Fluid compartments and sodium balance

The human body derives support and nourishment from the external environment but its function is relatively independent of it. This independence comes from the fact that the tissues are contained from the external environment and is constituted by a separate internal environment that consists mainly of fluids circulating the body. 60% of the body is composed of fluids much of which are extracellular fluids (interstitial fluids and vascular fluid).

Vascular fluid refers to the fluid portion of blood i.e blood plasma. Interstitial fluids are the fluids that surround the cells of the body which is filtered from Vascular fluid through capillaries. It brings nutrients to cells and drains waste products from them towards lymph vessels. Most cells are within 50 microns of a capillary which ensures rapid diffusion of nutrients from circulation to interstitial fluid to cells and vice versa. ECF is in  intimate contact with 4 organs that interface with external environment: alimentary canal, lungs, kidneys, skin.
Figure 1
The ionic composition of plasma and interstitial fluid is similar. Figure 1 demonstrates that plasma is slightly hypertonic compared to interstitial fluid. As explored in the cardiovascular system, this hypertonicity as well as higher hydrostatic pressure is important in the arteriolar area to allow net flow from plasma to interstitial fluid to occur. Protein content in plasma is higher than interstitial fluid as larger protein molecules cannot pass through the capillaries. As the ionic concentration drops due to diffusion over the capillary bed, protein concentration draws fluid back into vascular circulation in the venule side. 

Figure 2
Osmolarity is similar in ECF as ICF which is important for cell volume regulation (Figure 1 and 2). Figure 2 demonstrates that ICF has high K+, low Na+, Cl-, HCO3-, Ca2+. Na+,Cl- is the main ion in ECF and K+ is the main ion in ICF.

 As an example of feedback, increase in sodium intake will result in an increase in excretion of sodium (Figure 3). Stepping up intake from 7-15g Na+ will result in a positive sodium balance for the first 2 days after as loss<gain (due to the delay in the specific negative feedback mechanisms) . On the 4th day, sodium excretion will match intake to establish a new stable balance (as loss=gain) however there will be a residual percent increase in total body sodium which is the error signal that will continue to signal to the kidneys to excrete 15g/day instead of 7g/day.
Figure 3


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