The internal environment + two major fluid compartments

What is the internal environment

• Cells suspended in fluid inside the body

What is intracellular fluid (ICF)

• Fluid inside the cells

What is extracellular fluid (ECF)

• Fluid outside the cells

What are the two main parts of the ECF

• Interstitial fluid (around cells)

• Plasma (in blood)

Where is interstitial fluid found

• In the spaces between cells

Where is plasma found

• In the blood vessels

Total body water in a 70 kg person

• 40 L (≈60% of body weight)

Intracellular fluid (ICF) volume in a 70 kg person

• 25 L (≈40% of body weight)

Extracellular fluid (ECF) volume in a 70 kg person

• 15 L (≈20% of body weight)

Interstitial fluid (part of ECF) volume

• 12 L (≈80% of ECF)

Plasma (part of ECF) volume

• 3 L (≈20% of ECF)

What does plasma transport

• Plasma carries oxygen and nutrients through the bloodstream

How does plasma interact with interstitial fluid (ISF)

• Plasma leaks out of blood vessels to become ISF around the cells

How do substances move from ISF to cells

• Nutrients and oxygen travel from ISF directly into the cells

What is intracellular fluid (ICF) used for

• ICF supports chemical reactions that keep cells functioning

What percentage of body mass is fluid

• About 60% of total body mass is fluid

How does body fluid percentage change with age

• Babies and children have a higher fluid percentage; it decreases with age

What are body fluids (ICF + ECF)

• They are solutions made of a solvent and solutes

What is the solvent in body fluids

• Water

What are solutes in body fluids

• Substances dissolved in water (e.g., O₂, CO₂, K⁺, Na⁺)

What is a solution

• A solvent (water) + solute(s) (dissolved substances)

Why are solutes important in body fluids

• They allow chemical reactions, electrical activity, and fluid balance