Homeostasis

Question 1

Give 3 cellular functions underpinned by differences in ECF and ICF compositions.

Answer 1

• Electrical activity in the nervous system
• Muscle contraction
• Formation of urine in the kidney

Question 2

Give Fick’s Law of diffusion.

Answer 2

Jx = Px ([X]o – [X]i )

• Jx = flux
• Px = permeability coefficient, derived from membrane thickness, solubility of X in the membrane and diffusion coefficient.
• ([X]o – [X]i) = difference in concentration

Question 3

How do pores allow for movement of substances?

Answer 3

Always open
Simple diffusion down a concentration gradient

Porins in mitochondria, perforins in lymphocytes, aquaporins

Question 4

How do channels allow for movement of substances?

Answer 4

• Non-gated channels: potassium leak channels
• Gated channels: Na+ channels, K+ channels, Ca2+ channels, anion channels like Cl- channels
• Gated channels allow opening and closing by voltage, ligand or secondary messenger to allow movement of substances

Question 5

How do carriers allow for movement of substances?

Answer 5

• Transporter that does facilitated diffusion
• Specific binding of solute, change of conformation, release of solute
• Transporters can have 1 solute, more than 1 solute in the same direction or more than 1 solute in opposite directions.

Question 6

How do pumps allow for movement of substances?

Answer 6

Active transport of ions that utilises hydrolysis of ATP to move them against a concentration gradient.

• Sodium-potassium pump is ubiquitous and is present in all cells. Does secondary active transport – relies on a gradient set up by the active transport of another solute.
• Hydrogen-potassium pumps found in the stomach

Question 7

Describe the sodium ion concentration of ECF, ICF and blood plasma.

Answer 7

ECF = higher
ICF = lower
blood plasma = higher

Question 8

Describe the potassium ion concentration of ECF, ICF and blood plasma.

Answer 8

ECF = lower
ICF = higher
Blood plasma = lower

Question 9

Describe the chloride ion concentration of ECF, ICF and blood plasma.

Answer 9

ECF = higher
ICF = lower
blood plasma = lower

Question 10

Describe the protein concentration of ECF, ICF and blood plasma.

Answer 10

ECF = lower
ICF = higher
Blood plasma = higher

Question 11

When is electroneutrality achieved?

Answer 11

For electroneutrality in each fluid compartment, the sum of the anion concentration must equal the sum of the cation concentration.

Question 12

What is an electrochemical gradient?

Answer 12

When charged molecules diffuse down a chemical gradient, there is build up of charge. And so charged molecules then move down the electrical gradient in the opposite direction.
So, the driving force for passive transport is this electrochemical gradient.

Question 13

Define equilibrium potential.

Answer 13

Equilibrium potential – when the electrical gradient and the chemical gradient are equal for a particular ion.

Question 14

What is hypokalaemia and hyperkalaemia?

Answer 14

Hypokalaemia – too little K+. Causes muscle weakness and cardiac arrhythmias

Hyperkalaemia – too much K+. Causes cardiac arrhythmias

Question 15

Where is calcium in the body?

Answer 15

99% of calcium in the body is inorganic, mineralised matrix of bone as hydroxyapatite.

Intracellular in endoplasmic reticulum. 0.00002% free in the cytosol. Extracellular fluid contains 0.1% calcium at about 2.5mmol/l.

• 50% of this is ionised Ca2+ (the biologically active calcium)
• 5% complexed Ca2+ salts and bicarbonates
• 45% is protein bound

Question 16

What are some responses to calcium entering a cell as a secondary messenger?

Answer 16

When it enters the cell via calcium ion channels, calcium can activate intracellular processes and changes cellular function.
Cellular function is regulated by interaction with intracellular calcium binding proteins and calcium-sensitive protein kinases.

Biological responses include neurotransmitter release, contraction and secretion.

Question 17

How is calcium gained, exchanged and lost in the body?

Answer 17

• Calcium gained form diet
• Calcium exchanged in bone remodelling. There is more remodelling during growth or pregnancy.
• Calcium lost in urine, faeces and lactation

Question 18

Name the 3 hormones involved in calcium homeostasis.

Answer 18

• Parathyroid Hormone (PTH)
• Calcitonin
• Active vitamin D (1,25-dihydroxycholecaliferol or 1,25-DHCC, calcitriol)

Question 19

What is the parathyroid gland and what does it secrete?

Answer 19

PTG is a discrete area of tissue embedded in thyroid gland or near. Made up of chief cells which synthesise, store and secrete PTH.

Question 20

What is the effect of PTH in response to low calcium?

Answer 20

Low calcium stimulates PTH that acts on bone to release more calcium ions from the ECF in the bone.
Can also act in the longer term to break down hydroxyapatite crystals, the mineralised from of calcium in the bone, but this is more of a long term effect, and is done by osteoclasts.
PTH increases absorption of calcium in the kidney, reducing the amount that is lost in the urine.
PTH promotes the formation of active vitamin D, which enhances the absorption of calcium in the intestine.

Question 21

What do C-cells in the thyroid gland secrete?

Answer 21

Calcitonin is secreted. It is a peptide hormone that acts primarily on kidney and bone and reduces calcium ion levels.

Question 22

How does calcitonin decrease blood calcium ion concentration?

Answer 22

1. Reduces the calcium released from the skeleton into the plasma by acting on osteoclasts and inhibiting their activity.
2. Binds directly to osteoclast and causes them to shrink and detach from the bone, and so they cannot break down the bone anymore.
3. Calcitonin acts on the kidney to reduce absorption of calcium and increase the amount lost in urine.

Question 23

How is active vitamin D produced in the body?

Answer 23

• When cholesterol is exposed to UV light in the skin, it is converted from cholesterol to cholecalciferol, vitamin D3.
• Vitamin D3 can also be provided through the diet.
• In the liver, cholecalciferol is converted to 25-hydroxycholecalciferol.
• In the kidney, the enzyme, renal 1 alpha hydroxylase, produces active vitamin D. this stage is regulated by PTH.

Question 24

What is the effect of active vitamin D on blood calcium ion concentration?

Answer 24

Increases absorption of calcium ions from the intestine. It moves into gut epithelial cell and then moves into the nucleus, to cause the production of new proteins within the cell. This helps the cell deal with calcium, as the proteins could be calcium channels, calbindin, calcium pumps and exchangers. So allows calcium to move out of bloodstream and into the cells, to protect the bone.

Question 25

What happens to an animal with a vitamin D deficiency?

Answer 25

• If there is a reduction of vitamin D, there is a reduction of calcium ions absorbed from the gut and into cells.
• This leads to hypocalcaemia.
• This will cause an increase in PTH secretion from the PTG.
• This causes calcium to be reabsorbed from bone and the cartilage may become improperly mineralised.
• Leads to weak and deformed bones.

Question 26

Describe hyperparathyroidism.

Answer 26

PTG cells secrete unregulated, excessive amounts of PTH. Can be caused by cancer of chief cells.

Question 27

Describe secondary hyperparathyroidism.

Answer 27

Commonly caused by chronic renal failure. Decreased calcium re-absorption in the kidney and production of active vitamin D. Decreases absorption of calcium in the gut, leading to hypocalcaemia. Increases amount of PTH. Increased absorption of calcium in bones, causing bone fractures and deformities.

Question 28

What is parturient hypocalcaemia commonly called and how is it caused?

Answer 28

Milk fever in dairy cows.
A large amount of calcium is lost to the foetus or to the skeleton of the foetus in late pregnancy. There is a great loss of calcium reserves as lactation starts.

Question 29

Why do cows with milk fever develop paresis?

Answer 29

There is a point where the cow cannot get up and becomes recumbent and develops paresis (partial paralysis). This is because excitation-contraction coupling is lost at neuromuscular junctions.

Question 30

What can be done to prevent milk fever?

Answer 30

Ensure there is no inappropriate prenatal nutrition, often with calcium supplements, as this will inhibit normal PTG development necessary to meet the demands for lactation.

Question 31

How can milk fever be treated?

Answer 31

Calcium gluconate, slowly given intravenously.
Most cows will show improvement in clinical signs during treatment; become brighter, rumen contractility and peripheral circulation improve, and core body temperature normalises.
Some cows will relapse and will need to be re-treated.

Question 32

How is puerperal hypocalcaemia caused in bitches? What is the effect of this?

Answer 32

Caused by a large amount of calcium being lost during lactation. Low concentrations of calcium in the ECF has an excitatory effect on nerves and muscles. Causes tremors and tetany. If calcium is removed from outside of nerve cells, sodium channels become really activated with only a very small change in membrane potential. The nerve fibres become very excitable and the muscles become over-excited.

Question 33

Define homeostasis.

Answer 33

keeping internal environment of the body constant. Dynamic equilibrium where variables change but are continuously corrected and kept between narrow limits.

Question 34

What is the difference between positive and negative feedback systems?

Answer 34

A positive feedback system is a divergence from equilibrium and a negative feedback system is a maintenance of equilibrium.

Question 35

What do negative feedback systems consist of?

Answer 35

Controlled variables, receptors, a processor (integration centre) and effector mechanisms

Question 36

How can disturbances in homeostasis cause illness and death?

Answer 36

Homeostatic mechanisms maintain composition of extracellular fluid.
All cells bathed in ECF. ICF is affected by changes to ECF composition

Question 36

How can disturbances in homeostasis cause illness and death?

Answer 36

Homeostatic mechanisms maintain composition of extracellular fluid.
All cells bathed in ECF. ICF is affected by changes to ECF composition.

Question 37

What percentage of body content is ECF and ICF?

Answer 37

40% of the body is solid matter and 60% is fluid. 1/3 of this fluid is ECF and 2/3 is ICF

Question 38

What is ECF made up of?

Answer 38

Plasma, interstitial fluid, cerebrospinal fluid, fluid from the gastrointestinal tract, joint fluid and lymph

Question 39

What is the effect of adding solute to a fluid?

Answer 39

Physiochemical/colligative properties of solvent are changed by adding solute. The number not nature of particles affects this. Can cause freezing point or osmotic pressure of the fluid to change.

Question 40

Define molarity.

Answer 40

A unit of concentration that expresses the number of moles of solvent per unit of volume (1mol/L = 1 mol solute in 1 litre solvent)

Question 41

Define osmolarity.

Answer 41

A measure of activity of the solvent and is the number of osmoles per unit volume. An increase of osmolarity causes a decrease in solvent activity.

Question 42

Describe the Gibbs-Donnan effect.

Answer 42

Intracellular proteins have a negative charge and there is movement of positive charges into the cell down an electrical gradient, which can bring positively charged ions with it. This increases ICF activity and ICF becomes hypotonic and water is drawn into the cell. Sodium-potassium pump actively transports 3 Na+ out for every 2 K+ in to counteract.

Question 43

How does water move between ECF and IVF, intravascular fluid?

Answer 43

Capillary has high hydrostatic and oncotic pressures, these are Starling’s forces. Hydrostatic pressure forces water out and oncotic pressure draws water in from ECF by osmotic suction - where filtration rate is directly proportional to hydraulic drive.