Body water

Question 1

Name the different compartments that water is stored in the body and give the amounts

Answer 1

Approx 60% of body weight is water

-this is higher in neonates and children and less in elderly

The main compartments are:

• INTRACELLULAR (ICF) which has 2/3rds of TBW
• EXTRACELLULAR (ECF)

These are separated by the cell membrane

The main components of ECF are:

• Transcellular
• Intravascular
• Interstitial
• Bone and CT

Question 2

1. What does the intravascular fluid comprise of? (give percentages)
2. What is the amount in ml/Kg
3. What is the function of interstitial fluid?
4. What is its composition?

Answer 2

1. Water (90%), Protein (7%) and ions
2. 70-75ml/Kg
3. It is the largest ECF compartment and surronds cells acting as a transport medium between the ICF and intravascular fluid for nutrients and waste products to pass between the two.
4. It is smiliar to plasma but has NO PROTEIN. It includes the lymphatic system

Question 3

What is transcellular fluid?

Answer 3

This consists of fluid secreted but is seperate from the plasma by epithelial layers:

• GI fluid
• CSF
• Bladder urine
• Aqueous humour
• Bile
• Sweat

Question 4

Osmotic pressure

1. Define osmotic pressure
2. What is osmolality?
3. What is the normal plasma osmolality?
4. What is the equation for it?

Answer 4

1. If you have two solutions on opposite sides of a semi-permeable membrane, the hydrostatic pressure required to resist movement across is the OSMOTIC PRESSURE

The greater the number of osmoles the greater the amount of hydrostatic pressure required to resist this.

2. It is the number of osmoles of a solute/kg of water is known as OSMOLALITY. The greater the number of osmoles (osmotically active particles) the greater the osmolality
3. 280-305 mosmol/kg (this is the same for the ICF as the ISF as water moves freely)
4. Plasma osmolality= urea + K+ + (2xNa+)
   - osmolalty of the ECF is largely dependent on the Na+ conc

Question 5

Distribution in the ECF

Describe the mechanisms behind the flud distributions of the ECF

Answer 5

Fluid in the ECF moves via various mechanisms.

Blood vessels have fenestrations to allow free passage (except for proteins). Consequently, the distribution of water within the ECF does not depend on ionic differences but on the relative volumes of the different compartments.

The size of the compartment is determined by the number of solute particles rather than osmolality. More solute means more water is needed to maintain osmolality.

The heart generates an intravascular pressure greater than the interstitial pressure which encourages water to leave the vessels.

Opposing this is the oncotic pressure (osmotic effect of proteins), encouraging water to remain in the vessels.

Oncotic pressure remains relatively constant along a capillary such that the main factor affecting water movement is the change in the hydrostatic pressure along the vessel.

Question 6

How is what uptake regulated?

Answer 6

There are 3 regulatory processes:

• Sensors:
  
  • Osmoreceptors in the hypothalamus respond to changes in ECF tonicity and largely reflect the ECF sodium concentration. These receptors can respond to very small changes in tonicity of the order of 1-2%.
• Controllers
  
  • Central control takes place in the hypothalamus via a complex system of interconnecting pathways.
• Effectors
  
  • The regulatory system results in either:
    
    • Thirst - resulting in the conscious intake of more water, or,
    • Antidiuretic hormone (ADH) - this alters the renal reabsorption of water in the collecting ducts. Increased tonicity will result in the release of ADH from the pituitary. This increases the reabsorption of water preventing a diuresis and reducing the osmolality of the plasma.