Therefore, it is unsurprising to find overlapping mechanisms controlling cardiovascular function and body fluid homeostasis. On the other hand, heart failure may decrease water and sodium urinary excretion, promoting a fluid disorder in the body. Increased sodium levels in ECF increase the effective circulating volume, leading to an enhancement in cardiac output and blood pressure (BP). Furthermore, some pathological cardiovascular conditions may lead to changes in body fluid homeostasis. Whereas the osmolarity of ECF is regulated by water intake and renal water excretion, the volume is controlled by the sodium content in the ECF, which is determined by the amount of sodium intake and the amount of sodium excreted in the urine (Verbalis 2003).Ĭhanges in the water and sodium content of ECF may result in severe consequences to the cardiovascular system. The relationship between water and sodium in ECF may change both the osmolarity and volume of this compartment. Body sodium and fluid balance is achieved through mechanisms that control sodium intake and sodium urinary excretion. Sodium is the main electrolyte in ECF and sodium loss or gain is usually accompanied by an increase or decrease in water in this compartment in order to maintain sodium concentration. The mechanisms that keep the body fluid osmolarity within its narrow range (280–300 mOsm/L) depend on matching the water volume excreted by the kidneys and the fluid intake volume. The main source of body water in terrestrial vertebrates is from drinking and, once in the body, it is distributed between body fluid compartments. The physical barrier between the intracellular fluid and interstitial fluid is the cell plasma membrane, and the physical barrier between the interstitial fluid and plasma is the capillary endothelium.Body fluid homeostasis is maintained by a steady-state interchange of water between extracellular fluid (ECF) and intracellular fluid compartments. These fluid compartments are emphasized by the gray background color. If we examine this figure carefully, and if we consider the functional interconnections of the fluid compartments, it becomes clear that there are three major fluid compartments: (1) intracellular fluid, (2) interstitial fluid, and (3) plasma. Transcellular fluid generally comprises up to about 2.5% of the total body water (or up to ~1 L). Cerebrospinal fluid (the fluid bathing the brain and the spinal cord), intraocular fluids (aqueous and vitreous humors), inner ear fluids (endolymph and perilymph), pericardial fluid, peritoneal fluid, synovial fluids (in joints), as well as some other fluids are components of the transcellular fluid in the body. This fluid is generally separated from the plasma by an additional epithelial layer in addition to the capillary endothelium. Transcellular fluid is found in small amounts in different body regions and, in total, comprises a very small portion (though not unimportant) of the total body water. It is generally about 75% of the volume of ECF (or ~10.5 L). Interstitial fluid is the fluid that directly bathes the cells and tissues in the body. The extravascular compartment is further subdivided into two compartments: interstitial fluid and transcellular fluid. The volume of plasma is about 3 L and is generally about 25% of the volume of ECF. Plasma is the fluid portion of blood (i.e., whole blood minus the so-called formed elements red cells, white cells, and platelets). The fluid in the intravascular compartment is the fluid that is present in all of body blood vessels. These two compartments are separated by the capillary endothelium (lining of the walls of the capillaries). The extracellular fluid (ECF) compartment is further subdivided into the intravascular compartment and the extravascular compartment. Approximately 33% of the total body fluid (or ~20% of total body weight) is in the extracellular fluid (ECF) compartment. The intracellular fluid is separated from the extracellular fluid by the cell plasma membrane. Therefore, the total volume of ICF is ~28 L (cytoplasmic volume of all cells combined). Approximately 67% of total body water (or ~40% of total body weight) is in the intracellular fluid (ICF) compartment, which is the fluid that is present in the cytoplasm of all cells of the body. If we use 70 kg (155 lb) to represent the average adult male, the total volume of water in the body can be calculated to be about 42 L.
In a typical adult male human, approximately 60% of the total body weight is composed of water.
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