Editing Heart (section)

===Location and shape===
[[File:Real-time MRI - Thorax.ogv|thumb|[[Real-time MRI]] of the human heart]]
[[File:Heart near.png|thumb|left|upright|The human heart is in the middle of the [[thorax]], with its apex pointing to the left.<ref>{{cite web |url=http://www.bartleby.com/107/284.html |title=Gray's Anatomy of the Human Body – 6. Surface Markings of the Thorax |publisher=Bartleby.com |access-date=2010-10-18 |url-status=live |archive-url=https://web.archive.org/web/20101120145802/http://bartleby.com/107/284.html |archive-date=20 November 2010}}</ref>]]

The human heart is situated in the [[mediastinum]], at the level of [[thoracic vertebrae]] [[Fifth thoracic vertebra|T5]]–[[Eighth thoracic vertebra|T8]]. A double-membraned sac called the [[pericardium]] surrounds the heart and attaches to the mediastinum.<ref>{{cite book|last1=Dorland's|title=Dorland's Illustrated Medical Dictionary|date=2012|publisher=Elsevier|isbn=978-1-4160-6257-8|page=1461|edition=32nd}}</ref> The back surface of the heart lies near the [[vertebral column]], and the front surface, known as the sternocostal surface, sits behind the [[sternum]] and [[costal cartilage|rib cartilages]].<ref name="CNX2014"/> The upper part of the heart is the attachment point for several large blood vessels—the [[venae cavae]], [[aorta]] and [[pulmonary trunk]]. The upper part of the heart is located at the level of the third costal cartilage.<ref name="CNX2014"/> The lower tip of the heart, the '''apex''', lies to the left of the sternum (8 to 9&nbsp;cm from the [[midsternal line]]) between the junction of the fourth and fifth ribs near their [[articulation (anatomy)|articulation]] with the costal cartilages.<ref name="CNX2014"/>

The largest part of the heart is usually slightly offset to the left side of the chest ([[levocardia]]). In a rare congenital disorder ([[dextrocardia]]) the heart is offset to the right side and is felt to be on the left because the [[left heart]] is stronger and larger, since it pumps to all body parts. Because the heart is between the [[human lung|lungs]], the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart.<ref name="CNX2014"/>
The heart is cone-shaped, with its '''base''' positioned upwards and tapering down to the apex.<ref name="CNX2014"/> An adult heart has a mass of 250–350 grams (9–12&nbsp;oz).<ref>{{cite web|last1=Bianco|first1=Carl|title=How Your Heart Works|url=http://health.howstuffworks.com/human-body/systems/circulatory/heart1.htm|website=HowStuffWorks|access-date=14 August 2016|url-status=dead|archive-url=https://web.archive.org/web/20160729035858/http://health.howstuffworks.com/human-body/systems/circulatory/heart1.htm|archive-date=29 July 2016|date=April 2000}}</ref> The heart is often described as the size of a fist: 12&nbsp;cm (5&nbsp;in) in length, 8&nbsp;cm (3.5&nbsp;in) wide, and 6&nbsp;cm (2.5&nbsp;in) in thickness,<ref name="CNX2014"/> although this description is disputed, as the heart is likely to be slightly larger.<ref>{{cite journal |last1=Ampanozi |first1=Garyfalia |last2=Krinke |first2=Eileen |last3=Laberke |first3=Patrick |last4=Schweitzer |first4=Wolf |last5=Thali |first5=Michael J. |last6=Ebert |first6=Lars C. |title=Comparing fist size to heart size is not a viable technique to assess cardiomegaly |journal=Cardiovascular Pathology|date=7 May 2018 |volume=36 |pages=1–5 |doi=10.1016/j.carpath.2018.04.009 |pmid=29859507 |s2cid=44086023 |issn=1879-1336}}</ref> Well-trained [[athlete]]s can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle.<ref name="CNX2014"/>