Anatomy and physiology of the heart

This is just a short and somewhat simplified description of the heart’s anatomy and physiology. Only the parts directly connected and important for the electrocardiography have been described.

Table of contents

• Anatomy
• Surface anatomy/ Surface marking of the heart
• Physiology
• Types of cardiac muscle and their function
  • Heart conduction pathway
• Learn more
  • Web sites
  • Books
• References

Anatomy

The heart is a hollow muscular organ which functions as a pump distributing the blood through the blood vessels to all parts of the human body. This is of course very important because the function of the circulation is to maintain optimal conditions for survival and functioning of the cells, which is mainly achieved by distributing substrates and oxygen to the tissue, as well as collecting waste products and carbon dioxide. Secondary functions of the cardiovascular system are: control of blood flow to the skin and extremities to enhance retard heat loss, distribution of hormones and delivery of antibodies, platelets, and leukocytes to aid body defense mechanism.



The heart is located just behind the middle part of the front thorax wall and in most people approximately 2/3 of it are located on the left side and 1/3 on the right side. Its shape can be compared with that of a reversed cone. On the heart, just as on the cone, we can distinguish the wider end, or the base, and the blunt point called the apex. Heart base is directed upward, backward, and to the right, while the apex is directed downward, forward, and to the left. Imaginary line passing through the center of the base and the apex is called the axis of the heart (not to be confused with the electrical axis of the heart), and it too is following the same direction (up, right and back - down, left and front). It is important to be familiar with the axis of the heart, since it determines the placement of the electrodes during the electrocardiography.

Surface anatomy/ Surface marking of the heart

Knowing the surface anatomy of the heart can be useful in the locating certain structures of the heart from the skin surface.

Here is an analogy, the body is similar to a vast, almost featureless desert with various terrains i.e mounts, valleys, depressions, tunnels etc. We obviously need a way to locate our position in the desert & find our way through it.

A GPS (global positioning system) locates the relative position of a certain location on the earth's surface using landmarks sites closely related to the area in question,so this is what we need to get our way throuh the desert.

Surface marking/ anatomy works the same way as the GPS, by using 'landmarks' on the body surface to determine position & location of structures deep to the skin, e.g the heart.

EXAMPLE:
To locate the position & course of the Superior vena cava (SVC) in the thorax,
We take a 'landmark' point at the sternal end of the lower border of the right 1st costal cartilage.
We take a second 'landmark' at the sternal end of the upper border of the right 3rd costal cartillage.

We then joint this 2 points.
The line resulting from this corresponds to position & course of this vein starting from its formation (when the left & right brachiocephalic veins joins to from the SVC) till its termination (the vein will join with the heart to drain blood into the right atrium.

Now we will enter the crux of the matter, THE Surface Marking of cardiac structures......

As noted above the heart have sternocostal surface, i.e the heart surface the faces us when we look at the heart from the front/ anterior when then thoracic cage has been opened.
This surface appears as a square with upper, lower, right & left borders.......
knowing the positions of this borders & hence this surface would useful when we are trying to study the heart from the anterior...e.g in determining electrode placement on the chest wall & "knowing which electrode correponds to which part of the heart"

The right border can known by:
1) Taking a point, a finger's breath lateral to termination of the SVC (this is the second 'landmark' mentioned above for locating the SVC)

2) The 2nd 'landmark' is taken in the 4th right intercostal space 4cm (about the width of the index, middle & ring finger put together) from the mid-line)

3) The "Last landmark" is taken at the sternal end of the right 6th rib

A line joining this 3 landmarks drawn with a gentle convexity to the right represents
the RIGHT BORDER.

The lower border is marks by:
1) Taking the "Last landmark" for the RIGHT BORDER

2) Take the 'xiphisternal junction' (this is where you will feel some bony projection in the pit of the stomach OR it is the lowest ends of the sternum in the midline)

3)Thirdly, locate the 'apex beat'(see below) of the heart......this requires you to palpate for the heartbeat.

Join all this points to determine the LOWER BORDER of the heart........

Now we will mark the LEFT BORDER

1) Locate the 'apex beat'

2)Take a point a finger's breath (approx 1-1.5cm) from the sternal end of the lower border of the 2nd rib

Join this points with an upward convexity to mark the LEFT BORDER.....


The UPPER BORDER is very simple..........We just have to join the upper ends of the RIGHT & LEFT BORDERS.........

**** The apex beat is the strongest beat felt when palpating the heart in normal condition,
It can be located in the 5th intercostal space abut 10cm lateral to the midline on the left. For males, this is usually just below & slight left to the left nipple)*********







OK, now we will attempt to surface mark the orifices of heart.....

The heart is basically a hollow pump with 4 orifices:
a) PULMONARY orifice
- located on the right side, where the pulmonary trunk exits the heart
into the thorax
- Contains pulmonary valves

SURFACE MARKING
1. We draw a horizontal line 2.5cm (approx the width of the middle &
ring finger put together) in such a way, that half of the line is on
the upper border of left 3rd costal cartillage & the remaining half is
on the sternum

This line represents the position & orientation of the orifice/ valve
on the heart

b) AORTIC orifice
- located on the left side, where the aorta exits the heart into the
thorax
- Contains aortic valves

SURFACE MARKING
1. We draw a 3cm long line downwards & to the right, From the medial end
of left 3rd INTERCOSTAL SPACE

This line represents the position & orientation of the orifice/ valve
on the heart

c) LEFT ATRIOVENTRICULAR (Left AV) a.k.a MITRAL orifice
- The orifice separating the left atrium from the left ventricle
- Contains the mitral a.k.a bicuspid valve

SURFACE MARKING
1. We draw a 3cm long line downwards & to the right, From the sternal end
of left 4th costal cartillage

This line represents the position & orientation of the orifice/ valve
on the heart

d) RIGHT ATRIOVENTRICULAR (Right AV) a.k.a TRICUSPID orifice
- The orifice separating the right atrium from the right ventricle
- Contains the tricuspid valve

SURFACE MARKING
1. We draw a 4cm long line BEGINNING in the midline opposite the 4th
costal cartillage......passing downwards & slightly to the right.

This line represents the position & orientation of the orifice/ valve
on the heart


WE ARE DONE WITH THE SURFACE MARKING OF THE HEART BORDERS & ORIFICE.....


Just to add something useful.........
The coronary sulcus (groove that separates the atria from the ventricles & can
be seen on outer surface of the heart) can be marked by:
drawing a straight line starting from the sternal end of left 3rd costal
cartillage & ends in the sternal end of 6th costal cartillage

Please note: this surface markings is only a 'rough' guide to help locate
the position & orientation of these structure......
The structures might not "exactly" correspond to the surface marking,
due to individual variations or in pathological states of the heart...
Nevertheless, they are still useful in locating deep structure from the
skin surface.

Physiology

Although humans have one heart, from a functional point of view we can think of two pumps in series connected by the pulmonary and systemic circulations, the left heart and the right heart. Each of these “hearts” is further divided into two chambers. So the heart has four chambers in total, left and right atrium, as well as left and right ventricle. Atria, the pre-chambers, mainly function as the helpers of the ventricles by filling them with more blood before their contraction. The function of the right side of the 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). 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, since they produce the greatest part of the force needed to pump the blood. 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.

Types of cardiac muscle and their function

Cardiac cells

The heart is composed of three major types of cardiac muscle: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers. The atrial and ventricular types of muscle contract in the same way as skeletal muscle, and they are the workers responsible for contraction. On the other hand, the specialized excitatory and conductive fibers contract only feebly, but instead they exhibit automatic rhythmical electrical discharge in the form of action potentials and conduction of the action potentials through the heart. These fibers form an excitatory system that controls the rhythmical beating of the heart, telling the workers to contract.

Basically all cardiac muscle fibers can produce electrical discharge, but the rate of it varies. Atrial and ventricular muscle fibers have a much slower rate of discharge than the specialized excitatory and conductive fibers and that is why they normally do not have a chance to pace the heart. The fastest discharge comes from a part of the specialized excitatory and conductive fibers called the sinus (SA) node, which therefore is the normal pacemaker of the heart. The isolated SA node has a firing rate of 90-120 beats/min. The actual heart rate of an adult is approximately 70 beats/min because some intrinsic factors like vagal nerve activity slow it down a bit.

Heart conduction pathway

Normal conduction pathway in the heart starts in the SA node, located in the wall of the right atrium, because this is where the normal rhythmical impulse is generated and is then conducted to the atrioventricular (AV) node by the internodal pathways. From here the AV bundle conducts the impulse from the atria into the ventricles where the left and right bundle branches of so called Purkinje fibers conduct the cardiac impulse to all parts of the ventricles. Impulses are also conducted by the atrial and ventricular muscle fibers, the only difference is in the speed of conduction which is much slower in this case than in the case of specialized conductive muscle fibers. Another important thing to mention when we talk about impulse conduction is its slowing in the AV node. When the impulse is generated in the SA node, it spreads through both right and left atrium via the atrial muscle and some conductive muscle fibers, and atria simultaneously contract. But they do this just a fraction of a second before ventricles, because the impulse hasn’t yet spread through them since the AV node delayed it for about 0.13 seconds. This delay is important because it allows time for the atria to empty their blood into the ventricles before ventricular contraction begins, thus allowing for more blood to be pumped out.

Table 1. Cardiac Conductive Properties

Tissue	Fiber Diameter (µm)	Resting Membrane Potential (mV)	Conduction Velocity (m/s)
SA node	. . .	40-50	0.05
Atrial muscle	8-10	R70-80	0.3-0.5
Internodal tracts	F15-20	80-90	1.0
AV node	Variable	50	0.02-0.05
Purkinje fibers	70-80	70	2.0-4.0
Ventricular muscle	10-16	80	<1.0

Learn more

Web sites

• The Franklin Institute - The Human Heart
• Spencer S. Eccles Health Sciences Library - Knowledge Weavers Animations

Books

Guyton and Hall - Textbook of Medical Physiology

References

1. Guyton and Hall - Textbook of Medical Physiology
2. Z. Krizan. Human anatomy, review of the thorax, abdomen, pelvis and extremities. 3rd ed. Zagreb: Skolska knjiga; 1997. 38 p.
3. ganfyd.org - The free medical knowledge base
4. WikiMD - the free medical encyclopedia

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