Heart
The
heart is a hollow,
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. The term
cardiac (as in
cardiology) means "related to the heart" and comes from the
Greek καρδιά,
kardia, for "heart." The heart is composed of
cardiac muscle, an
involuntary muscle tissue which is found only within this organ.
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At 21 days after conception, the human heart rate begins beating at 75-80 beats per minute and accelerates linearly for the first month of beating. |
The human embryonic heart (EHR) 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). It 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 PregnanciesThere is no difference in male and female heart rates before birth.
[Terry J. DuBose Sex, Heart Rate and Age]In the human body, the heart is normally situated slightly to the left of the middle of the
thorax, underneath the
sternum (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 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 (8-11 oz), 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 right is a picture of a fresh human heart which was removed from a 64 year old British gentleman.
A
septum divides the right atrium and ventricle from the left atrium and ventricle, preventing blood from passing between them. An exception to this occurs in a developing fetus, the right and left atria are opened together. Valves, such as the tricuspid valve (Malenza valve), between the atria and ventricles (atrioventricular valves) maintain coordinated unidirectional flow of blood from the atria to the ventricles. The ventricular
systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Once all the blood empties from the ventricles, the pulmonary and aortic semilunar valves close. Finally, complete cardiac diastole involves relaxation of the atria and ventricles in preparation for refilling with circulating blood.
The function of the right side of the heart (see
right heart) is to collect deoxygenated 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,
left atrium, and the
left ventricle pumps it out to the body,
systemic circulation. On both sides, the lower ventricles are thicker and stronger than the upper atria.
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Anterior (frontal) view of the opened heart. White arrows indicate normal blood flow. |
The heart is a pump that beats continuously throughout life. It constantly supplies oxygen to the tissues and cells of the body and removes wastes from them. Oxygen-depleted or deoxygenated blood from the
systemic loop enters the right atrium through two great veins, the
superior vena cava which drains the upper part of the body and the
inferior vena cava that drains the lower part. The blood then passes through the
tricuspid valve to the right ventricle. The
right ventricle pumps the deoxygenated blood, through the
pulmonary loop, to the lungs, through the pulmonary semilunal valve into the "T" shaped pulmonary trunk into the right or left
pulmonary artery. In the lungs gas exchange takes places and the blood releases
carbon dioxide into the lung cavity and picks up
oxygen which is called
external respiration. The oxygenated blood then flows through four
pulmonary veins, two from each lung, to the left atrium. From the left atrium this newly oxygenated blood passes through the
mitral valve(also called the
bicuspid valve) to enter the left ventricle. The left ventricle then pumps the blood through the aortic semilunar valve to the
aorta to the systemic loop. Even the
lungs take some of the blood supply from the
aorta via
bronchial arteries.
The left ventricle is much more muscular, which is 1.3 - 1.5 cm (.5-.6 in) thick, than the right, 0.3 - 0.5 cm ( .1-.2 in) thick, as it has to pump blood around the entire body, which involves exerting a considerable force to overcome the vascular pressure. As the right ventricle needs to pump blood only to the lungs, it requires less muscle.
Even though the ventricles lie below the atria, the two vessels through which the blood exits the heart (the pulmonary artery and the aorta) leave the heart at its top (base) side.
The contractile nature of the heart is due to the presence of
cardiac muscle in its wall which can work continuously without fatigue. The heart wall is made of three distinct layers. The first is the outer
epicardium which is composed of a layer of flattened
epithelial cells and connective
tissue. Beneath this is a much thicker
myocardium made up of cardiac
muscle. The
endocardium is a further layer of flattened epithelial cells and connective tissue which lines the chambers of the heart.
The blood supply to the heart itself is supplied by the left and right
coronary arteries'', which branch off from the aorta.
Reference:
*
Heart PointCardiac muscle is self-exciting. It is a specialized muscle found nowhere else but in the heart because it has own conducting system. This is in contrast with
skeletal muscle, which requires either conscious or reflex nervous stimuli. The heart's rhythmic contractions occur spontaneously, although the
frequency or
heart rate can be changed by nervous or hormonal influences such as exercise or the perception of danger.
The rhythmic sequence of contractions is coordinated by the
sinoatrial and
atrioventricular nodes. The sinoatrial node, often known as the
cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation (See
action potential) that initiates atria contraction. Once the wave reaches the atrioventricular node, situated in the lower right atrium, it is conducted through the
bundles of His and causes contraction of the ventricles. The time taken for the wave to reach this node from the sinoatrial nerve creates a delay between contraction of the two chambers and ensures that each contraction is coordinated simultaneously throughout all of the heart. In the event of severe pathology, the
Purkinje fibers can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the other pacemakers and hence is overridden.
The heart also secretes
atrial natriuretic factor (ANF), a powerful
peptide hormone that affects the blood vessels, the adrenal glands, the kidneys, and the regulatory regions of the brain in order to regulate blood pressure and volume.
See cardiac arrest for emergencies involving the heartIf 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.
The hearts of
fish have only two chambers: one
atrium and one
ventricle. In fish, the system has only one circuit. The blood pumps through the gills and on to the bodily tissues before returning to the heart.
Amphibians and
reptiles have a three-chambered heart, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enters by separate atria, and is directed via a spiral valve to the appropriate vessel"aorta for oxygenated blood and pulmonary vein for deoxygenated blood. The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise.
Mammals (a class of vertebrate),
birds and
crocodiles show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four-chambered heart of birds evolved independently of that of mammals.
Invertebrates
The
earthworm doesn't have a heart; it has five aortic arches that serve the same purpose.
Many invertebrates, such as bivalves, exhibit an open circulatory system where blood flows freely in the body cavity. In these animals the blood usually collects in a series of specialised cavities where it is returned to the heart and is again released into the body.
Heartbeat
Smaller animals have faster heartbeats. This is evident within a species as well, as the young beat their hearts faster than the adults. See "Early development" above for information about the early human heart rates.The
Gray Whale's heart beats 9 times per
minute,
Harbour Seal 10 when diving, 140 when on land,
elephant 25,
human 72,
sparrow 500,
shrew 600, and
hummingbird 1,200 when hovering. These heart rates usually vary on the animal's surface area; an elephant with less surface area than a mouse loses less heat and requires less blood to be pumped throughout their body. An
ectothermic animal will usually have a slower, and more variable heartbeat than an
endothermic animal of similar size.
The hearts of
cattle,
sheep,
pigs 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.
Different species have different heart chambers. It can vary from one to four chambers (2 atria and 2 ventricle)
The heart may also be illustrated as an
icon (
♥), symbolizing
love. The heart is closely associated with
Valentine's Day, being used in cards and as boxes of chocolate.
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Artificial heart*
Atrium*
Blood pressure*
Cardiology*
Cardiothoracic surgery*
Cardiovascular pathology*
Circulatory system*
Echocardiography*
Electrical conduction system of the heart*
Haemodynamics*
Heart defects*
Heart rate*
Heart transplant*
Pulse*
Ventricle*
Ventricular hypertrophy*
Holiday heart syndrome*
Circle map -- simplified mathematical model of the beating heart.
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MUGA scan*
Cardiac stress test*
http://www.easyrashi.com/TEFILLIN/sternum.htm The position of the heart*
Very Comprehensive Heart Site*
The InVision Guide to a Healthy Heart An interactive website
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Heart and cardiovascular disease A info site on heart and cardiovascular disease.
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Conditions and ProceduresAn information resource guide on cardiac care.
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Coronary heart disease A resourceful site on coronary heart disease and rheumatic heart disease.
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Self Improvement Wednesday - ABC 702 Drive audio
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3D Animated Heart - A great resource to view and interact with the anatomy of a 3 dimensional heart
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Patient Queries - Heart A Library of Patient Queries related to Heart answered by Cardiologists & Physicians
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THE CIRCULATORY SYSTEM*
The position of the heart*
many heart graphics*
misconceptionszh-yue:心臟
