Body fluid compartments

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Body fluid compartments


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Body fluid compartments

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USMLE® Step 1 style questions USMLE

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A study is performed to understand the fluid compartments of the body. Researchers divide body fluid compartments into two main categories: intracellular and extracellular. Extracellular gets further subdivided into interstitial fluid and plasma. Which of the following statements is correct?  

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Fluid compartments p. 605


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Water is the key to life, and it takes up a big proportion of our body weight, typically around 60 percent! The precise amount of water depends on a person’s body composition.

Since fat doesn’t store any water, a person’s water content is inversely proportional to a person’s fat content.

So a really muscular and lean person would have a relatively high proportion of their body weight made up of water.

Additionally, females tend to have more fat than males and so on average tend to have lower proportion of their body weight made up of water.

Total body water can be subdivided into two major compartments, intracellular fluid which is fluid inside cells, and extracellular fluid which is fluid outside of cell like in the blood and in the interstitial tissue between cells.

Assuming that the total body water is about 60% of their body weight, roughly 2/3 of that, or 40% is intracellular fluid, and the other 1/3 or 20% is extracellular fluid. This is also known as the 60-40-20 rule.

Intracellular fluid is important for dissolving cations which are molecules with a positive charge, and anions which are molecules with a negative charge.

The major intracellular cations are potassium (K+) and magnesium (Mg2+), whereas the major anions are proteins and organic phosphates like ATP.

Fluid compartments always maintain the same concentration of positive charges as negative ones in order to stay electrically neutral - that’s called the principle of macroscopic electroneutrality. So for example, the K+ in the intracellular fluid is balanced out by negatively charged proteins and organic phosphates.

The extracellular fluid can be subdivided further into interstitial fluid, which is the fluid that can be found surrounding the cell, and plasma, which is the aqueous portion of blood.

The major cation in extracellular fluid, both in the interstitial fluid and in the plasma, is sodium (Na+) and the major anions are chloride (Cl-) and bicarbonate (HCO3-).

Now, the plasma makes up about 55% of the blood, while the remaining 45% is mostly made of red blood cells, white blood cells, and platelets.

Plasma is made up of around 90% water and 10% proteins like albumin which help transport hormones and minerals.


Water makes up an essential part of our body weight. In our body, water is distributed in two major fluid compartments: the intracellular and extracellular compartments. The intracellular compartment consists of all the fluids inside cells; whereas the extracellular compartment consists of all the fluids outside of cells. The extracellular compartment is further divided into three subcompartments, which are the plasma, the interstitial, and the transcellular subcompartments. Plasma is the liquid part of blood. The interstitial fluid consists of fluids which surrounds the cells, whereas the transcellular compartment includes fluids such as cerebrospinal fluid, which surrounds the brain and spinal cord; lymph, which is a clear fluid that helps circulate immune cells; and peritoneal fluid, which surrounds the organs in the abdominal cavity. About 60% of our body weight is water, 40% of it is intracellular, and 20% is extracellular. This is known as the 60-40-20 rule �.


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  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "The 'third space'--fact or fiction?" Best Pract Res Clin Anaesthesiol (2009)
  6. "Third-space fluid shift in elderly patients undergoing gastrointestinal surgery: Part 1: Pathophysiological mechanisms" Contemp Nurse (2002)

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