What are the symptoms of dehydration?
Sunken eyes
Lack of skin turgor
Low blood pressure
Concentrated Urine
Weight loss
Dry mouth
Rapid pulse
Refusing fluids
What causes dehydration?
Vomiting
Diarrhoea
Excessive sweating
Low water intake
Drainage from burns
Damage to thirst mechanism
Why do we need H2O?
Universal solvent
Chemical reactions occur in water
Found as major component of plasma and cells.
Lubricant of joints, organs
High ability to absorb and retain heat as stays in liquid state over broad range (0-100 0C) of temperature
Large amount of heat required to increase its temperature (this stabilises body temperature plus sweating carries a lot of heat away from body)
Where does our body water come from?
Drinks 1500ml
Food 750ml
Water of metabolism 250ml
How much water is in our body?
32-40L approx 60%
Where do we lose water from our bodies?
Urine 1500ml
Stools 100ml
Sweat 200ml
Respiratory loss 700ml
Where is the water in the body?
Intracellular fluid 25L=2/3=66%
Extracellular fluid/Interstitial fluid 12L=30%
Plasma 3L=7.5%
Joints, CSF 0.7L
What are aquaporins?
Semi-permeable membranes
Define simple diffusion?
Movement from an area of high concentration to an area of low concentration through a semi-permeable membrane.
This is passive.
Define defusion?
Allows substances to move in and out of cells e.g. organelles, large proteins, blood etc
What factors affect diffusion rate?
Temperature i.e. Increased temperature increases motion of particles
Molecular weight i.e. Larger molecular weight means particles move slower
Steepness of conc. gradient i.e. increased difference, increased rate
Membrane surface area i.e. increased area increased rate
Membrane permeability i.e. increased permeability, increased rate
Define osmosis?
Movement of water down its concentration gradient accross a semipermiable membrane
This is passive
What is a solution?
A homogenous mixture of one or more dissimilar substances
Usually a liquid (solvent) and a solute (electrolyte - dissolved particle)
What are the three types of solutions?
Aqueous solutions
Colloids
Suspensions
What is an aqueous solution?
Small molecules dissolve
Ionic compounds dissociate into anions (-ve) and cations (+ve)
Electrolytes are soluble inorganic molecules whose ions conduct electricity in solution
What is a colloid?
A solution where proteins or other large molecules as solute e.g. plasma or albumin
What is a suspension?
A solution with even larger molecules than colloid solutions
Particles settle if undisturbed
What is pH?
A low pH is acidic with a high amount of H+ ions, a high pH is basic with a high amount of OH- ions
The body has a pH of 7.35
What is osmotic pressure?
The pressure exerted by water in a solution
OR
Exerted by solutes drawing water into area of low water
OR
The force (thrust) of water movement into the higher concentration solution
OR
The amount of hydrostatic pressure required to stop osmosis
OR
Pull of fluid from high -> low concentration
e.g. NA has high osmotic pressure as it pulls water across semi permeable membrane
How can the concentration of solutions be measured?
- ) g/L - weight per volume
- ) % - weight in grams per 100ml
- ) Moles of solute per litre i.e. 1 mole = Number of atoms/molecules present in a sample equal to the molecular weight in grams
so 1 mole H2=1 mole O2=1 mole NaCl same concentration
Why use moles?
Gives better ability to comare as number of molecules in solution is more important
What is osmolarity?
Number of osmoles/L solution (H20)
Dependent on number of particles dissolved rather than the weigh of the molecule
What is osmolality?
Number of osmoles/kg (H20)
How do you measure osmotic concentration?
Osmoles/L or Kg
What is 1 osmole?
1 mole of dissolved particles i.e. 1 mole of NaCl contains 1 mole of Na+ ions and 1 mole of Cl- ions = 2 osmoles/L
What does iso-osmotic mean?
Having the same osmotic pressure e.g. 300 mosmol Glucose, 300 mosmol NaCl, 300 mosmol Plasma
Same osmolarity so isoosmotic to each other
mosmol=milli osmol=1/1000 osmol
Define tonicity?
Ability of a solution to affect the cell
Depends on concntration and permeability of the membrane to the solutes
What does isotonic mean?
Same tonicity
What does hypotonic mean?
Having a lower tonicity than a particular fluid
What does hypertonic mean?
Having a higher tonicity than a particular solution
What is the tonicity inside and outside the cell?
Isotonic
Concentration of electrolytes outside the cell is equal to that inside the cell
No water movement
No change in cell shape
What do hypotonic solutions in the body do?
Has low concentration of solutes that do not cross the membrane
High Water content (more water, less solute)
Cell absorbs water, swells and may burst (lysis)
NOT IDEAL
What do hypertonic solutions in the body do?
Low water concentration and high concentration of solutes that do not cross the membrane
Water moves out of cell to try and balance water/ion concentation
Cells lose water and shrivel up
NOT IDEAL
How does the body tell us we need more water?
Thirst
What happens in the body when body water decreases?
Osmoreceptors sense changing conc.
Baroreceptors sense changing pressure
These are communicated to thirst centre in hypothalamus
Hypothalamus tells posterior pituitary to release ADH to reabsorb more water from urine and sends message to make mouth drier which makes you feel thirsty and makes you drink
This is negative feedback
What are the treatments for dehydration?
Fluid therapy
Gelofusine (plasma expander, increases volume of fluid, isotonic to replace blood loss)
0.9% saline (0.9g saline in 100ml of water)
5% glucose/dextrose solution (5g glucose in 100ml of water)
What two problems can result from overhydration?
Hyponatraemia (low sodium)
Water intoxication
e.g. ecstasy - take too much water, dilutes body fluids, cell swell, lysis
What is hydrostatic pressure?
The force exerted on the membrane by water/fluid
In a resting cell this will be stable as intracellular=interstitial
What influences water movement?
Hydrostatic pressure
Movement of fluid between compartments through a selectively permeable membrane driven by hydrostatic pressure
What effect does fluid movement have?
Any fluid movement results in a change in osmotic pressure (electrolytes on one side of membrane) therefore cell will change size
Water will always follow salts