Study unit 1: Body water
Body water:
- Water is the most abundant molecule in the body
- Water is the solvent for all living matter
- 50-60% of the total body weight is water
Functions of water:
1. Solvent and transport medium
- Soluble substances: ions
- Insoluble substances: lipids
2. Absorption
- Digestive system
3. Excretion
- Through skin and kidneys
- Get rid of metabolic waste
4. Medium for chemical reactions/reagent in some chemical reactions
5. Ionization of atoms
- Determine electrical activity of cells
- Water so some degree ionizes itself to H+ and OH- ions
- Causes some molecules to ionize
6. Lubrication
- Joints and digestive system
7. Stabilizing body temperature
- Thermo-homeostasis: maintenance of a constant internal temperature due to the high
specific heat capacity of water.
→ Properties of water that makes it suitable:
o Great heat capacity
o Good heat conduction
o High latent temperature of evaporation
Water content in body varies according to:
- Age
- Sex
- % body fat
, Water content as percentage of total body mass by age and sex:
AGE MALE FEMALE
Infant 65% 65%
1-9 62% 62%
10-16 59% 57%
17-39 61% 51%
40-59 55% 47%
60+ 52% 46%
Terminology
- Intracellular fluid (ICF): water inside cell
- Extracellular fluid (ECF): water in all the spaces outside cell
o Interstitial fluid (intercellular): in between.
o Blood plasma
o Trans-cellular fluid (inside hollow organs). Lymph, cerebrospinal fluid, fluid of
gastrointestinal tract, intra-ocular fluids, etc.
- Intravascular fluid: inside blood vessels
- Extravascular: outside blood vessels
- The volume of water in the body is controlled by a delicate balance between:
o Intake
o Excretion
o Reabsorption
NB slide 13 and 14
- The body fluids are in two compartments: the extracellular fluid (ECF) and intracellular fluid
(ICF). The ECF and ICF are in osmotic equilibrium but have very different chemical
composition.
o Intracellular fluid is 2/3 of the total body water volume. Material moving into and
out of the ICF must cross the cell membrane.
o Extracellular fluid includes all fluid outside the cells. It makes up 1/3 of the body fluid
volume and consists of:
▪ Interstitial fluid (IF), which lies between the circulatory system and the cells,
is 75% of the ECF volume.
▪ Plasma, the liquid matrix of the blood, is 25% of the ECF volume. Substances
moving between the plasma and the interstitial fluid must cross the leaky
exchange epithelium of the capillary wall.
Water loss
- The body has no water reserves
o 5% water loss – thirsty
o 10% water loss – sick
o 20% water loss – death
- No water intake: 4-6 days (death)
- Minimum intake: 0.5-1 liter/day
- Compulsory urine loss: 300-500 ml/day
o (Anuria- no urine passed)
Body water:
- Water is the most abundant molecule in the body
- Water is the solvent for all living matter
- 50-60% of the total body weight is water
Functions of water:
1. Solvent and transport medium
- Soluble substances: ions
- Insoluble substances: lipids
2. Absorption
- Digestive system
3. Excretion
- Through skin and kidneys
- Get rid of metabolic waste
4. Medium for chemical reactions/reagent in some chemical reactions
5. Ionization of atoms
- Determine electrical activity of cells
- Water so some degree ionizes itself to H+ and OH- ions
- Causes some molecules to ionize
6. Lubrication
- Joints and digestive system
7. Stabilizing body temperature
- Thermo-homeostasis: maintenance of a constant internal temperature due to the high
specific heat capacity of water.
→ Properties of water that makes it suitable:
o Great heat capacity
o Good heat conduction
o High latent temperature of evaporation
Water content in body varies according to:
- Age
- Sex
- % body fat
, Water content as percentage of total body mass by age and sex:
AGE MALE FEMALE
Infant 65% 65%
1-9 62% 62%
10-16 59% 57%
17-39 61% 51%
40-59 55% 47%
60+ 52% 46%
Terminology
- Intracellular fluid (ICF): water inside cell
- Extracellular fluid (ECF): water in all the spaces outside cell
o Interstitial fluid (intercellular): in between.
o Blood plasma
o Trans-cellular fluid (inside hollow organs). Lymph, cerebrospinal fluid, fluid of
gastrointestinal tract, intra-ocular fluids, etc.
- Intravascular fluid: inside blood vessels
- Extravascular: outside blood vessels
- The volume of water in the body is controlled by a delicate balance between:
o Intake
o Excretion
o Reabsorption
NB slide 13 and 14
- The body fluids are in two compartments: the extracellular fluid (ECF) and intracellular fluid
(ICF). The ECF and ICF are in osmotic equilibrium but have very different chemical
composition.
o Intracellular fluid is 2/3 of the total body water volume. Material moving into and
out of the ICF must cross the cell membrane.
o Extracellular fluid includes all fluid outside the cells. It makes up 1/3 of the body fluid
volume and consists of:
▪ Interstitial fluid (IF), which lies between the circulatory system and the cells,
is 75% of the ECF volume.
▪ Plasma, the liquid matrix of the blood, is 25% of the ECF volume. Substances
moving between the plasma and the interstitial fluid must cross the leaky
exchange epithelium of the capillary wall.
Water loss
- The body has no water reserves
o 5% water loss – thirsty
o 10% water loss – sick
o 20% water loss – death
- No water intake: 4-6 days (death)
- Minimum intake: 0.5-1 liter/day
- Compulsory urine loss: 300-500 ml/day
o (Anuria- no urine passed)
