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."

early development

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 at 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. ^*

After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 bpm (+/-25 bpm) during the 15 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: http://www.obgyn.net/us/us.asp?page=/us/cotm/0001/ehr2000

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 (breastbone). The heart is usually felt to be on the left side because the left heart (the left part of the heart) is stronger (it pumps the blood out). The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs. Betwen the pericardium and the heart there is a serous fluid to reduce friction during heart contractions. The apex is the blunt point at the base of the heart. A stethoscope can be placed directly over the apex so that the beats can be counted. 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. 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. Valves, such as the 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 from the body and pump it into the lungs (pulmonary circulation)so that carbon dioxide can be dropped off and oxygen picked up. This happens through a process called diffusion. The left side (see left heart) collects oxygenated blood from the lungs and pumps it out to the body (systemic circulation). On both sides, the lower ventricles are thicker than the upper atria.

Oxygen-depleted or deoxygenated blood from the body 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 to the lungs, through the pulmonary artery. In the lungs gaseous 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 pulmonary veins 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 aorta to the entire body. 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 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.

Regulation of the cardiac cycle

Cardiac 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.

Other physiological functions

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.

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.

The hearts of other animals

Vertebrates

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 actually have a "heart". It has five aortic arches that can 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. 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 (2 atria and 2 ventricle)

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.

External links

Very Comprehensive Heart Site
The InVision Guide to a Healthy Heart An interactive website
Heart and cardiovascular disease A info site on heart and cardiovascular disease.
Coronary heart disease A resourceful site on coronary heart disease and rheumatic heart disease.
Self Improvement Wednesday - ABC 702 Drive audio

Cardiovascular system | Thorax | Organs

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