Editing Fatty acid (section)

==Circulation==

===Digestion and intake===
{{Main|Digestion#Fat digestion}}
[[Short-chain fatty acid|Short-]] and [[medium-chain fatty acids]] are absorbed directly into the blood via intestine capillaries and travel through the [[portal vein]] just as other absorbed nutrients do. However, [[long-chain fatty acids]] are not directly released into the intestinal capillaries. Instead they are absorbed into the fatty walls of the intestine [[Intestinal villus|villi]] and reassemble again into [[triglyceride]]s. The triglycerides are coated with [[cholesterol]] and protein (protein coat) into a compound called a [[chylomicron]].

From within the cell, the chylomicron is released into a [[lymphatic]] capillary called a [[lacteal]], which merges into larger lymphatic vessels. It is transported via the lymphatic system and the [[thoracic duct]] up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left [[subclavian vein]]. At this point the chylomicrons can transport the triglycerides to tissues where they are stored or metabolized for energy.

===Metabolism===
{{Main|Fatty acid metabolism}}
Fatty acids are broken down to CO{{sub|2}} and water by the intra-cellular [[mitochondria]] through [[beta oxidation]] and the [[citric acid cycle]]. In the final step ([[oxidative phosphorylation]]), reactions with oxygen release a lot of energy, captured in the form of large quantities of [[Adenosine triphosphate|ATP]]. Many cell types can use either [[glucose]] or fatty acids for this purpose, but fatty acids release more energy per gram. Fatty acids (provided either by ingestion or by drawing on triglycerides stored in fatty tissues) are distributed to cells to serve as a fuel for muscular contraction and general metabolism.

====Essential fatty acids====
{{Main|Essential fatty acid}}
Fatty acids that are required for good health but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has a double bond [[Omega-3 fatty acid|three carbon atoms]] away from the methyl end; the other has a double bond [[Omega-6 fatty acid|six carbon atoms]] away from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side.<ref>{{cite book|last=Bolsover|first=Stephen R.|title=Cell Biology: A Short Course|url=https://books.google.com/books?id=3a6p9pA5gZ8C&pg=PA42+|date=15 February 2004|publisher=John Wiley & Sons|isbn=978-0-471-46159-3|pages=42ff|display-authors=etal}}</ref> Two essential fatty acids are [[linoleic acid]] (LA) and [[α-Linolenic acid|alpha-linolenic acid]] (ALA). These fatty acids are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer-chain [[omega-3 fatty acid]]s — [[eicosapentaenoic acid]] (EPA) and [[docosahexaenoic acid]] (DHA), which can also be obtained from fish. Omega−3 and [[Omega-6 fatty acid|omega−6]] fatty acids are [[Biosynthesis|biosynthetic]] precursors to [[Cannabinoid#Endocannabinoids|endocannabinoids]] with [[Nociception|antinociceptive]], [[anxiolytic]], and [[Nervous system|neurogenic]] properties.<ref>{{Cite journal|last1=Ramsden|first1=Christopher E.|last2=Zamora|first2=Daisy|author-link3=Alexandros Makriyannis|last3=Makriyannis|first3=Alexandros|last4=Wood|first4=JodiAnne T.|last5=Mann|first5=J. Douglas|last6=Faurot|first6=Keturah R.|last7=MacIntosh|first7=Beth A.|last8=Majchrzak-Hong|first8=Sharon F.|last9=Gross|first9=Jacklyn R.|date=August 2015|title=Diet-induced changes in n-3 and n-6 derived endocannabinoids and reductions in headache pain and psychological distress|journal=The Journal of Pain|volume=16|issue=8|pages=707–716|doi=10.1016/j.jpain.2015.04.007|issn=1526-5900|pmc=4522350|pmid=25958314}}</ref>

===Distribution===
{{Main|Blood fatty acids}}
Blood fatty acids adopt distinct forms in different stages in the blood circulation. They are taken in through the intestine in [[chylomicron]]s, but also exist in [[very low density lipoprotein]]s (VLDL) and [[low density lipoprotein]]s (LDL) after processing in the liver. In addition, when released from [[adipocytes]], fatty acids exist in the blood as [[free fatty acids]].

It is proposed that the blend of fatty acids exuded by mammalian skin, together with [[lactic acid]] and [[pyruvic acid]], is distinctive and enables animals with a keen sense of smell to differentiate individuals.<ref>{{cite web
 | url=https://www.sciencedaily.com/releases/2009/07/090721091839.htm
 | title=Electronic Nose Created To Detect Skin Vapors
 | date=July 21, 2009
 | website=Science Daily
 | access-date=2010-05-18
}}</ref>