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The heart and lungs, from an older edition of *Gray's Anatomy*.

The heart is a pear shaped, muscular organ in vertebrates, responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods^[1] The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek καρδία, kardia, for "heart." The heart is composed of cardiac muscle, an involuntary muscle tissue which is found only within this muscle.^[2]

Early development

At 21 days after conception, the human heart rate begins beating at 70-80 beats per minute and accelerates linearly for the first month of beating.

The human embryonic heart begins beating approximately 21 days after conception, or five weeks after the last normal menstrual period (LMP), which is the date normally used to date pregnancy. The human heart begins beating at a rate near the mother’s, about 75-80 beats per minute (BPM). The embryonic heart rate (EHR) then accelerates linearly for the first month of beating, peaking at 165-185 BPM during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, an increase of 100 BPM in the first month. [1]

After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 BPM (+/-25 BPM) during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 BPM) BPM at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 LMP weeks is:

Age in days = EHR(0.3)+6

See: Embryonic Heart Rates Compared in Assisted and Non-Assisted Pregnancies

There is no difference in male and female heart rates before birth.^[3]

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Structure

File:3DScience Human Heart.jpg

Anterior (frontal) view of the opened heart. Arrows indicate normal blood flow. Image provided courtesy of www.3dscience.com.

In the human body, the heart is usually situated to the left of the middle of the thorax, underneath the breastbone (see diagrams). The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts). The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs. The pericardium is a double membrane structure containing a serous fluid to reduce friction during heart contractions. The mediastinum, a subdivision of the thoracic cavity, is the name of the heart cavity.

The apex is the blunt point situated in an inferior (pointing down and left) direction. A stethoscope can be placed directly over the apex so that the beats can be counted. This physical location is between the sixth and seventh rib, just to the left of the sternum [2]. In normal adults, the mass of the heart is 250-350 g (9-12 oz), or about three fourths the size of a clenched fist, but extremely diseased hearts can be up to 1000 g (2 lb) in mass due to hypertrophy. It consists of four chambers, the two upper atria (singular: atrium ) and the two lower ventricles. On the left is a picture of a fresh human heart which was removed from a 64-year-old British male.

The function of the right side of the heart (see right heart) is to collect de-oxygenated blood, in the right atrium, from the body and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange). This happens through a passive process called diffusion. The left side (see left heart) collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body. On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.

First aid

See cardiac arrest for emergencies involving the heart

If a person is encountered in cardiac arrest (no heartbeat), cardiopulmonary resuscitation (CPR) should be started, and help called. If an automated external defibrillator is available, this device may automatically administer defibrillation if this is indicated.

Food use

The hearts of cattle, sheep, pigs, chickens and certain fowl are consumed as food in many countries. They are counted among offal, but being a muscle, the taste of heart is much more like regular meat than that of other offal. It resembles venison in structure and taste.

As a symbol

The heart was historically seen by some as the seat of the soul and the organ responsible for human thought. Even though it is now known that the heart has nothing to do with thought or love, people still carry on using the term "heart" metaphorically when talking about love. When used in this metaphorical sense, the heart is often illustrated as an icon (♥).

The term "heart" can also refer to the core or center of anything e.g. "The heart of the matter".

There has been some recent research on the Cellular memory of the heart, particularly with heart transplant patients. ^[4] ^[5] This study shows how the heart contains thinking and memory skills, interacting heavily with the brain. This, though, is not widely accepted by the scientific community.

References

^ The American Heritage Stedman's Medical Dictionary. "KMLE Medical Dictionary Definition of heart". {{cite web}}: External link in |author= (help).
^ The American Heritage Stedman's Medical Dictionary. "KMLE Medical Dictionary Definition of cardiac". {{cite web}}: External link in |author= (help)
^ Terry J. DuBose Sex, Heart Rate and Age
^ Sylvia, Claire (1997). A Change of Heart. New York, New York: Little, Brown and Company. ISBN 0-316-82149-7.
^ Pearsall, Doctor Paul; et alii (1999). The Heart's Code. New York, New York: Broadway Books. ISBN 0-7679-9942-8.

External links

3D Animated Heart with Anterior Cut - life-like 3D human heart animation with anterior cut.
3D Animated Heart Beat - life-like 3D human heart animation.
American Heart Month - National month devoted to discussion of heart disease.

eMedicine: Surgical anatomy of the heart
Very Comprehensive Heart Site
The InVision Guide to a Healthy Heart An interactive website
Self Improvement Wednesday - ABC 702 Drive audio
3D Animated Heart - A great resource to view and interact with the anatomy of a 3 dimensional heart
The circulatory system
The position of the heart
American Heart Association

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[1] The American Heritage Stedman's Medical Dictionary. "KMLE Medical Dictionary Definition of heart". {{cite web}}: External link in |author= (help).

[2] The American Heritage Stedman's Medical Dictionary. "KMLE Medical Dictionary Definition of cardiac". {{cite web}}: External link in |author= (help)

[3] Terry J. DuBose Sex, Heart Rate and Age

[4] Sylvia, Claire (1997). A Change of Heart. New York, New York: Little, Brown and Company. ISBN 0-316-82149-7.

[5] Pearsall, Doctor Paul; et alii (1999). The Heart's Code. New York, New York: Broadway Books. ISBN 0-7679-9942-8.

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 There is no difference in male and female heart rates before birth.<ref>Terry J. DuBose [http://www.obgyn.net/english/pubs/features/dubose/ehr-age.htm Sex, Heart Rate and Age]</ref>
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 ==Structure==