Homeostasis

Question 1

Define hydrostatic pressure

Answer 1

The pressure difference between capillary blood and interstitial fluid, this moves water and solute from plasma into the interstitial fluid

Question 2

Define oncotic pressure

Answer 2

Osmotic pressure exerted by proteins, notably albumin, in a blood vessel’s plasma (blood/liquid), the movement of water from interstitial fluid into capillaries

Question 3

What is an odema

Answer 3

When there is excess water in intercellular spaces.
This could be due to low levels of albumin.
Normally hydrostatic pressures push fluid out of the capillaries, and oncotic pressures (largely due to albumin) pull the fluid back in. (Oncotic pressures are higher than hydrostatic) Low levels of albumin mean that oncotic pessures are lower and so the fluid is not drawn back in, it stays in the intercellular spaces forming an odema.
normally 85% reabsorbed, 15% to lympatics

Question 4

What causes an inflammatory odema?

Answer 4

Increased vascular permeability due to trauma, infection, burn, allergy (more water and solutes diffuse out of the cell due to hydrostatic pressure, do not diffuse back in due to decreased osmotic pressure) 
Lymphatic obstruction (malignancy, chronic infection)
Venous obstruction (thrombosis, malignant infiltration)

Question 5

What is Henderson hasselbalch’s equation?

Answer 5

[H+] X [HCO3-] = K X [C02] X [H20]

∴[H+] X [HCO3-] = K X p[C02]

pH = pk + log ([HCO3-] / [C02])

Question 6

What reaction does carbonic anhydrase catalyse?

Answer 6

CO2 + H20 ⇌ H2CO3 ⇌ HCO3- + H+
carbonic anhydrase converts carbonic acid and bicarbonate ions in the blood, when the blood reaches in lungs it is converted back into CO2 which we then breathe out.

Question 7

Name some natural buffers in the body

Answer 7

a) Bicarbonate
- removes CO2 from lungs
- produced by the kidney

b) Proteins
- albumin contains weak acidic and basic groups
- protein matrix of the bone buffers large amounts of H+

eg, Haemoglobin

• binds to both Co2 and H2O so acts as a powerful buffer
• deoxygenated has a higher affinity for Co2 and H+ when deoxygenated so high affinity in the tissues

When carbonic acid dissociates it gives off H+
H+ binds to Hb = HHb
Bicarbonate ions (from carbonic acid) pass back into plasma in exchange for Cl- ions so no net loss of gain of -ve charge
Process reverses in lungs - CO2 released

c) phosphates

Question 8

Respiratory acidosis

Answer 8

When PaCO2 is above the upper limit of normal >6kPa

Net gain for H+ ions due to hyperventilation (often anxiety or severe asthma)
High Co2 = results in high H+ and high HCO3-

Question 9

Respiratory alkalosis

Answer 9

Excessive excretion of CO2,

Question 10

Metabolic acidosis

Answer 10

Excessive gain of H+ ions due to any reason other than respiratory eg, due to diarrhoea = excess loss of HCO3- = net gain of H+ or Lactic acid due to excercise

High H+ levels activates respiratory reflexes,(chemoreceptors) decreased ventilation = less CO2 = less H+ and higher pH

Question 11

Metabolic alkalosis

Answer 11

Excessive loss of H+ ions due to any reason other than respiratory eg, due to vomitting, loss of H+ from HCL in stomach
Low H+ levels activates respiratory reflexes, increased ventilation = more CO2 = more H+ ions

Question 12

Name 3 sources/losses of H+ ions in the body

Answer 12

1) CO2 - huge amounts of hydrogen ions generated due to oxidative metabolism
2) non-volatile acids - organic and inorganic acids from sources other than co2. catabolism of proteins, etc phosphoric acid, sulfuric acid, lactic acid
The metabolism of these acids both generates and utilises H+. High protein diets = 40-80mmole of H+ produced a day.
3) Third source/loss of H+ is in GI secretions.
GI secretions - high alkaline losss off HCO3, so (net) H+ gain
Vomitus - high H+ loss due to HCL in stomach
4) Kidneys can either add or remove H+

Question 13

What are the two main ways the body reacts to changes in H+? How fast is each reaction?

Answer 13

1) Respiratory system - fast, hypo or hyper ventilation

2) Kidneys - alter H+ conc, by excreting HCO3-, ‘reabsorption’ or production of ‘new’ HCO3.

Question 14

Describe 3 methods of HCO3 ‘reabsorption’ due to high H+ levels

Answer 14

1.
a)Carbonic anhydrase catalyses the combination of H2O and CO2 to H2CO3, this dissociates to H+ and HCO3-.
b) HCO3 moves out of the cell via facilitated diffusion into the plasma blood stream
c) H+ in secreted into the lumen
d) Here it combines with filtered HCO3- and generates CO2 and H20 (and the cycle can begin again)
=Filtered HCO3- gone, plasma has uptaken one HCO3- molecule
2. H+ combines with non-bicarbonate molecule buffer, eg HPO4 2-
a)Carbonic anhydrase catalyses the combination of H2O and CO2 to H2CO3, this dissociates to H+ and HCO3-.
b) HCO3 moves out of the cell via facilitated diffusion into the plasma blood stream
c) H+ is secreted into the lumen
d) Here H+ combines with HPO4 -2 to form H2PO4 -, this is excreted
= plasma has a net gain of HCO3-
3. GLUTAMINE
a) Tubular cells uptake glutamine from interstitial fluid or tubular lumen, and split it into NH4- and HCO3-
b) NH4- is secreted into the tubular lumen via Na+/NH4- counter-transportation and excreted
c) HCO3- is reabsorbed into the plasma

Question 15

Define homeostatis

Answer 15

Homeostasis is the ability to maintain a constant internal environment in response to environmental changes.

The nervous and endocrine systems control homeostasis in the body through feedback mechanisms involving various organs and organ systems.

Question 16

What is osmotic pressure?

Answer 16

Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in water by osmosis.