Homeostasis & The Kidney

• What does the environment of an organism do all the time?


  The environment of an organism changes all the time, with some conditions changing slowly, and others dramatically.
• How do organisms respond to environmental change?

  Some animals maintain their internal environment and their normal activities, more-or-less whatever the external conditions are like.
• What do we call these organisms?

  We call these organisms' regulators (e.g. mammals and birds) with their high, almost constant body temperature over a wide range of external temperatures.
• What is homeostasis?


  Homeostasis is the maintenance of a constant internal environment.
• What is mammalian tissue fluid made up of?

  Mammalian tissue is made up of a collection of cells bathed in tissue fluid.
• What should the composition of mammalian fluid be kept at + what is it irrespective of?

  The composition of this fluid + therefore the blood must be kept constant, irrespective of the external conditions outside the body.
• What are some of the features of the blood and tissue fluid that must be kept within narrow limits? (6)

  - Blood glucose concentration

  - Temperature

  - pH

  - Water potential

  - The concentration of ions (e.g. sodium, potassium + calcium)

  - Oxygen levels
• What must the work of organs be to achieve homeostasis + what does it depend on?

  The work of a number of organs must be coordinated to achieve homeostasis, and it depends upon the continual

  feedback of information from a variety of receptors.
• What is negative feedback?

  Negative feedback is the mechanism of delivering homeostasis.

  
• What type of control is negative feedback?


  It is a type of control in which the conditions being

  regulated are returned to set values as soon as they are

  detected to have deviated.
• What are the components of a negative feedback system? (4)

  - A detector device that measures the value of the variable (a sensor).

  - These detectors (receptors) can be in the brain or localized throughout the body.

  
  

  - A control unit to which this information is transmitted.

  - Here the incoming data are compared with a pre-set value.

  - These monitors are usually in the brain.

  
  

  - When the value is below or above the required value, the control unit activates an effector device.

  - This corrective mechanism brings about the changes required to return the factor to its normal level.

  
  

  - Once it returns to normal levels, the control unit switches off the response.
• What is the function of the components of a negative feedback system + how does this differ in reality?

  - This is to try and ensure that there is no over-correction.

  - However, in reality, negative feedback control does typically involve some degree of 'overshoot'.
• What gives a greater degree of control in homeostasis?

  Having separate systems controlling deviations in different directions from the norm gives a greater degree of control.
• How can communication between the sensors/receptors + the monitor + to the effectors be conducted?

  Communication between the sensors/receptors and the monitor, and then to the effectors can be by nervous (temperature) or hormonal (blood glucose) control.
• What are the two important roles in mammals?

  - Excretion

  - Osmoregulation
• What is excretion?


  Excretion is the removal of the toxic waste products of metabolism.
• What is the main toxic waste substance + where is it produced?

  The main toxic waste substance is urea; this nitrogenous waste is produced during deamination in the liver.
• What is deamination?


  Deamination is the breakdown of excess amino acids (and nucleic acids), as these cannot be stored within the body.
• What is another waste product in the kidney + where is it produced? (2)

  - Other toxic products also excreted by the kidney include creatinine.

  
  

  - This is produced by the breakdown of creatinine phosphate (a molecule important in ATP synthesis) in muscles.
• What is osmoregulation?


  Osmoregulation is the control of the water potential of body fluids.
• Use the function of kidneys to give an example of negative feedback.


  The kidneys maintain the volume and water potential of the blood, within optimal narrow levels, and as a result the volume and concentration of the urine produced by the kidneys will vary widely. This is an example of negative feedback.
• Where do the kidneys receive blood through?

  The kidneys receive blood through the renal artery (this contains blood with a high concentration of urea).

  
• What drains the kidneys?

  Drained via the renal vein (the blood in this vessel has been "cleaned").
• What is the blood supply of the kidney in comparison to other organs?

  Have the largest blood supply of blood of any organ, about 1200cm3 of blood each minute.
• What level of blood pressure do the kidneys require + what happens when it changes?

  Require a blood supply at high pressure if they are to function properly - they fail quickly if the pressure drops.
• Explain the journey in which urine takes after being made in the kidney.

  Urine made in the kidney flows to the ureter and is pushed by peristalsis to the bladder where it is stored and then released, when full, by relaxing the sphincter muscles at the base of the bladder allowing urine to pass into the urethra and out.
• What components does the kidney consist of? (5)

  - A thin outer covering layer - the renal capsule

  
  

  - An outer cortex

  
  

  - Inner medulla (sometimes called the medullary pyramids, due to their shape)

  
  

  - Millions of nephrons which traverse both the cortex + medulla, along with the nephrons blood supply, which come from the renal artery that branches progressively into tiny arterioles, each serving a single nephron.

  
  

  - A large central pelvis that the pyramids drain in to.
• What is the nephron?


  The nephron is the functional unit of the kidney that allows excretion and osmoregulation to take place.
• What is the nephron responsible for?

  It is responsible for the production of urine and it is estimated that each kidney contains 1 million nephrons, each of which is around 3cm long.
• What processes does the nephron use to purify the blood? (3)

  - Ultrafiltration (pressure filtration)

  - Selective reabsorption of nutrients

  - Concentration
• What are the 6 regions of a nephron?

  1. Renal corpuscle (containing the glomerulus and Bowman's capsule)

  2. Proximal convoluted tubule

  3. Descending limb of the loop of Henle

  4. Ascending limb of the loop of Henle

  5. Distal convoluted tubule

  6. Collecting duct
• What does the cortex contain?

  The cortex contains the renal capsule, and the proximal and distal convoluted tubules.

  
• What does the medulla contain?

  The medulla contains the loop of Henle and collecting duct.

  
• What is the renal corpuscle the site of?

  Ultrafiltration
• What is the glomerulus?

  The Glomerulus is a dense network of capillaries held in an invagination of the Bowman's capsule.
• Where does the blood enter the glomerulus + what does the difference in diameters create? (2)

  - Blood enters the glomerulus from the wider afferent arteriole and then leaves via the narrow efferent arteriole.

  
  

  - The difference in diameters creates a high blood pressure which aids filtration via pores in the glomerular capillaries.
• What is the barrier between the blood in the capillaries + the capsular space is composed of?

  A single layer of capillary endothelial cells with very small spaces between the cells (50-100nm).
• What is the Bowman's capsule?


  A cup shaped capsule containing the glomerulus.
• Where is the capsular space?


  Between the inner and outer linings of the capsule there is a capsular space.
• What are podocytes + describe their structure? (4)

  - The inner layer of the capsule is composed of cells called podocytes.

  
  

  - These cells do not fit tightly together.

  
  

  - They have many foot-like projections in contact with the basement membrane.

  
  

  - It is between these gaps that the filtrate can flow.