Emulsifiers

Emulsifiers are substances that can mix oil and water by being one large molecule that has nonpolar (only carbon and hydrogen like most of fat molecules)  parts for dissolving fats + other non-polar substances and polar parts (basically other elements besides carbon and hydrogen) to dissolve water, metals and charged organic substances.
In food industry these molecules are used to avoid fat and water separating but bodies use similar molecules to keep fats dissolved in water and to avoid waterproof oil droplets from forming in blood vessels.

Animal and plant cells many brownish-yellowish lecithin type molecules and above substance is one such example. It has long areas that bind preferably to fats and somewhat long area in black and red that binds polar substances. Substances of this class are used in food as natural emulsifiers.

Soap is also one type of emulsifier with typical long non-polar area together with charged very small area which demonstrates how little of it has to be charged to help dissolve charged particles like dust.

Sodium stearoyl lactylate is example of emulsifier approved for use in food.

Bile acids illustrated above. One of the roles for bile is to dissolve fat in digestive tract so it wouldn't try to float on top. Emulsifiers in bile are produced from cholesterol with charged parts usually at opposite ends of molecule. One common molecule that is added to cholesterol by body is taurine that is common in energy drinks. Emulsifiers in bile are toxic to body and one study mentioned that starting from 10 micro-moles of some bile acid per liter started to show some minor damage to biliary cells from biliary tract themselves although large scale damage was not seen even with 5 times that concentration. More hydrophobic ones seemed more damaging.     

Emulsifier are also potentially toxic by dissolving cell membranes as their non-polar part tries to be in cell membranes while polar area is more attracted to water outside membrane which combined with thermal motion and bumping together with other molecules can pry cell membranes open.

One the other hand huge number of proteins seem mainly safe although they have some emulsifier role. For example every receptor on surface of cell has non-polar parts that go through membranes and polar parts that stay outside membrane. Probably every protein has enough differently charged amino acids to turn entire protein into emulsifier. If body didn't have emulsifiers its fat could float up to maybe brain which sounds deadly but i don't know any health problem causing that as body has enough emulsifiers to avoid it and even if some mutation caused such oil layering then they wouldn't be likely to survive in fetal development to reach live birth.  

In bloodstream lipoproteins are freely moving partially water soluble proteins that carry around fat molecules. Some of them are connected to longevity. In general lipoproteins carry fats and cholesterol between liver and body tissues (some move them out of liver while other lipoproteins carry fats back to liver). Low density lipoproteins (LDL) carries cholesterol from liver to other tissues and it is sometimes called "bad cholesterol" lipoprotein (although "bad" and "good" is misleading as cholesterol is same but direction of transport between liver-body determines these "good-bad" labels). "Good cholesterol" lipoproteins are also called high density lipoproteins (HDL) as they are smaller than LDL (diameter difference is ~2-5 fold). HDL seem helpful because they carry cholesterol from body to liver and in general people that for some reason have relatively more HDL than other people tend to have less cardiovascular diseases.