Body Fluids

Question 1

Why is water a vital molecule?

Answer 1

Water is a vital molecule because:
•Water is the medium for all metabolic processes
•Water is involved in diffusion of nutrients/waste products
•Universal solvent
•Stable volume, BP and composition of the body fluids is essential for metabolism (homeostasis)
Water content of body fluid is reduced in the elderly and the overweight

Question 2

How is body fluid distributed?

Answer 2

Body fluid is divided into 3 compartments
60% of body weight is water
40% of body weight is intracellular water (ICF)
20% of body weight is extracellular water (ECF)
The ECF is subdivided into the plasma (4% is water) and interstitial fluid (16% is water)

Osmosis determines water movement between ICF + ECF

Question 3

How would body fluids be distributed in a 70kg human?

Answer 3

60% of 70= 42L of body fluid altogether
40% of 70=28L
20% of 70= 14L (11+3 in the ECF)

Question 4

Describe the composition of body fluids in the ECF and ICF

Answer 4

composition of the fluids is so different to create the electrical gradient for muscle and nerve potential

Question 5

Why and how is body fluid composition different in the ECF and ICF?

Answer 5

The v different composition of the ECF and ICF are vital for cellular functions, eg depolarisation of excitable cells

The plasma membrane maintains these compositional differences. It has a v low electrolyte permeability as most electrolytes are polar, so can’t pass through the non-polar centre of the phospholipid bilayer

Barrier between ICF & ECF = The Cell Wall

Question 6

How much fluid do we need?

Answer 6

Question 7

What does the movement of molecules depend on?

Answer 7

Depends on pressure
Ionic pressure
Osmotic pressure

Depends on the nature of the barrier
Cell membrane
Capillary wall

Question 8

What is the difference between the cell membrane barrier and the capillary wall barrier?

Answer 8

Cell membrane barrier: Lipid bilayer w proteins.
Impermeable to most ions and proteins; has pumps and highly specified voltage gated channels.
Only water can move freely, driven by osmotic Pa

Capillary wall barrier: Endothelial cells, w semi permeable pores
Allows water and electrolytes, but not proteins
Large molecules can’t get through the barrier so it pulls water in
Only large molecules (e.g. proteins) can exert an osmotic pa across it. Hydrostatic pa drives water movement

Easy diffusion between interstitial and plasma means they have nearly identical ion composition.

Question 9

What is an osmole?

Answer 9

Osmoles are a unit describing the conc of a solute in body fluids
1 Osm= 1mol of osmotically active particles (6.02x10^23)
Osmoles depend on the dissociation of the solute
For example, glucose doesn’t dissociate, therefore 1mol of glucose is equal to 1 Osm
NaCl on the other hand does dissociate into Na and Cl, therefore 1mol of NaCl is equal to 2 Osm

Question 10

Describe osmolarity and osmolality

Answer 10

Osmolality and osmolarity are used interchangeably. They describe the conc of body fluids but have technical differences:
Osmolality= conc of osmotically active particles per kg of water (Osm/kg)
Osmolarity is the concentration of osmotically active particles per litre of water (Osm/L).
Osmolarity changes w temp as water expands/contracts.
The osmolarity of the ECF and the ICF is very similar (~300mOsm/L) to prevent large fluid shifts between the compartments

Question 11

What is hydrostatic and therefore osmotic pressure?

Answer 11

Hydrostatic pressure= pa exerted on a membrane by a stationary fluid. As water diffuses into a compartment, volume in the compartment ⇡, which ⇡ hydrostatic Pa

Osmotic pressure= the hydrostatic pa needed to prevent the inward osmosis of water. It reflects the tendency of water to move passively down a conc gradient

Question 12

How are hydrostatic and osmotic pressure linked?

Answer 12

They are linked via Starling’s principle of fluid exchange. This states that hydrostatic pa drives filtration of fluid out of a capillary, whilst the osmotic pa drives reabsorption

Osmosis is driven by an osmotic pa/osmolality gradient. Water moves from an area of low osmolality (dilute solution) to high osmolality (conc solution) (this is from a low to high osmotic pa)

Osmosis stops when hydrostatic + osmotic pa reach eqm

Question 13

State the calculation for osmotic Pa

Answer 13

Question 14

State the definition for tonicity

Answer 14

Tonicity = effect of a solution [and solutes that cannot cross] on osmotic movement of water across cell membrane

Question 15

What is the difference between a hyper and hypotonic solutions?

Answer 15

A hypotonic solution has a lower solute conc than the intracellular fluid – this causes water to move into the cell, causing it to expand

A hypertonic solution has a higher solute conc than inside the cell – this draws water into the ECF, causing the cell to shrink

Question 16

What happens in the presence of too much or too little water?

Answer 16

Too little water leads to diarrhoea and low bp (90/40 mmHg)

Too much water can lead to kidney failure. Patient unable to get rid of water, placed on dialysis. Bp=200/100 mmHg. Blood vessels are rigid- can’t expand to accommodate more fluids. ⇡ fluid in the blood vessels = ⇡ BP

Question 17

Is there a large difference in osmolarity between ICF, Interstitium and blood vessels?

Answer 17

• Osmolarity between intracellular, interstitium and blood vessels= v similar
• Causes water to move freely across both cell membrane and capillary wall
• When more water is added to the system (taking a drink) the osmotic pressure changes

An osmotic pa of 7atm stops flow. Therefore, osmotic pressure of bodily fluids = 5.800 mmHg/7 atm

Question 18

What is the difference between isosmotic and isotonic? Give examples

Answer 18

Isosmotic=conc of water in and outside the cell is the same

Isotonic=conc of solutes in and outside the cell is the same

150 mOsm NaCl is isosmotic and isotonic
300 mOsm glucose is isosmotic, not isotonic
300 mOsm urea is isosmotic but not isotonic

Question 19

Draw a diagram to explain the influence of a solution’s osmolality on cell size

Answer 19

Question 20

What can be a cause of rapid changes in osmolarity?

Answer 20

Clinical note: rapid changes in serum sodium concentration (osmolarity) can cause swelling of brain cells and cause confusion and fits

Question 21

What are the different ways to measure volumes?

Answer 21

Question 22

Salt and water balance depends on Osmoregulation and volume regulation. Describe osmoregulation

Answer 22

Osmoregulation: maintain osmotic equilibrium between ICFV and ECFV
Na is the principal electrolyte contributing to ECFV osmolarity. Body achieves osmoregulation by adding/removing water, not Na
Plasma osmolarity rises: more water needed. Kidneys respond my producing small volume of conc urine (water retention)
Plasma osmolarity falls: too much water. Kidneys respond by producing large volume of dilute urine (water excretion)

Question 23

Salt and water balance depends on Osmoregulation and volume regulation. Describe volume regulation

Answer 23

Volume Regulation: maintain adequate ECFV to support plasma volume
Specific to the control of the circulating plasma volume
Changes detected by stretch and pa receptors in CVS.
Fall in blood volume is opposed by sodium retention; water follows osmotically, restoring volume
Although total body sodium amount may be increased, conc and osmolarity is little changed bc the retained sodium brings more water with it.

Question 24

Describe oedema

Answer 24

Fluid accumulation in the interstitium is called oedema
Fluid accumulation in the interstitium lungs causes shortness of breath
Fluid accumulation in the interstitium of lower limbs causes discomfort

Question 25

What are the causes of oedema?

Answer 25

Causes of oedema:
Increased capillary fluid pressure
Decreased capillary osmotic pressure
Leaky membrane
Blocked lymphatics

Question 26

What do these images represent?

Answer 26

Oedema due increased capillary pressure

Question 27

What does this image represent?

Answer 27