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Heart
The heart is a muscular organ responsible for pumping blood through the
blood vessels by repeated, rhythmic contractions, or a similar structure in
the 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 organ.[2]
Early development
The human heart beats more than 3.5 billion times in an average lifetime.
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.[3]
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
There is no difference in male and female heart rates before birth.[4]
Structure
In the human body, the heart is usually situated in the middle of the thorax
with the largest part of the heart slightly to the left (although sometimes
it is on the right, see dextrocardia), 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. It is located posterior to the 5th intercostal space
in the left mid-clavicular line. In normal adults, the mass of the heart is
250-350 g (9-12 oz), or about three quarters 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.
Starting in the right atrium, the blood flows through the tricuspid valve to
the right ventricle. Here it is pumped out the pulmonary semilunar valve and
travels through the pulmonary artery to the lungs. From there, blood flows
back through the pulmonary vein to the left atrium. It then travels through
the bicuspid valve to the left ventricle and on to through the aortic
semilunar valve to the aorta. The aorta forks, and the blood is divided
between major arteries which supply the upper and lower body. The blood
travels the arteries to the smaller arterioles, then finally to the tiny
capillaries which feed each cell. The (relatively) deoxygenated blood then
travels to the venules, which coalesce into veins, then to the inferior and
superior vena cavae and finally back to the right atrium where the process
began.
The heart is effectively a syncytium, a meshwork of cardiac muscle cells
interconnected by contiguous cytoplasmic bridges. This relates to electrical
stimulation of one cell spreading to neighboring cells.
First aid
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. Usually, if there is enough
time, the victim can be rushed to the hospital where he or she will be cared
for by a cardiologist, a doctor who specializes in the heart and lungs.
Food use
The hearts of cattle, sheep, pigs, chickens and certain fowl are consumed in
many countries. They are counted among offal, but being a muscle, the taste
of heart is like regular meat. 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. The Romans for example noticed that
emotions often corresponded with changing heart rates such as fear, love and
excitement. While it is now known that the heart has nothing to do with
thoughts or emotions as such, 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". The center of the world, the magma, is the "heart of
the world."
The Aztec civilization used the heart as a sacrificial token during the
sacrifice of a human being. The priest used a stone knife to cut into the
thoracic cavity and remove the heart, upon which it would be placed on a
stone altar as an offering to the gods. The greatest sacrifice under the
reign of Montezuma involved the removal of the hearts of over 12,000 enemy
soldiers.
References
1. ^ The American Heritage Stedman's Medical Dictionary. KMLE Medical
Dictionary Definition of heart..
2. ^ The American Heritage Stedman's Medical Dictionary. KMLE Medical
Dictionary Definition of cardiac.
3. ^ http://www.obgyn.net/us/us.asp?page=/us/cotm/0001/ehr2000
4. ^ Terry J. DuBose http://www.obgyn.net/english/pubs/features/dubose/ehr-age.htm
Sex, Heart Rate and Age]
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