Fluid balance and its distribution

Body fluid or bio fluid 

Human body is  composed of 60-65 % of water .

Water being the most important nutrient of the body serves as solvent , a mode of transport  & a substance for major metabolic reactions.

fluid & electrolyte balance is vital in maintaining homeostasis.

Distribution of body fluid 

1). Intracellular fluid/ cellular fluid     

It's the fluid present inside the cells.

40% of total body weight 

70%of total body fluid 

Potassium is largest content in cellular fluid .

2). Extracellular fluid 

Divided into 2 sites

a). Interstitial sites :

• space between the cells
•  15 % of total body weight 
• 24% of total body fluid 

b). Intravascular sites 

• Blood plasma 
• 5% of total body fluid 
• 6% of total body fluid 

Composition of body fluid 

1). Electrolyte : they are compounds carry charge & water soluble .

Cations  

Positively charged electrolyte 

Na+ , k+ ,ca²+ &  mɡ²+ .

Anion          

Negatively charged electrolytes 

Cl- , Hco₃-², po₄- ³ , so₄- ²

Electrolyte measured in milliequivalents per liter [MEq/L ]

Serum sodium level = 135- 145 MEq/L 

Serum potassium level = 3.5-5.3MEq/L

Serum calcium level = 4-5 MEq/L

Serum magnesium level = 1.5 to 2.4

Serum chloride level = 100-106 MEq/L

Serum phosphate level = 3.5-4.5 mg/dl

Serum bicarbonate level 

Arterial blood bicarbonate value : 22-26MEq/L

Venous blood bicarbonate value : 24-30MEq/L 

2). Minerals 

They catalyse nerve responses, muscle contraction & metabolism .

Calcium , magnesium ,zinc , iron & phosphorus etc. 

3). Cells : RBC,WBC, platelet .

Fluid intake 

Average fluid

intake of an adult is 2600 ml per day .

Thirst sensation is produced by following :

1. Hypothalamus 
2. Dry pharyngeal mucus membrane
3. Angiotensin 
4. Increased plasma concentration 
5. Decrease blood volume 
6. Decrease plasma osmolarity

Fluid output 

Kidneys are the major regulatory organs of fluid balance .

Kidney filters 150 L plasma everyday

& Produce 1500 ml of urine . 

The fluid requirements per kilogram of body weight at different ages :

• Newborn : 80 to 100 ml per kg of body weight
• O to 1 yr : 100 to 150 ml per kg of body weight
• 1 to 2 yrs : 100 to 125 per kg of body weight
• 2 to 10 yr : 75 to 100 ml per kg of body weight
• 11 to 18 yr : 50-75 ml per kg of body weight
• Adults: 40 ml per kg of body weight.

Example : an adult weight 70 kg of body weight requires 

40 × 70 ml = 2800 ml fluid per day .

Fluid requirement in an infant is high due to 

1). It has greater

metabolic rate so Greater fluid loss

2). Immature kidney so less efficient

3). Body surface area is proportionally larger 

4). Rapid respiratory rate 

      RR - 40 to 60 per minute 

Clinical aspects 

1). Body Fluid as vector 

Biofluid can be vector for infection disease such as sexually transmitted disease or blood borne disease .

2). Lab evaluation : 

Body fluid is analysed  to find microbes, inflammation , cancer etc.

Methods of sampling of body fluid 

a). Blood sampling 

• Arterial blood sampling : radial artery puncture 
• Venous blood sampling also called venipuncture 

b). Lumbar puncture for cerebrospinal fluid 

C). Paracentesis for peritoneal fluid 

d). Thoracocentesis for pleural fluid 

e). Amniocentesis for amniotic fluid