B3.3 - Maintaining internal environments

• Homeostasis

  Homeostasis is maintaining a constant internal environment to allow metabolic reactions to proceed at appropriate rates.
• It is important for an organism to maintain a constant internal environment in response to internal and external environmental changes while maintaining optimum internal conditions for enzyme action. If the homeostatic limits are exceeded, the organism may die.
• Some examples of these internal conditions include:

  • Water content (of an individual cell or of the body fluids of an organism)
  • Body temperature
  • pH
  • Blood pressure
  • Blood glucose concentration
• Body Temperature is Controlled by the Hypothalamus


  All enzymes work best at a certain temperature. The enzymes in the human body work best at about 37 °C:

  • It contains receptors that are sensitive to the blood temperature in the brain. It also receives impulses from receptors in the skin (nerve endings) that provide information about the external temperature.
  • When the hypothalamus detects a change, it causes a response in the dermis (deep layer of the skin):
• • Negative feedback mechanisms in homeostasis help maintain conditions in the body within an optimal narrow range; any movement away from ideal conditions results in changes occurring which bring them back.
  • This involves detecting that the level of a substance or a condition has gone above or below normal levels, which triggers a response to bring the level back to normal again.
• Negative feedback control loop:

  A) receptor

  B) stimulus

  C) nervous

  D) brain

  E) spinal cord

  F) effector

  G) receptors

  
• When you're too hot...

  1. Erector muscles relax, so hairs lie flat.
  2. Lots of sweat (containing water and salts) is produced. When the sweat evaporates it transfers energy from your skin to the environment, cooling you down.
  3. Blood vessels close to the surface of the skin dilate (widen). This is called vasodilation. It allows more blood to flow near the surface, so it can transfer more energy into the surroundings, which cools you down.
• When You're Too Cold...

  1. Erector muscles contract. Hairs stand on end to trap an insulating layer of air, which helps keep you warm.
  2. Very little sweat is produced.
  3. Blood vessels near the surface of the skin constrict (narrow). This is called vasoconstriction. It means less blood flows near the surface, so less energy is transferred to the surroundings.
  4. When you're cold you shiver too (your muscles contract automatically). This needs respiration, which transfers some energy to warm the body.
• To hot:

  A) receptors

  B) erector

  C) gland

  D) dilate

  
• Too cold:

  A) erect

  B) reduced

  C) sweat

  
• Blood glucose levels are regulated by the pancreas and the liver using hormones insulin and glucagon.

  • Insulin stimulates cells to take in glucose from the bloodstream, particularly liver and muscle cells.
  • In liver and muscle cells, excess glucose is converted into glycogen, a polymer of glucose, for storage.
• The pancreas acts as an endocrine gland, secreting the two hormones:

  • Insulin: Converts glucose to glycogen, which is insoluble so can be stored for later use.
  • Glucagon: Converts glycogen to glucose for respiration.

  Both hormones act to maintain the blood glucose levels within the optimum limits as part of a negative feedback cycle.
• If the blood glucose concentration gets too low:

  • Cells in the pancreas detect the decreased blood glucose levels.
  • The pancreas produces the hormone glucagon and secretes it into the blood.
  • Glucagon causes the glycogen stored in the liver to be converted into glucose and released into the blood.
  • This increases the concentration of glucose in the blood back to normal levels, at which point the pancreas stops secreting glucagon.
• If the blood glucose concentration gets too high:

  • Cells in the pancreas detect the increased blood glucose levels.
  • The pancreas produces the hormone insulin, secreting it into the blood.
  • Insulin stimulates muscles and the liver to take up glucose from the bloodstream and store it as glycogen (a polymer of glucose).
  • This reduces the concentration of glucose in the blood back to normal levels, at which point the pancreas stops secreting insulin.
• Type 1 diabetes:


  -Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin to control blood glucose levels.

  -Type 1 diabetes is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.
• Type 2 diabetes:


  -Type 2 diabetes occurs when the body's cells no longer respond effectively to insulin produced by the pancreas.

  -While the person still produces insulin, their cells become resistant to it and do not respond as well as they should.

  -This can lead to uncontrolled high blood glucose levels.

  -Obesity is a significant risk factor for Type 2 diabetes, as obese individuals may consume diets high in carbohydrates, leading to overproduction of insulin and therefore development of insulin resistance.

  -A carbohydrate-controlled diet and exercise are treatments for Type 2 diabetes.
• Tissue fluid

  Fluid surrounding body cells, squeezed out of blood capillaries to supply cells with what they need
• Tissue fluid water potential is different to fluid inside a cell this means that water moves into or out of cell by osmosis
• If tissue fluid water potential is higher than the water potential inside the cell, there will be a net movement of water into the cell by osmosis. If too much water moves into the cell then the cell may burst - this is called lysis.
• If water potential inside the cell is higher then there will be a net movement of water out of the cell and into the tissue fluid. This causes the cell to shrink.
• If tissue fluid and cell water potentials are roughly the same then the cell will stay the same
• Controlling water content of blood (and therefore tissue fluid) is really important to keep cells functioning normally
• Kidneys

  -They maintain the water level in the body (vital for maintaining blood pressure).-Kidneys vary the volume and concentration of urine and hence water excreted.

  -They get rid of toxic waste products (such as urea) and control the levels of other substances in the body.
• Regions of the kidney:

  -Cortex - the outermost region.

  -Medulla - the inner section of the kidney.

  -Each kidney contains around a million tiny structures called kidney tubules.

  -The tubules start in the cortex of the kidney, loop down into the medulla and back up to the cortex.

  -The Tubules (nephrons) drain into the innermost part of the kidney (the renal pelvis) and the urine collects there before it flows into the ureter to be carried to the bladder for storage.
• Kidney structure:

  A) adrenal

  B) renal artery

  C) renal vein

  D) cortex

  E) Medulla

  F) ureter

  G) nephrons

  H) in

  I) out

  
• A kidney tubule has several sections:

  1)Bowman's capsule

  2)Proximal convoluted tubule

  3)Loop of Henle

  4)Distal convoluted tubule

  5)Collecting duct

  -Surrounding the tubule is a network of capillaries with a knotted section, called the glomerulus, which sits inside the Bowman's capsule.
• Kidney Tubule:

  A) proximal convoluted tubule

  B) distal convoluted tubule

  C) glomerulus

  D) bowman's capsule

  E) loop of henle

  F) collecting duct

  
• This is what happens at each tubule...

  Blood flows through the glomerulus at high pressure, to force substances out of blood, and small molecules including water, sugar, salt and urea are filtered out into the bowman's capsule. The liquid then flows along the tubule and useful substances are selectively reabsorbed:

  • All the sugar is reabsorbed.
  • Sufficient salt is reabsorbed. Excess salt isn't.
  • Sufficient water is reabsorbed (osmosis), according to the level of the hormone ADH.
  • Whatever isn't reabsorbed forms urine, which is excreted by the kidneys and stored in the bladder.
• • The concentration of urine is controlled by the hormone ADH.
  • This is released into the bloodstream by the pituitary gland.
  • The brain monitors the water content of the blood and instructs the pituitary gland to release ADH into the blood according to how much is needed.
  • ADH makes the kidney tubules more permeable so that more water is reabsorbed back into the blood.
  • The whole process of water content regulation is controlled by negative feedback. This means that if the water content gets too high or too low a mechanism will be triggered that brings it back to normal.
• Kidney negative feedback:

  A) water loss

  B) pituitary gland

  C) ADH

  D) reabsorb

  E) water

  
• • Using negative feedback, the amount of water in your body can be closely regulated.
  • The more water your kidneys reabsorb, the less water will pass out as urine so you'll produce a smaller volume of urine.
  • Your kidneys will still excrete all the waste products they need to though, so your urine will be more concentrated (as it contains the same amount of waste substances but less water).
• The volume and concentration of urine depends on the water content of the blood
• Water content of the blood


  Can vary with temperature and other changes that affect the water potential of the blood (osmotic challenges)
• Sweating and dehydration when it's hot or when you exercise


  1. Sweat contains water, so sweating causes water loss

  2. Losing more water than you take in causes dehydration

  3. This triggers more ADH to be released

  4. The kidney will reabsorb more water so only a small volume of concentrated urine will be produced

  5. The brain also triggers feelings of thirst when you're dehydrated, which helps to restore water balance in the body
• Excess water intake


  The kidney responds by excreting more water meaning lots of dilute urine
• High salt intake


  The kidney responds by excreting more salt, which will produce concentrated urine