Encyclopedia
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 means "related to the heart" and comes from the Greek ?a?d??,
kardia, for "heart." The heart is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ.
Early development
The human embryonic heart begins beating approximately 21 days after conception, or five weeks after the last normal menstrual period , 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 . It then accelerates linearly for the first month of beating, peaking at 165-185 bpm during the early 7th week, . 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.
After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 bpm during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 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+6
See:
There is no difference in male and female heart rates before birth.
Structure
In the human body, the heart is normally situated slightly to the left of the middle of the
thorax, underneath the sternum . The heart is usually felt to be on the left side because the
left heart is stronger . 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 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 . In normal adults, the mass of the heart is 250-350
g , but extremely diseased hearts can be up to 1000 g in mass due to
hypertrophy. It consists of four chambers, the two upper
atria 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 is to collect deoxygenated blood, in the right atrium, from the body and pump it, via the right ventricle, into the lungs so that carbon dioxide can be dropped off and oxygen picked up . This happens through a passive process called
diffusion. The left side 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.
Regulation of the cardiac cycle
Cardiac muscle is myogenic . It is a specialized muscle found nowhere else but in the heart because it has its 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
waves or
nerves can be changed by nervous frequency 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 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
Other physiological functions
The heart also secretes atrial natriuretic factor , 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.
First aid
See cardiac arrest for emergencies involving the heartIf a person is encountered in cardiac arrest ,
cardiopulmonary resuscitation 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 other animals
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 most
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 artery 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 ,
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 ratio of surface area to body mass; an elephant with relatively less surface area than a mouse loses proportionally less heat and requires comparatively 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.
Food use
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
As an icon
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.
References
See also
External links
- An interactive website
- - ABC 702 Drive audio
- - A great resource to view and interact with the anatomy of a 3 dimensional heart
