Endocardium

{{short description|Innermost layer of tissue lining the chambers of the heart}}

{{Infobox anatomy

| Name = Endocardium

| Latin = endocardium

| Image = Gray493.png

| Caption = Interior of right side of heart

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File:Blausen 0470 HeartWall.png

The endocardium ({{plural form}}: endocardia) is the innermost layer of tissue that lines the chambers of the heart. Its cells are embryologically and biologically similar to the endothelial cells that line blood vessels. The endocardium also provides protection to the valves and heart chambers.{{cite journal |last1=Brutsaert |first1=D. L. |last2=Andries |first2=L. J. |title=The endocardial endothelium |journal=American Journal of Physiology-Heart and Circulatory Physiology |date=October 1992 |volume=263 |issue=4 |pages=H985–H1002 |doi=10.1152/ajpheart.1992.263.4.H985 |pmid=1415782 }}

The endocardium underlies the much more voluminous myocardium, the muscular tissue responsible for the contraction of the heart. The outer layer of the heart is termed epicardium and the heart is surrounded by a small amount of fluid enclosed by a fibrous sac called the pericardium.{{cite book |last1=Tran |first1=Dan B. |last2=Weber |first2=Carly |last3=Lopez |first3=Richard A. |title=StatPearls |date=2025 |publisher=StatPearls Publishing |chapter-url=http://www.ncbi.nlm.nih.gov/books/NBK545195/ |chapter=Anatomy, Thorax, Heart Muscles |pmid=31424779 }}

==Function==

File:Endocardium and subendocardium histology.pngThe endocardium, which is primarily made up of endothelial cells, controls myocardial function. This modulating role is separate from the homeometric and heterometric regulatory mechanisms that control myocardial contractility.{{cite journal |last1=Milgrom-Hoffman |first1=Michal |last2=Harrelson |first2=Zachary |last3=Ferrara |first3=Napoleone |last4=Zelzer |first4=Elazar |last5=Evans |first5=Sylvia M. |last6=Tzahor |first6=Eldad |title=The heart endocardium is derived from vascular endothelial progenitors |journal=Development |date=November 2011 |volume=138 |issue=21 |pages=4777–4787 |doi=10.1242/dev.061192 |pmid=21989917 |pmc=3190386 }} Moreover, the endothelium of the myocardial (heart muscle) capillaries, which is also closely appositioned to the cardiomyocytes (heart muscle cells), is involved in this modulatory role.{{Cite web |title=Endothelial Dysfunction |url=https://stanfordhealthcare.org/medical-conditions/blood-heart-circulation/endothelial-dysfunction.html |access-date=2023-02-23 |website=stanfordhealthcare.org}} Thus, the cardiac endothelium (both the endocardial endothelium and the endothelium of the myocardial capillaries) controls the development of the heart in the embryo as well as in the adult, for example during hypertrophy. Additionally, the contractility and electrophysiological environment of the cardiomyocyte are regulated by the cardiac endothelium.{{cite journal |last1=Brutsaert |first1=Dirk L. |last2=De Keulenaer |first2=Gilles W. |last3=Fransen |first3=Paul |last4=Mohan |first4=Puneet |last5=Kaluza |first5=Grzegorz L. |last6=Andries |first6=Luc J. |last7=Rouleau |first7=Jean-Lucien |last8=Sys |first8=Stanislas U. |title=The cardiac endothelium: Functional morphology, development, and physiology |journal=Progress in Cardiovascular Diseases |date=November 1996 |volume=39 |issue=3 |pages=239–262 |doi=10.1016/s0033-0620(96)80004-1 |pmid=8970576 }}

The endocardial endothelium may also act as a kind of blood–heart barrier (analogous to the blood–brain barrier), thus controlling the ionic composition of the extracellular fluid in which the cardiomyocytes bathe.{{cite book |last1=Dotiwala |first1=Ary K. |last2=McCausland |first2=Cassidy |last3=Samra |first3=Navdeep S. |title=StatPearls |date=2025 |publisher=StatPearls Publishing |chapter-url=http://www.ncbi.nlm.nih.gov/books/NBK519556/ |chapter=Anatomy, Head and Neck: Blood Brain Barrier |pmid=30137840 }}