1.1 Body Fluid
Let's take a look at human body fluid. I think this is one of the topics that should be handled at the beginning of studying human physiology. Today we will discuss basics of body fluid including its composition and volume.
Body fluid is a solution of water, ions and other substances. In normal adult human body, body fluid takes about 60% of the weight. This percentage can vary from approximately 50~70%. Muscles contain more water than fat tissues, making this percentage changeable according to the muscle and fat ratio. This total body fluid can be divided into two main compartments: Intracellular fluid (ICF) and Extracellular fluid (ECF).
1) Intracellular Fluid (ICF)
As we can tell by its name, ICF is the fluid inside the cells. ICF takes two-third of the body fluid, meaning 40% of the body weight. The ICF contains large amount of K+, PO43-. ICF is also rich in proteins that contribute to the cell's structure and function. However, ICF has low concentration of Na+, Cl-, HCO3-, and Ca2+.
2) Extracellular Fluid (ECF)
In contrast to the ICF, ECF is the fluid outside the cell. ECF takes the remaining one-thirds of the body fluid, meaning 20% of the body weight. ECF has opposite ion composition to the ICF. ECF has high concentration of Na+, Cl-, HCO3-, Ca2+, and low concentration of K+, PO43-, proteins. ECF includes nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains waste products produced by the cells such as carbon dioxide and urea.
I. Subcompartments of ECF
ECF can be further divided into subcompartments.
- Interstitial Fluid (ISF): This is the fluid that surrounds the cells in the spaces between cells. ISF constitutes about 75% of the ECF, meaning 15% of the total body weight.
- Plasma: The liquid part of the blood, which makes up about 25% of the ECF and 5% of the total body weight. Plasma has similar composition to ISF but with higher protein content, especially albumin.
- Transcellular Fluid: Transcellular fluid is a small but significant part of the ECF. It encompasses the fluids found in various body spaces that are lined by epithelial cells. Pleural, peritoneal, pericardial, synovial, intraocular and cerebrospinal fluid are all transcellular fluids.
3) Major Ions comparison
Table 1. Major Ion composition of ICF and ECF
We can notice different ionic composition of ICF and ECF. Major cation of ICF is potassium ion, and major anions of ICF are phosphate ion and proteins. In ECF, on the other hand, sodium and calcium ions are major cations, and chlorine and bicarbonate ions are major anions. These various charged particles balance each other out making both ICF and ECF electrically neutral. Cations and anions of ICF balance each other out while those of ECF also balance each other out. However, it is to note that even though ICF and ECF are electrically neutral respectively, there are localized differences in charge distribution across the cell membrane, in other words, membrane potential. We will discuss membrane potential in following chapters.
Today we studied compartments of body fluid and their ionic compositions. Let's take a look at how these different types of body fluids are separated next time.
