Before I can talk about possible problems with TPN at the End-of-Life, I need to lay a quick foundation describing what the “normal” components are in the blood when we’re healthy, and how those components function to help maintain the balance of fluids in the blood and in the tissues. The scientific term for this is “osmolarity” – which in the most general sense, refers to the concentration of the various components dissolved in water. But in addition to their concentration, it also is related to how big the molecules are that make up these components. The reason that “osmolarity” is so important is that it plays a very significant role in determining whether the blood is loosing water to the surrounding tissues, or pulling it in. If the blood is loosing water to the tissues it can result in dehydration, edema, or in severe cases, anasarca.
In this blog entry, I’ll give you both some definitions of the terms I just introduced, as well as some examples. In my video blog, I will be showing how these processes work using both some animation, and a short time-lapse video.
So, what are the major players among the blood components? Well, of course there are inorganic salts (primarily table salt) and sugar (glucose), as well as a number of other organic components and proteins. When it comes to managing the amount of water in the circulatory system, all of the components just mentioned play a role, but one of the more important components is a protein called albumin (which is manufactured by the liver). When the level of albumin in the blood serum drops, the water normally contained in the blood and blood vessels begins to leak out of the blood and into the tissue. The “why” for this leakage is due, in part, to the change in the balance of osmolarity between the blood and the fluids in the tissue surrounding the blood vessels.
Osmolarity is one of those “scientific” terms, which as I mentioned earlier, generally refers to the concentration of various substances in water. The Wikipedia entry for Osmolarity is too complex for the purposes of my blog. The very high-level explanation I’ll offer is that it’s a way of determining the total concentration of multiple substances in a sample of water. If there are two samples with different osmolarity, the one that has a higher value will tend to draw water from the lower value.
This flow is called osmosis – and the direction is determined by the osmolarity. In short, water moves from the lower to the higher concentration in an attempt to make the two samples equal. (Remember the old adage: Nature abhors a vaccuum? Well, Nature “hates” any imbalance!). Another way to think of osmosis is that if there are two samples with water, each with different substances dissolved, the water will tend to flow from the sample with more water to the one with less water. Think of it as water flowing “downhill.”
Well then, when we have blood in the circulatory system with fewer dissolved substances in it than the fluid surrounding the cells in the tissue – the water will migrate from the blood, through the walls of the blood vessels, and into the fluid in the tissue. The net result is that you end up “dehydrated” from the perspective of the circulatory system, but swollen from the resulting increase in water in the tissue – this is edema.
Stay Well -NurseBob
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