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for starters. . .hydrostatic pressure is the pressure in the pipes (the veins and arteries); colloidal pressure is the pressure exerted by what is in the tissues of the body. these two pressures work against each other. what results will be fluid balance.
[quote=Daytonite;3384369]For starters. . .hydrostatic pressure is the pressure in the pipes (the veins and arteries); colloidal pressure is the pressure exerted by what is in the tissues of the body. These two pressures work against each other. What results will be fluid balance.
My understanding is that hydrostatic pressure is the pushing power of circulating blood and fluid. ie, Fluid pushes itself through the normal "leaks" in the small spaces between the thin cells of the capillary walls. Then the fluid goes into the tissues, cells, etc. outside of the capillaries. This is hydrostatic pressure.
Colloid osmotic pressure is just the opposite, and it is exerted by the protein albumin. Albumin is normally too large to pass through the capillary walls, so it stays inside the capillaries. The structure of albumin is such that it pulls fluid towards itself. (How and why it does this is probably beyond my own ability to understand or explain, but just know that it does this. lol.) This effect of albumin is colloid osmotic pressure. This keeps too much fluid from leaving the capillaries.
The simplified version is that hydrostatic pressure pushes fluid out of the capillaries, while colloid osmotic pressure keeps fluid inside the capillaries.
There's a good visual about this in one of my books. Wish I could show it to you, esp since I had a problem understanding this myself one time.
So where does sodium come in to maintain the fluid balance?
Water always 'follows' solutes. Think of the water trying to seek a balance of solutes inside and outside the vessel. If the body retains or has too much sodium, water will follow, and result in increased capillary volume, increased blood volume, and hence, increased BP. Conversely, if sodium is lost (via kidneys or restricted diet) less water will enter vessels, and blood volume and pressure decreases.
I had to clear this up for my A&P teacher (she is good, but for the life of her, she could not get the point out to the "whole" class).
Hydrostatic pressure= pressure exerted by the water or liquid. Imagine a big pipe filled with water. The pressure at the top is minimal compared to the water pressure at the bottom (due to the "top" water stacking on the "bottom" water). Now imagine laying this pipe down on it's side. Instead of having gravity and "top" water creating the pressure, you have a pump (you guessed it, your heart). This pressure as stated above; forces the liquid into the interstitial space so the cells can make use of the "stuff" it carries.
Osmotic pressure= it's not really "pressure". It's more like a balancing of water. The higher concentration goes to the lower concentration until both are equal. Also stated above, the albumin is what causes this balancing act to happen. Think of it as a sponge. It pulls/attracts liquids back into the veins.
As for sodium: sodium naturally "attract" water. Wherever sodium is, water follows (This is the reason why the angeiotenson/aldonsterone effect increases BP, it retains/attract water; keeping it from being exported from the body. The greater the blood volume, the greater the BP. The more you have to pump = the stronger the pump will be, kinda)
Hydrostatic and oncotic pressure work in opposition of each other but all tissue compartments have both...what counts is the DIFFERENCE between the two, this difference is what determines which way fluid will go. As circulation proceeds from the arterial side of the capillary to the venous side of the capillary fluid will either be lost to the interstitial space (under normal circumstances a small amount of intravascular fluid is always left in the interstitium and is later drained via the lymphatic system), or excess interstitial fluid may be drawn into the venous intravascular space (from the interstitutium). The latter may be facillitated by urinary retention of sodium, thus providing an osmotic gradient for fluid to move from tissue to vascular space. Another scenario could be from the IV administration of mannitol, a large non-soluble molecule that increases intravascular oncotic pressure and causes fluid to leave tissues and enter the vascular space (this is why mannitol is given to patients with pathological cerebral edema/increased intracranial pressure-removing fluid from the cerebral tissue helps rectify the problem although it should be noted an intracranial hemmorage or non-intact cerebral blood-brain barrier is an absolute contraindication to the use of mannitol as it will go into the cerebral parynchama and attract fluid to the already compromised tissue). Osmotic (sodium) pressure of intravascular space is regulated primarily through feedback mediated release of aldosterone which results in urinary retention of sodium thereby causing retention of fluid as well. Under normal circumstances the result is increased BP due to fluid retention and a small quantity of concentrated urine. Pathology that can interrupt this process is diabetes insipidus or SIADH. The takeaway for you should be that not none of these processes work alone, they all complement each other, the key to a comprehensive understanding is not only how they work seperatly but also ho they work together.