Homeostasis, pregnancy and fertility

• Homeostasis is the maintenance of a constant internal environment
• The internal environment in mammals includes conditions:

  • body temperature
  • blood glucose concentration
  • salt concentration
  • pH
  • water potential
• Physiological control systems often rely on negative feedback mechanisms

  • deviations from the norm result in corrective processes being stimulated to restore the norm
  • once the norm has been re-established the corrective mechanisms are inhibited by negative feedback
  • negative feedback systems maintain an optimal internal state in the context of a dynamic equilibrium
• Thermoregulation can be used as an example of a physiological control system which operates to maintain a constant internal environment
• Thermoregulation in mammals

  • mammals are endotherms and are homeothermic within a narrow range
  • hypothalamus contains the thermoregulatory centre which receives nerve impulses from temperature receptors in the skin in addition to its own thermoregulatory monitoring the blood
  • hypothalamus contains two thermoregulatory centres
  • impulses reach each centre from the temperature receptors in the skin via sensory neurons
  • when stimulated each centre will transmit impulses to other parts of the body along motor neurones to stimulate corrective mechanisms to return to normal body temperature
• • Heat loss centre: anterior hypothalamus detects a rise in temperature
  • Heat gain centre: in the posterior hypothalamus detects a fall in temperature
• An increase in temperature

  • vasodilation
  • increased sweating
  • decreased metabolic activity
  • no shivering
  • body hairs flattened
  • behavioral responses
• An increase in temperature: vasodilation

  • nerve impulses in parasympathetic nerves from the hypothalamus causes smooth muscle in the skin arterioles to relax
  • shunt vessels constrict which increases blood flow to the surface capillaries leading to dilation of the capillaries
  • more heat is lost from the skin radiation
• An increase in temperature: increased sweating

  • sweat glands remove water from the blood and secrete it onto the skin surface
  • water uses body heat to evaporate producing a cooling effect
  • some mammals don't sweat and instead pant
• An increase in temperature: behavioral responses

  • moving into the shade
  • laying down exposure of a large surface area for heat loss
  • removal of outer layers of clothing
• A decrease in temperature

  • vasoconstriction
  • increased metabolic rate
  • shivering
  • piloerection
  • sweating inhibited
  • behavioral responses
• The liver

  • carries out many metabolic activities
  • important role in protein and glucose metabolism
  • essential for homeostasis
• The liver is supplied with blood from two vessels:

  1. the hepatic portal vein supplying high concentrations of digested food products
  2. hepatic artery supplying oxygenated blood
• Blood leaves the liver by the hepatic vein that transports deoxygenated blood to the vena cava
• Functions of the liver

  1. transamination: amino group from one amino acid can be transferred to a different keto-acid to produce a new amino acid
  2. deamination: involved the enzymatic removal of the amino group from the amino acid along with hydrogen to form ammonia
• Ornithine cycle

  • ammonia is highly toxic and combines with CO2 in the ornithine cycle to form urea
  • urea is soluble and transported in the blood from the liver to the kidneys where it is excreted in urine
  • keto acid produced is either used in respiration or is converted to carbohydrate and stored as glycogen
• Increase in blood glucose

  1. rise in blood glucose level following a meal is detected by the pancreas which secretes insulin from beta cells in the islets of Langerhans into the bloodstream
  2. insulin travels to the liver stimulating and increase in permeability to glucose
  3. the liver will use glucose, glycerol and amino acids in glycolysis and aerobic respiration
  4. excess glucose becomes glycogen by glycogenesis carried out by insulin, glycogen synthase is active and glycogen phosphorylase is inactive
  5. blood glucose levels falls to set off a negative feedback system
• Decrease in blood glucose

  1. blood glucose fall significantly stimulating the secretion of glucagon from alpha cells in the islets of Langerhans
  2. glucagon travels to the liver and stimulates enzymes to breakdown glycogen into glucose by glycogenolysis
  3. when glycogen levels are exhausted the liver converts amino acids, fatty acids and glycerol into glucose by gluconeogenesis
• Adrenaline on the liver

  • binds and initiates a series of reactions leading to the activation of enzymes
  • similar action to glucagon
  • acts in muscle where it stimulates glycogenolysis
  • the glucose phosphate this produces cannot be releases to increase blood glucose
  • supplies glucose for immediate use in glycolysis by the muscle only
• Hypoglyceamea

  • blood glucose falls too low
  • sweating, trembling, blurred vision, poor concentration
  • if untreated person can have a fit and enter a coma
• Hyperglycaemia

  • blood glucose rises too high
  • sickness, drowsiness, stomach pains
  • person could enter a coma
• Type 1 diabetes

  • caused by the loss of beta cells
  • individuals cannot produce insulin
  • managed by a low sugar diet and insulin injection
  • affects children and young adults
  • males and females equally affected
• Type 2 diabetes

  • caused by the tissue becoming less sensitive to the insulin in the circulation
  • managed by diet and symptoms often decrease with weight loss
  • affects older people
  • people who are overweight
  • affects more women than men
• Gestational diabetes

  • caused by pregnancy, usually disappears after childbirth
  • glucose in the urine is a symptom but also aggravated by high blood pressure towards the end of pregnancy
• Unless blood glucose level is managed by diet and insulin, it may reach approximately twice the normal levels:

  • may surpass the renal threshold
  • glucose cannot be recovered from the filtrate
  • glucose appears in the urine of a diabetic
• Osmoregulation is the maintenance of the water potential of the blood within restricted limits
• The kidneys

  • filter the blood (ultrafiltration) and then selectively reabsorb useful substances e.g. glucose, amino acids and water
  • metabolic waste, excess ions and varying amounts of water are left behind to form urine
  • the functional unit of the kidney is the nephron
• The nephron

  • the cortex (outer part) of each kidney contains the glomerulus, Bowman's capsule, proximal convoluted tubule and distal convoluted tubule
  • the loop of Henle and collecting duct of each nephron extend down into the medulla (inner part)
• The nephron

  • the cortex (outer part) of each kidney contains the glomerulus, Bowman's capsule, proximal convoluted tubule and distal convoluted tubule
  • the loop of Henle and collecting duct of each nephron extend down into the medulla (inner part)
• Ultrafiltration (part one)

  • occurs between glomerulus and Bowman's capsule of each nephron and results in the formation of glomerular filtrate
  • each glomerulus is supplied with blood by an affected arteriole which branches from the renal artery
  • the filtrate is forced out of the glomerulus into the renal capsule by the high blood pressure created by the contraction of the left ventricle
  • the blood pressure is further increased due to the afferent arteriole being wider than the efferent arteriole which take blood away from each glomerulus
• Ultrafiltration (part 2)

  • the barrier between blood in the capillary and filtrate in the renal capsule consist of two cells making three filtration layers
  • the endothelial cells of the capillary have pores allowing plasma to pass through but hold back blood cells
  • the endothelial cells are attached to the basement membrane (the main fine filter) and holds back the large plasma proteins in the plasma allowing only small molecules to pass through
  • the epithelium cells of the wall of the renal capsule are podocytes which extended to the basement membrane and allow filtrate to pass
• Ultrafiltration (step 3)

  • the glomerular filtrate which enters the renal capsule has small soluble components of the blood
  • blood cells, large plasma proteins and some water remain and enter the efferent arteriole
  • each efferent arteriole forms a capillary network around the rest of each nephron where useful substances from the filtrate are selectively reabsorbed
  • filtered blood eventually leaves the kidney by the renal vein
• Proximal convoluted tubule

  • all glucose and amino acids are reabsorbed back into the blood as the filtrate passes through the PCT
  • reabsorption is initially by facilitated diffusion and then active transport into the cells lining the PCT
  • most of the mineral ions and water are reabsorbed
  • absorption of water occurs via osmosis ; active transport of glucose lowers the water potential in the blood so water moves down its water potential gradient
• The cells lining the PCT have several adaptation for reabsorbing substances:

  • microvilli for large surface area
  • numerous mitochondria to provide ATP from aerobic respiration for active transport
  • carrier proteins in the cell-surface membranes for active transport
• The loop of Henle

  • ensures that a concentration gradient of sodium and chloride ions is created in the medulla of the kidneys
  • concentration of sodium and chloride ions increases deeper down the medulla
  • the increase in concentration of ions created a water potential gradient down the medulla with a lower water potential at the base of the loop
  • this enables more water to be reabsorbed by the collecting duct from the filtrate by osmosis
• Loop of Henle mechanism

  • the descending limb is permeable to water so some water leaves by osmosis into the tissue fluid and enters the surrounding blood capillaries
  • the loss of water increases the ion concentration in the filtrate at the base of the loop causing sodium and chloride ions to move from the lower part of the ascending limb into the medulla by diffusion
  • the cells in the upper part of the ascending limb then actively transports sodium and chloride ions from the filtrate into the surrounding tissue
  • ascending limb is impermeable to water
• Distal convoluted tubule and collecting duct (part one)

  • permeability to water is increased by ADH
  • ADH attaches to specific receptors on the cells of the DCT and collecting duct and stimulate aquaporins
  • aquaporins enable more water to be reabsorbed from these structures by osmosis down a water potential gradient
  • water that leaves the distal tubule and collecting duct passes their medullary tissue fluid and then into the surrounding blood capillaries which eventually join to form the renal vein
• Distal convoluted tubule and collecting duct (part two)

  • the increase in concentration of sodium and chloride ions down the medulla ensures that there is always a lower water potential in the medulla through which filtrate in the collecting duct travels
  • the amount of water that is reabsorbed from the distal convoluted tubule and the collecting duct depends on the water potential of the blood
  • the water potential of the blood and the body fluids must be maintained at a constant level
• Osmoreceptors are present in the hypothalamus and are sensitive to the water potential of the blood
• Decreases in water potential of the blood

  • if the water potential of the blood plasma decreases the osmoreceptors in the hypothalamus are stimulated
  • when the osmoreceptors are stimulated more ADH is produces in the hypothalamus and travels down neurons to the posterior pituitary gland
  • more ADH is released into the blood by the posterior pitutry gland
  • the permeability of the distal tubule and collecting duct are increased by ADH
  • more water is reabsorbed from the filtrate by osmosis into the blood leading to less water in the urine