Anatomy of the Human Heart

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Introduction

The heart is a muscular pump that serves two functions; to collect oxygen-poor blood from the tissues of the body and pump this blood to the lungs to pick up oxygen and release carbon dioxide and to collect oxygen-rich blood from the lungs and pump this blood to all of the tissues of the body.

It is important to note that, the blood pumped by the heart also circulates many other important substances such as:

1.     Nutrients from digestion are collected from the small intestine and pumped through the circulatory system to be delivered to all cells of the body.

2.      Hormones are produced from one type of tissues and distributed to all cells of the body. The circulatory system carries waste materials (salts, nitrogenous wastes, and excess water) from cells to the kidneys, where they are extracted and passed to the bladder.

3.     The pumping of interstitial fluid from the blood into the extracellular space is an important function of the heart. Excess interstitial fluid is then returned to the circulatory system via the lymphatic system.

Anatomy

The general shape and orientation of the heart are that of a pyramid that has fallen over and is resting on one of its sides. Placed in the thoracic cavity, the apex of this pyramid projects forward, downward, and to the left, whereas the base is opposite the apex and faces in a posterior direction.

The average adult’s heart is about 5.5 inches (14 centimeters) long and 3.5 inches (9 centimeters) wide, or approximately the size of one’s fist and also varies with body size. In an infant, the heart is about a thirtieth of total body weight. In an average adult, the heart is about one three-hundredth of total body weight; this equals about 11 ounces (310 grams) in males and 8 ounces (225 grams) in females.

Location of the Heart

The heart is roughly in a plane that runs from the right shoulder to the left nipple. It lies in the protective thorax, posterior to the sternum and costal cartilages, and rests on the superior surface of the diaphragm.  The heart is located between the two lungs in the space referred to as the mediastinum("that which stands in the middle"). The human heart assumes an oblique position in the thorax, one-third of the heart is located on the right side, while two thirds is located on the left side of midline. The base is located below the third rib as it approaches the sternum (note that the sternal angle occurs at the level of the second rib). The base is directed superiorly to the right of midline and posterior. The pointed apex projects to the left of midline and anterior. Thus, the heartbeat can be most easily palpated at the left fifth intercostal space, 8-9 cm from the midsternal line (just inferior to the left nipple).

Nerve Supply

The sympathetic and parasympathetic autonomic nervous supplies to the heart form the cardiac plexus which is located close to the arch of the aorta. The fibres from the cardiac plexus accompany the coronary arteries and reach the heart, with most of them terminating at the SA node, AV node and a much less dense supply to the atrial and ventricular myocardium [11].

In general,the parasympathetic vagal fibres are inhibitory and reduce the heart rate and stroke volume. The sympatheticnerves act as accelatory nerves increasing both the heart rate and stroke volume [11]. The afferent nerves run

along sympathetic pathways via both cardiac accelerator nerves and thoracic splanchnic nerves to reach the intermediolateral horn of T1–T4 of the spinal cord [11].

The noradrenergic or the sympathetic nervous system is mainly involved with increasing the heart rate (chronotropy),

contractility (ionotropy) and the speed of conduction (dromotropy) in the cardiac muscle fibres and the conduction tissue; and the transmitter involved is mainly nor-epinephrine [6]. The SA node receives most

of it nerve fibres from the right-sided thoracic sympathetic

ganglia and the right vagus [8]. The AV nodes

and ventricles receive their nerve supply form the leftsided

thoracic sympathetic ganglia and the left vagus,

which is mainly because SA node develops from the

structures on the right side of the embryo and the AV

node develops from the structures on the left side of the

embryo [8].

The sympathetic effects are mediated mainly by the

adrenergic receptors, which includes β-1 and β-2 adrenergic

receptors [2, 11]. β-1 receptors are found mainly

in the SA node and AV node, and the ventricular myocardium

acts via activation of adenylate cyclase and an

increase in cAMP (cyclic adenosine monophosphate)

concentration in the cell to mediate the above mentioned

sympathetic effects [2, 11]. β-2 receptors are

mainly found in the vascular smooth muscles in addition

to the bronchial smooth muscle and wall of the GI

tract and the bladder. The mechanism of action is same

as that of β-1 receptors, i.e., increase in cAMP levels but

they cause relaxation of the vascular smooth muscle and

are involved in regulation of blood flow and systemic

blood pressure [2, 11].

The cholinergic or the parasympathetic nervous system

effects in the heart are opposite to the ones mentioned

above and the transmitter involved is mainly

acetylcholine [8, 12, 13]. The vagi supply the parasympathetic

fibres to the heart via the cardiac plexuses. The

parasympathetic effects are mediated via the muscaranic

receptors, which act by inhibition of adenylate cyclase

and hence decrease the intracellular cAMP levels

and result in a decrease in heart rate, contraction and

conduction velocity [8, 12, 13].

The autonomic centres in the CNS, mainly the vasomotor

centre of medulla and the hypothalamus regulate

the balance between the level of sympathetic and the

parasympathetic output to the cardiovascular system,

depending on the afferent inputs from the periphery

and the CNS [7, 8, 12, 13]. There is normally a tonic

vagal discharge in humans, which overrides the moderate

tonic discharge in the cardiac sympathetic nerves

[7, 8, 12, 13]. Both the sympathetic and the parasympathetic

fibres in the splanchnic thoracic nerves and the

vagi carry afferent input mediated via baroreceptors and

chemoreceptors to the autonomic centres in the CNS,

in addition to the efferent output from the CNS. These

afferents and efferents are involved in mediation of cardiovascular

reflexes as baroreceptor and chemoreceptor

reflexes [7, 8, 12, 13].

The receptors of the autonomic nervous system to

the heart are the target of numerous drugs used in the

treatment of various cardiovascular disorders in both

acute and chronic settings [8, 12, 13].

References

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