Body Fluids (Thursday 02/11/17)

Question 1

Fluid Balance

Answer 1

Fluid balance consists of two elements: EXTERNAL AND INTERNAL BALANCE.

Is how the body handles water and sodium. When water and sodium in and out is equal, you have balanced regulation. Kidney maintains fluid osmalality (no. of osmaticly active particles) and fluid volume is controlled by the kidney.

Obligate Production: 300ml of urine made despite not drinking any water.

Question 2

Internal Balance

Answer 2

o Internal balance describes distribution of fluid to fluid compartments within body.

Water can distribute into different compartments:

1. Intracellular: in the cells water makes up 40% of your body weight.
2. Extracellular: Interstitial (all the water surrounding the cells is 15% of your body weight) and Plasma space (5% of your body weight).

All compartments freely permeable to H2O

H2O will move in presence of osmotic gradient (hight to low water gradient).

Number of Osmotically active particles determine compartment size: osmotic gradient is determined by the no. of osmotically active particles in the compartments)

• In the body comparments all the compartments have the same osmalality (280 OsM)
• In cells the majority of the osmotically active particles are potassium and proteins (-ve charge).
• In the interstitial space and the plasma the major cation is sodium, the anions are chloride and HCO3. But plasma also has protein anions.
• The difference between interstitial and plasma is that the plasma has protein. So their difference in size is due to protein: Starling’s Forces.
• So basically the intracellular space is different to the extracellular space because the cells can actively gain or lose osmotically active particles to protect volume gain/loss.
• Regulatory Volume Increase: if a cell loses water and shrinks, it will make itself gain sodium and pottassium so water will follow.
• Regulatory Volume Decrease: If the cell swells and needs to lose water, it will taurine, and as a result water will be lost too.

Question 3

Starling’s Forces

Answer 3

Amount of fluid moving in and out of blood vessels is determined by colloid osmotic pressure- which drives fluid in (osmotic gradient), and hydrostatic pressure- drives fluid out (forse of fluid drops from arterial to venous end of the vein.) This means net movement in a capillaries is zero. This is because fluid out at first, then fluid in, cancel each other out. This means that if have loss of protein, colloid osmotic pressure would decrease, so if less protein in plasma, then less fluid moves from the interstitial space to the capillaries, so have oedema.

Question 4

Importance of controlling fluid volume

Answer 4

• In cells need to prevent cells from shrinking/ swelling. Especially the brain and the brain stem against swelling. If brain shrinks, blood vessels tear.
• Blood: important for blood pressure, make sure organs are perfused sufficiently.
• Interstitium: need to control fluid volume to prevent oedema.
• Changes in fluid balance which outstrip regulatory responses are fatal. If fluid volume changes happen too rapidly.

Question 5

How to measure osmolality

Answer 5

Osmolality measured clinically by 2 times the plasma sodium.

Question 6

Renal Control Mechanisms

Answer 6

Kidney maintains plasma volume (effective circulating volume) by controlling the amount of Na within the body. Effective circulating volume is sensed as vascular ‘stretch’ by a series of stretch receptors throughout the vasculature.

Question 7

Iso-omotic Dehydration Clinically

Answer 7

• When people vomit, burn victims (water in blisters not circulating) diahorrhea and blood loss. So extracellular volume (interstitual and plasma) will go down, but there have been no ionic changes so osmality stays the same. In intracellular nothing changes.
• Initially fluid is lost from the plasma. As the fluid loss from the plasma continues, fluid moves from the interstitial compartment driven by Starlings Forces so that both plasma and interstitial volumes decrease in size = decrease in ECF volume
• There is no change in osmolality of the ECF as fluid loss is isosmotic
• In absence of change in osmolality, there is no change in ICF volume or osomolality.

Question 8

Hypo-osmotic Dehydration

Answer 8

• Hyposmotic Dehydration:
  
  • Initially fluid is lost from plasma contains more salt than water (hypersmotic).
  • As a result the plasma volume decreases and becomes hyposmotic in relation to the interstitial space.
  • Water shifts from the plasma to the interstitial space driven by the osmotic gradient between the plasma and the interstitial space. The Interstitial space volume increases BUT the osmolality decreases – As a result the ECF volume becomes hyposmotic in relation to the Intracellular space (ICF)
• Water leaves shifts from the interstitial space to the intracellular space driven by both osmotic gradient. The Intracellular space volume increases BUT the osmolality decreases – increase in ICF volume and decrease in ICF osmolality
• Overall there is an net decrease in ECF volume ( due to loss of fluid from plasma to outside) and decrease in ECF osmolality BUT a net increase in ICF volume and a decrease in ICF osmolality
• Causes: Renal loss of NaCl due to adrenal insufficiency, hyposmotic dehydration due to heavy loss of hyposmotic sweat.

Question 9

Hyper-osmotic Dehydration

Answer 9

• Initially fluid is lost from plasma contains more water than salt (hyposmotic).
• As a result the plasma volume decreases and becomes hyperosmotic in relation to the interstitial space.
• Water leaves the interstitial space driven by both osmotic gradient and by Starlings forces between the plasma and the interstitial space . The Interstitial space volume decreases BUT the osmolality increases – decrease in ECF volume and increase in ECF osmolality
• As a result the ECF volume becomes hyperosmotic in relation to the Intracellular space (ICF)
• Water leaves the intracellular space driven by the osmotic gradient between ECF and the Intracellular space. The Intracellular space volume decreases BUT the osmolality increases – decrease in ICF volume and increase in ICF osmolality
• Causes: fever; diabetes insipidus, diabetes mellitus, excess loss from skin and/or breath, decreased intake of water

Question 10

External Balance

Answer 10

o Balance between fluid and Na entering body and fluid and Na leaving body

o Important since amount of fluid in body determines fluid osmolality and amount of Na in body determines fluid volume

o Fluid and Na entering body is variable and dependent upon diet and behaviour

o On average net gain of fluid per 24 hours is 2500ml

o Water in drinks 1200ml; water in food 1000ml water generated by metabolism 300ml

o Fluid loss matches fluid gain to give external balance

o Fluid loss – Insensible loss (not controlled) loss in expired air, loss as sweat and loss in faeces 800 ml.

o Fluid loss – regulated loss of fluid by kidney makes difference ~1500ml

o Renal loss can vary from 300ml (obligate loss) up to maximum of ~12-15l/day

o Renal loss varies to match changes in insensible loss to fluid intake and renal loss to changes in fluid intake

o Renal system can be overwhelmed leading to pathophysiology.