response
    • Within the hypothalamus there are thermoregulatory centres of which:
      • Heat gain centre: activated by a fall in blood temperature. It controls mechanisms that increase body temperature
      • Heat loss centre: activated by an increase in blood temperature. It controls mechanisms that decrease body temperature
    • Hypothalamus measures blood temperatures running through it
    • Thermoreceptors measure temperature at the skin. They send impulses along the autonomic nervous system to the hypothalamus

Kidneys

Kidneys

  • Nephron Structure

    • Long tubules along with bundles of capillaries where the blood is filtered are through the nephrons
    • one million nephrons per kidney
    • The top section of the kidney lies in the renal cortex whereas the lower section is in the medulla

    Ultrafiltration

    “Ultrafiltration is the movement of blood being forced at high pressure against the basement membrane, which optimises filtration”

    Structure:

    • Ultrafiltration occurs in the glomerulus and the glomerular filtrate (GF) before passing into the Bowman’s capsule.
    • Renal artery branches into one million arterioles where each one enters into a renal (Browman’s) capsule of a nephron.
    • The afferent arteriole divide to give the glomerulus which later forms the efferent arteriole.
    • The arteriole leading into the glomerulus is wider than the one leading out, therefore the resulting blood pressure is high in the capillaries of the glomerulus
    • The Capillary wall and the capsule walls are formed from a epithelial cells, with gaps between them. Therefore, the molecules with a molecular mass of < 68,000 are filtered out of the blood to form a filtrate in the renal capsule.
    • The inner lays of the renal capsule is lined with podocyte cells with regular intervals between them. This is optimised to allow filtrate to pass between the gaps. Filtrate passes through the gaps: not through them.
    • Endothelium of the glomerular capillaries too have spaces between them (approx. 100nm wide between cells)

    Process:

    • The renal artery splits into arterioles which run between different nephrons, called the afferent arteriole.
    • One of the branches of renal capillaries enters into the Renal’s (Bowman’s) capsule
    • The Renal capillaries form a ‘knot’ inside the capsule, the
    • The result of the high pressure , which causes resistance by:
      • Capillary epithelial cells
      • Connective tissue and epithelial cells of the capillary
      • Epithelial cells of the renal capsule a
      • Hydrostatic pressure of the fluid in the renal capsule space
      • Low water potential of the blood when in the glomerulus
    • The total resistance will prevent the filtrate from leaving the glomerular capillaries.
    • The high pressure results in the plasma moving out of the blood
    • What remains in the capillaries is only blood cells and large proteins (e.g. antibodies)
    • The kidneys produce about 180 litres of glomerular filtrate per day.
    Before Ultrafiltration After Ultrafiltration
    Red blood Cells Red blood Cells
    White blood cells White blood cells
    Platelets Platelets
    Other Large Proteins Large Proteins
    Glucose
    Water
    Urea
    Salts
    Blood plasma
    • Useful products such as glucose, salts, water etc. are reabsorbed back into the blood through the next step, Selective Reabsorption

    Selective Reabsorption

    “The absorption of some of the components of the glomerular filtrate back into the blood as the filtrate flows through the nephrons of the kidney.”

    • Selective reabsorption occurs as the glomerular filtrate flows along the proximal convoluted tubules (PCT) through the loop of Henle and along the distal convoluted tubule (DCT)
    • Useful substances leave the tubules of the nephrons and enter the capillary network
    • Epithelium of the wall of the PCT contains microvilli which proves a large surface area for the reabsorption of useful materials from the glomerular filters (in the tubules) into the blood
    • Useful solutes (i.e. glucose) are reabsorbed along the PCT via active transport and facilitated diffusion
    • Water enters the blood by osmosis
      • Water potential of blood is lower than the filter
      • Water is absorbed from the PCT, Loop of Henle, DCT and collecting duct
    • Filtrate which remains is urine which passes through the ureter to the bladder

    Urine

    • Urine comprises mostly of water and dissolves salts, urea and other substances (notably hormones and excess vitamins)
    • Urine should not normally contain proteins or blood cells
      • Due to their large size they should not be able to pass out normally
    • Glucose is actively reabsorbed back into the blood so it is not usually found in blood either
Introduction

Introduction

  • “The maintenances of a constant internal environment”

    • The internal environment is made up of tissue fluid
    • Maintaining the features of this fluid at the optimum levels protect the cells from changes in the external environment which gives the organism a degree of impendence
    • Homeostasis includes the maintaining of the chemical makeup, volume and other features of blood and tissue fluids within restricted limits
    • It ensures that all cells meet their needs to function normally despite external changes
    • Homeostasis is the ability to return to a set point and so maintain organisms in a balanced equilibrium

    Importance

    • Enzymes controlling biochemical reaches within cells and proteins are sensitive to changes in pH and temperature, resulting in lower effectiveness or denaturing if requirements are not met
    • Changes to the water potential of the blood and tissue fluids may cause cells to shrink/expand as a result of water leaving/entering by osmosis. Therefore, the cell cannot function normally.
    • Maintenance of blood glucose levels is essential in ensuring a constant water potential and for respiration of the cells
    • Enables the organisms to be less dependent on the environment

    Control Mechanisms

    • Set Point: The desired level for optimum performance
    • Receptor: Detects any deviations from the set point and informs the controller
    • Controller: Coordinates information from various receptors and seconds instructions to an effector
    • Effector: Brings about the change required to return the system back to set point
    • Feedback Loop: Informs the receptors of the change to the system brought about by the effector
Diabetes

Diabetes

  • Type 1

    Type 2

    Immune system attacks the β-cells in the islet of Langerhans 

    Glycoprotein receptors on body cells lose responsiveness to inulin

    Insulin is not produced by the body

    Insulin is produced by the body, however cells do not respond to it. In some cases pancreas may become ineffective of producing insulin)

    Diagnosed typically in young people

    Diagnosed typically in adults

    Insulin Dependent

    Insulin independent

    Control of Diabetes

    Type 1

    Type 2

    Controlled by injections of insulin (must be injected due to being a protein it will get digested in alimentary canal)

    Controlled by regulating the intake of carbohydrates in diet and regular exercise

    Injected 2-4 times a day

    Can be supplemented by injections of inulin or drugs which stimulate insulin production

    Dosage must match to glucose intake

    Also can be supplemented by drugs which reduce rate of glucose uptake by cells from the intestine

Control of Blood Glucose Concentrations

Control of Blood Glucose Concentrations

  • Normal Blood glucose concentration: 90 mg/100cm-3
  • Monitored by the pancreas
  • Increases due to consumption of carbohydrates
  • Decreases due to exercise

Hormonal Control of Blood Glucose Concentration

  • Blood glucose concentration is the result of the hormones insulin and glucagon
  • Insulin and glucagon are secreted by islet of Langerhans found in the pancreas.
  • 2 types Islet of Langerhans
    • Alpha Cells
      • Secretes glucagon into the blood
    • Beta cells
      • Secretes insulin into the blood

Insulin

  • Lower blood glucoses concentrations (when too high)
  • Binds to specific receptors on cell membranes on muscle cells and hepatocytes (liver cells)
  • Increases permeability of muscle-cell membranes to glucose so that the cells take up more glucose
    • It does this by increasing the number of channel proteins on the cell membrane
  • Glycogenesis is the activation of an enzyme in muscle and liver cells that convert glucose à glycogen
    • Cells are able to store glycogen in cytoplasm

Glucagon

  • Raises blood glucose concentration (when too low)
  • Binds to specific rececptors on the cell membrane of liver cells
  • Glycogenolysis is the activation of an enzyme that break down Glycogen à Glucose

  • Gluconeogenesis is the activation of a different enzyme which is involved in the formation of glucose from glycerol and amino acids

Overall Process of glycogenolysis and gluconeogenesis

Rise in Glucose Blood Concentration

  • Pancreas detect blood glucose concentration is too high
  • Beta cells secrete insulin and the alpha cells stop secreting glucagon
  • Insulin binds to receptors on liver and muscle cells
  • Liver and muscle cells respond to decrease the blood glucose concentration (glycogenesis)
  • Blood glucose concentrations return back to normal

Fall In Glucose Blood Concentration

  • Pancreas detect blood glucose concentration is too low
  • alpha cells secrete glucagon and beta cells stop secreting insulin
  • Glucagon binds to receptors on liver
  • Liver respond to increase the blood glucose concentration (glycogenolysis)
  • Blood glucose concentrations return back to normal

Glucose Transporters

  • Type of channel protein
  • Allows glucose to be transported across cell membrane
  • Skeletal and cardiac muscles contain a type of glucose transporter (GLUT4)
  • Insulin levels are low:
    • GLUT4 is stored in vesicles in the cytoplasm
  • Insulin Levels are high:
    • GLUT4 moves to the cell membrane
    • Glucose can be transported into the cell using the GLUT4 protein via facilitated diffusion

Adrenaline

  • A hormone that is secreted from the adrenal gland (kidneys)
  • Secreted when low concentration of glucose in blood during stressed activity
  • Adrenaline binds to receptors on cell membrane of liver cells to:
    • Activates glycogenolysis
    • Inhibits glycogenesis
    • Activates glucagon secretion
    • Inhibitors insulin secretion
  • Overall adrenaline increase the glucose concentration for muscle response
  • Type of second messenger
    • Both adrenaline and glucagon activate glycogenolysis inside a cell despite there only being binding to receptors on the outside of the cell
    • Second messenger model is the binding of the hormone to cell receptors which activate an enzyme on the inside of the cell membrane which bring about a response