Biology Unit 3.7 - Homeostasis and the kidney

Question 1

What is homeostasis?

Answer 1

Maintenance of a constant internal environment, containing cells and tissue fluids that bathe the cells

Question 2

What is the purpose of homeostasis?

Answer 2

Protects cells from changes in the external environment, ensuring they function normally through keeping the body in a dynamic equilibrium

Question 3

What is negative feedback?

Answer 3

Output from an effector that reduces the effect of stimulus and restores the system to a set point:

• Set point is the norm at which the system operates
• Receptor detects the level of a factors and its deviation from the set point
• Instructions sent to coordinator
• Communication with one or more effectors to carry out corrective procedures
• Factors return to the set point

Question 4

What is excretion?

Answer 4

Removal of wastes made by the body

Question 5

What are the kidney’s two main functions?

Answer 5

Blood filtration - excretion of nitrogenous waste

Osmoregulation - control of water potentional of the body’s fluids

Question 6

How is urea produced?

Answer 6

Excess protein is borken down; amino acids are deaminated in the liver producing ammonia which is converted into urea, where it is then removed from the blood through ultrafiltration

Question 7

What are the functional parts of the nephron?

Answer 7

• Bowman’s capsule containing a capillary know
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule

Question 8

How does blood enter and leave the capillary knot?

Answer 8

In through the afferent arteriole - Out through efferent arteriole

Question 9

What processes occur in he nephron?

Answer 9

• Ultrafiltration
• Selectove reabsorption
• Osmoregulation

Question 10

Explain ultrafiltration by the Bowman’s capsule…

Answer 10

Filtration of solutes under high pressue into the lumen of the Bowman’s capsule, where small molecules pass into the Bowman’s capsule and blood is seperated from the space inside by two cell layers and a basement membrane

Question 11

How does the glomerulus enable ultrafiltration?

Answer 11

• First cell layer is the wall of the capillary that contains many small gaps (fenestrations)
• Allow substances to dissolve in blood plasma
• Extracellular basement membrane acts as a sieve between the blood and nephron
• Stops large protein molecules from getting through

Question 12

How does the Bowman’s capsule enable ultrafiltration?

Answer 12

Wall is made up of squamous epithelial cells called podocytes - highly specialised with pedicels which wrap around the capillary forming gaps called filtration slits

Question 13

How is hydrostatic pressure generated?

Answer 13

Filtration under high pressure, due to contraction of the left ventricle and the wider diameter of the afferent arteriole

Question 14

What is the purpose of the hydrostatic pressure?

Answer 14

Forces solutes through the fenestrae, basement membrane, and filtration slits, and into the cavity of the Bowman’s capsule

Question 15

What is selective reabsorption?

Answer 15

Process where substances such as glucose, amino acids, and salts are reabsorbed into the blood plasma

Question 16

What is the role of the proximal convoluted tubule in selective reabsorption?

Answer 16

Carries filtrate away from Bowman’s capsule where glucose and amino acids are reabsorbed by the blood capillaries surrounding the tubule

Question 17

How do microvilli affect selective reabsorption?

Answer 17

Highly folded to increase the surface area for reabsorption

Question 18

How do basal channels affect selective reabsorption?

Answer 18

Folded to increase surface for reabsorption

Question 19

How do the many mitochondria affect selective reabsorption?

Answer 19

Provide ATP by aerobic respiration needed for active transport

Question 20

How do ribosomes affect selective reabsorption?

Answer 20

Protein synthesis - enzymes, transport enzymes, etc

Question 21

How are salts reabsorbed?

Answer 21

Some through passive facilitated diffusion, with most occurring through active transport

Question 22

How is glucose reabsorbed?

Answer 22

Enters the cell by co-transport with sodium ions using facilitated diffusion, where they then bind to a binding site within a carrier protein, changing its tertiary shape, allowing the substances to enter the cytoplasm

Glucose the diffuses across the cytoplasm by simple diffusion, before leaving through either facilitated diffusion through a carrier protein or active transport though a pump

Question 23

How is water reabsorbed?

Answer 23

90% is reabsorbed from the glomerular filtrate through passive osmosis

Question 24

How are urea and small molecules reabsorbed?

Answer 24

50% is reabsorbed into the blood by diffusion

Question 25

What is the glucose threshold?

Answer 25

Normal circumstances - proximal convoluted tubule reabsorbs all the glucose

Concentration is too high - number of proteins becomes a limiting factor, glucose passes down the loop of Henle into the collecting duct before leaving the body through urine

Question 26

What is the mechanism of water reabsorption?

Answer 26

Loop of Henle concentrates salts in the tissue fluid, lowering water potential, allowing water to move by osmosis out of the loop. This concentrates the urine making it hypertonic.

This is achieved using the principle of a hairpin counter-current multiplier

Question 27

Describe the hair-pin counter current multiplier…

Answer 27

• Sodium and calcium ions are pumped from upper regions of the ascending limb by active transport
• Water potential of the tissue fluid decreases in the medulla of the kidney
• Water is lost from the descending limb by osmosis
• Loss of water cocentrates the filtrate, where it is most concentrated at the apex
• Sodium and calcium ions diffuse out of the filtrate as it ascends the loop of Henle, before being pumped out by active transport as concentration in the loop decreases

Question 28

What is osmoregulation?

Answer 28

Homeostatic control of body water achieved by negative feedback

Question 29

What are receptors, coordinators and effectors?

Answer 29

Receptors (located in the hypothalamus) are responsible for detecting changes
Posterior lobe of the pituitary gland acts as the coordinator
Walls of the collecting ducts act as the effector

Question 30

Explain hormonal control by ADH…

Answer 30

• Hormones secreted by endocrine glandsby the endocrine system
• Permeability of distal convoluted tubule and collecting duct is controlled by hormones
• Determines whether urine is hypertonic or hypotonic
• ADH makes the walls of the collecting duct more permeable to water leading to more absorption
• Increase in ADH leads to a production of small volumes of hypertonic urine comparable to that of the tissue fluids near the apex of the loop of Henle

Question 31

What are aquaporins?

Answer 31

Intrinsic membrane proteins with a pore through which water molecules move, whose availability is increased by ADH:

• ADH binds to receptor membranes
• Adenyl cyclase catalyses the production of AMP
• Vesicles contain aquaporins fuse with the cell membrane
• Aquaporins are incorporated into the membrane
• Water molecules move in single file through the aquaporins into the cell, down a water potential gradient

Question 32

How is nitrogenous waste disposed of in different environments?

Answer 32

• Aquatic - ammonia diffuses out across the gills and is quickly diluted non-toxic levels
• Birds and insects - Excrete uric acid, which allows organisms to live in dry environments
• Mammals - Excrete urea, whose production requires energy, but is less toxic, and tissues can tolerate it in higher concentrations

Question 33

Why is the loop of Henle important?

Answer 33

Enable mammals to live in arid areas, as it allows more water to be reabsorbed by osmosis, and more concentrated urine to be produced

Organisms that live in the dessert have a particularly long loop of Henle

Question 34

What are metabolic water sources?

Answer 34

Substances produced from the breakdown of food during respiration in cells, which reduces water loss by evaporation

Question 35

What are the common causes of kidney failures?

Answer 35

• Diabetes - High glucose concentrations resulting in glomeruli losing protein into filtrate, where some link together triggering scarring, which is called glomerulosclerosis
• High blood pressure - Damage the capillaries of the glomerulus and prevents ultrafiltration
• Auto-immune disease - Body makes anitbodies against its own tissues
• Infection - Cause inflammation and scarring of the tissues
• Crushing injury - Kidneys are not protected by the skelton, meaning crushing injuries can damage the tissues

Question 36

How do substances move during dialysis?

Answer 36

• Urea moves from a high conentration in the blood to a low concentration in the dialysis fluid by dffusion
• Salts move from a high concentration in the blood to a low concentration in the dialysis fluid by diffusion
• Glucose has no net movement
• Amino acids have no net movement

Question 36

What can be done to regulate solute concentrations?

Answer 36

• Reducing intake of certain nutrients, namely protein
• Use drugs to reduce blood pressure
• Use drugs to control the concetration of potassium and calcium ions
• Dialysis
• Kidney Transplant

Question 37

What is haemodialysis?

Answer 37

• Blood is taken from an artery and run through long narrow fibres of selectively permeable dialysis tubing
• Fibres surrounded by dialysis fluid
• Pores in the dialysis tubing allow small molecules to pass out of the blood plasma
• Large molecules are too large to pass through the pores
• Blood and dialysis fluid run through in opposite directions, enhancing diffusion out by a counter-current mechanisms
• Heparin is added to thin the blood preventing clothing
• Sensor detects haemoglobin released
• Patients use a machine for several hours and days a week

Question 38

What is continuous ambulatory peritoneal dialysis (CAPD)?

Answer 38

• Allows patient to walk and carry on with normal activities
• Drains 1-3 dm cubed bag of dialysis fluid through a catheter
• Peritoneum is the membrane lining the body cavity and has a rich supply of capillaries, which acts as the dialysis membrane

Question 39

Compare the dialysis machine and a healthy kidney?

Answer 39

Selectively permeable membrane - Glomerulus/capillary knot and Bowman’s capsule
Tube carrying blood into the dialysis machine - Renal artery
Dialysis fluid - Filtrate in the nephron
Tube carrying blood out of the dialysis machine - Renal vein

Question 40

Explain compatibility…

Answer 40

The donor and recipient must be compatible in their ABO blood group and in most of their human leucocyte antigens

Question 41

Explain a kidney transplant…

Answer 41

Transplanted kidney is placed in the lower abdomen, where the renal artery and vein emerging from the kidney are attached to the iliac artery and vein

Circulation of the new kidney is then restored where the kidney then resumes a healthy pink colour

Question 42

Explain immunosuppresents…

Answer 42

Transplant recipient must take immunosuppresent durgs for the rest of their life to avoid rejection

May make the recipient more susceptible to infection, especially of the urinary tract, which can be counteracted by long-term low-dose antibiotics

May increase the risk of cancers, especially skin cancer and lymphoma