Fluid Compartments of the Body

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Two Compartments of Body Fluids:

• Body fluids are present in two main compartments inside the body.

• The two compartments are "inside" and "outside" cells.

Extracellular and Intracellular Fluid:

• 1/3 is extracellular fluid, and 2/3 is intracellular fluid.

• The body is in fluid balance: water and solutes are correctly proportioned among various compartments.

• Processes such as filtration, absorption, diffusion, and osmosis provide a means to move fluid among the compartments.

Water Movements in Compartments:

• The concentrations of solutes provide the means to move water among the compartments. Water follows solute.

• Most solutes in the body are electrolytes.

• The kidneys can excrete urine of various concentrations, a very important concept as far as fluid balance is concerned.

• Some electrolytes present are: Na, K, Cl, bicarbonate, and protein.

• Water flows between compartments based on osmolarity.

• High interstitial fluid osmolarity draws water out of cells, low interstitial fluid osmolarity draws water into cells.

Changes in Osmolarity:

• Changes in osmolarity often result from changes in concentration of Na and Cl.

• Water moves such that intracellular osmolality = extracellular osmolality.

• Fluid balance between intravascular fluid and interstitial fluid is maintained by the colloid osmotic pressure balance by hydrostatic pressure in kidneys.

Water Intoxication:

• A state in which excessive body water causes cells to become hypotonic and to swell dangerously. This is caused by replacement of fluid loss by plain water, causing water to move into cells from ICF.

Water Dehydration:

• When water loss > water gain, a condition called dehydration results.

• Dehydration is a decrease in volume and an increase in osmolarity of body fluids - stimulates thirst. Water moves out of cells into ECF causing cell shrinkage.

Thirst Center:

• Thirst center in hypothalamus: stimulates thirst.


• Angiotensin II : Stimulates aldosterone secretion.

• Aldosterone: Increases water absorption by kidneys.

ANP and ADH:

• ANP: Increases loss of water in urine.

• ADH: Decreased loss of water in urine.

Cholera Toxin:

• Cholera stimulates secretion of NaCl and water.

• 5-10 liters of diarrhea per day.

• Toxin stimulates NaCl secretion by epithelial cells in Crypts of Lieberkuhn.

• ECF water loss. Acidosis, dehydration, hypovolemia, coma, circulatory failure, death.

More Water Body Notes:

• Colon: main function is to convert liquid ileal to solid feces.

• Diarrhea: an increase in frequency, volume, and fluid content of feces caused by increased motility of and decreased absorption of the intestines.

• Frequent diarrhea causes: electrolyte imbalance and dehydration.

Additional Reading:

Basic Endocrinology

1. Introduction to Endocrinology
2. Hypothalamic-pituitary system
3. Adrenal Hormones
4. Antidiuretic Hormone (ADH) and ECF Regulation
5. Endocrine Pancreas
6. Growth Hormone
7. Adrenal Medulla
8. Hormonal Control of Calcium and Phosphate
9. Thyroid Hormones
10. Hormones of Male Reproduction
11. Hormones of Female Reproduction
12. Fluid Compartments of the Body
13. Notes on Hypothalamus Anterior Pituitary and Thyroid
14. Additional Notes on Female Reproduction
15. Hormonal Signaling Pathways
16. FAQ on Adrenal Hormones
17. FAQ on Male Reproduction
18. Synthesis and Deficiencies of Adrenal Hormones
19. Significance of Glycosylated Hemoblogin (HbA1c)
20. Significance of Measuring Albumin while with Calcium Levels
21. Stepwise Approach to Treatment of Ascites
22. How to differentiate between Diabetes Insipidus vs Psychogenic Polydipsia

Related Topics

1. Histology of the Endocrine System
2. Histology of the Male Reproductive System
3. Histology of the Female Reproductive System

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