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Why does the apex of the human heart usually point to the left?

Why does the apex of the human heart usually point to the left?



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In the majority of human beings, the apex of the heart (left ventricle) points towards the left side of the body. Sometimes however (approx. 1/12000 births), a person is born with a condition known as "Dextrocardia", in which the apex of the heart points to the right side of the body instead.

Is there an evolutionary reason as to why the human heart usually points to the left side as opposed to the right side of the body?

(Note: Please don't answer with, "because there is a notch in the left lung", because I will simply reply with, "is there an evolutionary reason for the apex of the heart AND the cardiac notch being on the left as opposed to the right side of the body").


From a quick look at the paper @ChinmayKanchi links to (Palmer, 2004) it seems that:

All living vertebrates possess a heart that is conspicuously asymmetrical and normally displaced toward the left (Fishman & Chien, 1997).

So the heart orientation seems to be evolutionary conserved in vertebrates (as are many fundamental traits), and no specific explanation is needed for humans.

This is said with the reservation that human anatomy is not my subject field, and the refered paper also digs deeper into the molecular basis for the orientation/symmetry of organisms. For instance, it also says that:

Second, the molecular pathway directing hearts leftward-the nodal cascade-varies considerably among vertebrates (homology of form does not require homology of development) and was possibly co-opted from a preexisting asymmetrical chordate organ system.

so the molecular mechanisms governing this seems to differ between species. This could indicate that there is a selective pressure molding species into the same heart orientation. This is pure speculation on my part though.

I also want to mention that evolutionary outcomes doesn't have to have a "reason" (i.e. a selective advantage). Much depends on chance events and evolution can only act on what is present at the moment (i.e. is restricted by earlier evolutionary history).


I think this deals with the steps of Heart development during organogenesis, together with the influence of external factors (such as the lung development and the hemodynamics), which by nature would lead heart to occupy the optimal positions for it and for those interrelated organs. Basically, the organs tend to develop on the midline, or symmetrically to the midline, but due to cardiac tube contortion during the development, it changes the position.

You can check the details on wiki (https://en.wikipedia.org/wiki/Heart_development)


I think it is because of the reason that the left ventricle pumps blood throughout body, thus is bigger than the right ventricle. So, actually, the human heart is along the center septum, but seems just a bit to the right. 1.5 cm to be precise.


Both the observation and palpation of the point of maximal impulse (PMI) of heart is part of a complete cardiac exam. These exam findings can yield important information about the heart such as a laterally displaced PMI in an enlarged heart.

Shown is the curve created by the PMI measured by an apex-cardiogram. Although now used infrequently, this device can be used to help diagnose ventricular abnormalities and is shown here to better understand the normal PMI.

(A = left ventricular filling C = systolic wave E = systolic peak O = start of rapid filling (mitral valve open) F = start of slow filling [diastasis].)


Heart Valves

The locations of auscultation center around the heart valves. The aortic, pulmonic, tricuspid, and mitral valves are four of the five points of auscultation. The fifth is Erb’s point, located left of the sternal border in the third intercostal space. The aortic point is located right of the sternal border in the second intercostal space. The pulmonic point is to the left of the sternal border in the second intercostal space. The sound that emits from the aortic and pulmonic points is the S2 “dub” of the typical “lub-dub” heartbeat. The S1 and S2 sounds are present in normal heartbeat patterns.

The tricuspid point is found left of the sternal border in the fourth intercostal space, and the mitral point is located midclavicular on the left side of the chest in the fifth intercostal space. Both the tricuspid and the mitral points are where the S1 “lub” can be heard. The base of the heart is where the aortic and pulmonic S2 sound will be loudest. The apex is where the tricuspid and mitral S1 sound is loudest upon auscultation. The apex region will also be where S3 and S4 sounds(extra heart sounds not usually noted in normal assessments) and mitral stenosis murmurs may be auscultated, if present.


Why does the apex of the human heart usually point to the left? - Biology

In order to sustain viability, it is not possible for nutrients to diffuse from the chambers of the heart through all the layers of cells that make up the heart tissue. Thus, the coronary circulation is responsible for delivering blood to the heart tissue itself (the myocardium). The normal heart functions almost exclusively as an aerobic organ with little capacity for anaerobic metabolism to produce energy. Even during resting conditions, 70 to 80% of the oxygen available within the blood circulating through the coronary vessels is extracted by the myocardium. It then follows that because of the limited ability of the heart to increase oxygen availability by further increasing oxygen extraction, increases in myocardial demand for oxygen (e.g., during exercise or stress) must be met by equivalent increases in coronary blood flow. Myocardial ischemia results when the arterial blood supply fails to meet the needs of the heart muscle, for oxygen and/or metabolic substrates. Even mild cardiac ischemia can result in anginal pain, electrical changes (detected on an electrocardiogram) and the cessation of regional cardiac contractile function. Sustained ischemia within a given myocardial region will most likely result in an infarction.

As noted above, as in any microcirculatory bed, the greatest resistance to coronary blood flow occurs in the arterioles. Blood flow through such vessels varies approximately with the fourth power of these vessels' radii hence, the key regulated variable for the control of coronary blood flow is the degree of constriction or dilatation of coronary arteriolar vascular smooth muscle. As with all systemic vascular beds, the degree of coronary arteriolar smooth muscle tone is normally controlled by multiple independent negative feedback loops. These mechanisms include various neural, hormonal, local non-metabolic and local metabolic regulators. It should be noted that the local metabolic regulators of arteriolar tone are usually the most important for coronary flow regulation these feedback systems involve oxygen demands of the local cardiac myocytes. In general, at any one point in time, coronary blood flow is determined by integrating all the different controlling feedback loops into a single response (i.e., inducing either arteriolar smooth muscle constriction or dilation). It is also common to consider that some of these feedback loops are in opposition to one another. Interestingly, coronary arteriolar vasodilation from a resting state to one of intense exercise can result in an increase of mean coronary blood flow from approximately 0.5 to 4.0 ml/min/gram.

As with all systemic circulatory vascular beds, the aortic or arterial pressure (perfusion pressure) is vital for driving blood through the coronaries, and thus needs to be considered as another important determinant of coronary flow. More specifically, coronary blood flow varies directly with the pressure across the coronary microcirculation, which can be essentially considered as the aortic pressure, since coronary venous pressure is near zero. However, since the coronary circulation perfuses the heart, some very unique determinants for flow through these capillary beds may also occur during systole, myocardial extravascular compression causes coronary flow to be near zero, yet it is relatively high during diastole (note that this is the opposite of all other vascular beds in the body).

Oxygenated blood is pumped into the aorta from the left ventricle. This is where it enters the right and left main coronary arteries, and subsequent branching feeds the myocardial tissue of all four chambers of the heart (see Figure 7). The ascending portion of the aorta is where the origins (ostia) of the right and left coronaries reside specifically, they exit the ascending aorta immediately superior to the aortic valve at the sinus of Valsalva. Blood flow into the coronary arteries is greatest during ventricular diastole when aortic pressure is highest and it is greater than in the coronaries. Typically the right coronary artery courses along the right anterior atrioventricular groove just below the right atrial appendage and along the epicardial surface adjacent to the tricuspid valve annulus. It traverses along the tricuspid annulus until it reaches the posterior surface of the heart, where it then commonly becomes the posterior descending artery and runs toward the apex of the left ventricle. Along its course, a number of branches emerge, most notably those that supply the sinus node and the atrioventricular node hence blockage of such vessels can lead to conduction abnormalities. Additionally, several marginal branches run to the right ventricular and right atrial epicardial surfaces. The left main coronary artery typically bifurcates quickly upon exiting the ascending aorta into the left circumflex and left anterior descending arteries. The left circumflex artery runs under the left atrial appendage on its way to the lateral wall of the left ventricle. Along the way, it spawns a number of branches that supply the left atrial and left ventricular walls. In some cases, a branch will course behind the aorta to the superior vena cava such that it can supply the sinus node. The left anterior descending artery supplies a major portion of the ventricular septum, including the right and left bundle branches of the myocardial conduction system, and the anterior and apical portions of the left ventricle.

Figure 7. Drawing of the coronary arterial circulation in the human heart. The normal human hears does not typically elicit collateralization each area of myocardium is usually supplied by a single coronary artery. Ao = aorta LAD = left anterior descending artery LCx = left circumflex artery PA = pulmonary artery RCA = right coronary artery.

Coronary arteries are so vital to the function of heart whenever disease states are associated with flow restriction through the coronary arteries, and subsequently the remainder of the coronary circulations (capillaries and veins), the effects on cardiac performance are quite dramatic and often fatal. Coronary artery disease (CAD) is generally defined as the gradual narrowing of the lumen of the coronary arteries due to coronary atherosclerosis. Atherosclerosis is a condition that involves thickening of the arterial walls from cholesterol and fat deposits that build up along the endoluminal surface of the arteries. With severe disease, these plaques may become calcified and so large that they produce stenoses within the vessels, and thus permanently increase the vascular resistance which is normally low. When the walls of the coronary arteries thicken, the cross-sectional area of the arterial lumen decreases resulting in higher resistance to blood flow through the coronary arteries. This steady decrease in cross-sectional area can eventually lead to complete blockage of the artery. As a result, oxygen and nutrient supply to the myocardium drops below the demand of the myocardium. As the disease progresses, the myocardium downstream from the occluded artery becomes ischemic. Eventually, myocardial infarction (or known as a MI) may occur if the coronary artery disease is not detected and treated in a timely manner.

Myocardial ischemia not only impairs the electrical and mechanical function of the heart, but also commonly results in intense, debilitating chest pain known as angina pectoris. However, anginal pain can often be absent in individuals with coronary artery disease when they are resting (or in individuals with early disease stages), but induced during physical exertion or with emotional excitement.


Congenital heart disease happens when something goes wrong while the heart is forming in a baby that's still in the womb. The heart abnormality sometimes leads to problems right after birth, but other times there aren't any symptoms until you become an adult.

Septal abnormalities are among the most common congenital heart problems. These are holes in the wall that separates the left and right sides of your heart. You can get a procedure to patch the hole.

Another type of abnormality is called pulmonary stenosis. A narrow valve causes a decrease in the flow of blood to your lungs. A procedure or surgery can open or replace the valve.

In some babies, a small blood vessel known as the ductus arteriosus doesn't close up at birth as it should. When this happens, some blood leaks back into the pulmonary artery, which puts strain on your heart. Doctors can treat this with surgery or a procedure or sometimes with medication.

Sources

Myers, R., Heart Disease: Everything You Need to Know, Firefly Books Ltd, 2004.

Verheugt, F. Tonkin, A., Atherosclerosis and Heart Disease, Taylor & Francis Group 1st edition, 2003.


Heart Dissection

The mammalian heart is the central organ of the circulatory or cardiovascular system. It pumps blood to the body’s organs and tissues delivering oxygen and nutrients, while transporting wastes away.

Dissection of a preserved sheep or pig heart offers students an excellent opportunity to learn about mammalian heart anatomy. While dissecting, students can also explore how blood is pumped through the heart. Preserved sheep and pig hearts, while smaller and larger respectively, are similar in structure and function to the human heart, making this dissection great for many labs𠅏rom basic biology to human anatomy courses.

Use the instructions below to investigate the internal and external anatomy of the preserved sheep heart. For more detailed dissection instructions and information, check out Carolina® dissection kits.

The mammalian heart is the central organ of the circulatory or cardiovascular system. It pumps blood to the body’s organs and tissues delivering oxygen and nutrients, while transporting wastes away.

Dissection of a preserved sheep or pig heart offers students an excellent opportunity to learn about mammalian heart anatomy. While dissecting, students can also explore how blood is pumped through the heart. Preserved sheep and pig hearts, while smaller and larger respectively, are similar in structure and function to the human heart, making this dissection great for many labs𠅏rom basic biology to human anatomy courses.

Use the instructions below to investigate the internal and external anatomy of the preserved sheep heart. For more detailed dissection instructions and information, check out Carolina® dissection kits.


Explain why in the heart the wall of the left ventricle is thicker than the wall of the right ventricle?

This is because the left ventricle pumps oxygenated blood round the entire body while the right ventricle only pumps blood to the lungs which is a much shorter distance. Because the left ventricle needs to pump the blood further it needs to generate more force during contraction in order to do this. This extra force is generated due to the additional muscle found in the left ventricle wall compared to the right ventricle wall. Additionally, the blood being pumped to the lungs from the right ventricle needs to be at a lower pressure in order to prevent damage to the many thin capilaries the blood goes through in the lungs. This helps to explain why the wall of the right ventricle is thinner.


What determines the rhythm of your heart?

When a doctor checks your heartbeat, have you ever wondered how it stays so regular? Or what's gone wrong when someone has to get a pacemaker? When it comes to the heart, timing is critical. Without a strong heartbeat, blood can't get to where it needs to go, and a heartbeat needs to be steady in order to be strong.

To understand what sets the beat of your heart, and why that rhythm is so important, it's first helpful to understand what exactly a heartbeat is and what it does.

A "beat" is a contraction of the heart. Each time a section of the heart contracts, it forces blood from one point to another. It goes like this:

  • When blood returns to the heart from the rest of the body, it flows into the right atrium (1). The blood has been supplying oxygen throughout the body and needs a refill.
  • The right atrium fills with this blood, which then flows into the right ventricle (2), as well. The right ventricle is going to send the blood into the lungs for an oxygen fill-up.
  • To get as much blood into the right ventricle as possible, the right atrium contracts, pushing all of the blood down into the ventricle.
  • Once the right ventricle is full, it contracts, forcing the blood into the lungs.
  • Once the blood has picked up oxygen, it moves from the lungs to the left atrium (3), and then down into the left ventricle (4). The atrium contracts and then the ventricle contracts, like on the right side.
  • The right and left atriums actually contract at the same time. The right atrium pushes oxygen-low blood into the right ventricle, and the left atrium pushes oxygenated blood from the previous cycle into the left ventricle.
  • When the left ventricle contracts, it sends the blood to the rest of the body.
  • The blood eventually returns to the right atrium, low on oxygen, and the process starts again.

In­ each heartbeat, the atrium has to contract first, or else the ventricles will be low on blood and their contractions won't be effective.

What exactly keeps the pace? Since your heart is your body's engine, it makes sense that it might work something like the engine in your car: It starts with a spark.

SA node and AV Node: Setting the Pace

Your heart is more like a car engine that you might realize. Loosely speaking, the heart's chambers are the pistons, the contraction of those chambers is the piston stroke, and the ignited gas is the blood that keeps everything going. The heart even has a sparkplug. An electrical impulse triggers each contraction and sets the timing of the whole process.

When someone needs a pacemaker, it's usually because there's a problem with these electrical impulses, which weakens the heartbeat, causing all sorts of issues. If the heart can't get enough blood pumping through the body, the body -- and especially the brain -- suffers from lack of oxygen. An artificial pacemaker sends out electrical impulses to mimic the heart's natural pacemaker, the sinoatrial node (SA node), located in the right atrium.

The SA node is a group of cells that generates electrical current. It sends out an electrical charge at some set interval -- say, once every second, which would establish the low-end normal heart rate of 60 beats per minute (60 to 80 is a healthy heart rate). These impulses are the "sparks" that cause the right atrium to contract, starting the whole string of events that gets blood pumping in waves through your body. It's this electrical impulse that sets the rhythm of your heart. Whenever the SA node sends out a charge, your heart beats. When you need more blood pumping, like when you need more oxygen to climb steps or run a mile, the SA node shortens its electrical-discharge interval.

There are actually two pacemakers. The SA node is the primary the atrioventricular node (AV node), located in a bundle of tissues on the border between the right atrium and the right ventricle, is the secondary. When the SA node sends out an electrical impulse, the first place it goes is to the AV node. While the SA node sets the rhythm of your pulse, the AV node sets the rhythm of your heart contractions. It delays the signal on its way to the ventricle, giving the atrium time to contract first. It holds it up for about a tenth of a second [source: Signalife]. If the atrium and the ventricle contracted at the same time, the ventricles would push out their blood before they were totally full, resulting in low blood pressure, among other problems.

When the heart's electrical system misfires, it's called atrial fibrillation. Basically, what happens is the heart starts generating electrical impulses in more than one place, not just in the SA node. This messes everything up and can result in a pulse well above the 60 to 80 range that a healthy heart generates. With too many triggers, the right atrium can't possibly contract fully each time, meaning it never gets a full pump of blood into right ventricle, and the body gets deprived of blood. An artificial pacemaker stabilizes the system by taking over the job of sending out electrical impulses, getting the heart back into a regular rhythm.

For more information on the heart, atrial fibrillation and related topics, look over the links on the next page.


Differential diagnosis

The following is a very simple approach to the differentiation of some of the more common and simpler problems of identifying murmurs on auscultation:

    , aortic sclerosis and pulmonary stenosis (including effective pulmonary stenosis as with an atrial septal defect or hyperdynamic circulation) all produce a crescendo-decrescendo systolic murmur. Aortic stenosis is transmitted well to the carotids. Aortic sclerosis almost never occurs before 50 years of age and the patient is usually much older. It may be transmitted to the apex and axillary line. Pulmonary stenosis should not produce such a flat pulse wave as the others and the murmur may reduce on inspiration. starts at the beginning of systole and is a harsh sound of almost constant amplitude, best heard at the apex and transmitted to the axilla. is early diastolic and best heard at the aortic area with the patient sitting forward in expiration. It is only if regurgitation is severe that a collapsing pulse and low diastolic blood pressure will be found. Mitral stenosis is becoming rarer these days. It is late diastolic and best heard in the mitral area.
  • An innocent murmur in pregnancy is only systolic. It is a typical crescendo-decrescendo murmur that may be transmitted to the carotids. It may change with posture. There is a bounding pulse. There is no cardiac history including any shortness of breath on exertion. If in doubt, echocardiography provides a safe and reliable diagnosis.

10 Amazing Animal Heart Facts

They can be as big as a piano or too small to see without a microscope. They may beat as much as 1,000 — or as little as six — times a minute.

They are animal hearts and they’re extraordinary.

Yes, the human heart is pretty astonishing, too. The thing has its electrical impulse, so with enough oxygen it can beat when outside of the body.

But then again, we just have one of them. The octopus has three. And it just gets more amazing from there.

The cheetah is one of the fastest land animals, but its resting heart beat is about 120 beats per minute, similar to a jogging human. Here’s the difference: While it takes some time for a human heart to reach its limit, usually 220 BPM, the cheetah can go up to 250 BPM in just a few seconds.

A cheetah’s heart beat can go up to 250 BPM in just a few seconds.

The cheetah has a bit of competition, however, with the Etruscan shrew . The smallest known mammal by mass, the Etruscan shrew weighs in at under 2 grams and has a 25 beats per second heart rate. That’s a 1,500 BPM. It’s also kind of cute .

The human heart is about the size of a fist — and a cow’s heart is the size of a human head. The largest animal heart is the blue whale’s , which has been weighed at about 400 pounds (and it is not the size of a small car , contrary to popular belief).

But the animal with the largest heart-to-body-mass ratio is somewhat surprising: the dog . Compare a dog’s heart to its body mass and it’s a .8 percent ratio. Almost all other animals — including elephants, mice and humans — have a .6 percent ratio. Another animal with a ratio larger than most mammals is the dog’s ancestor, the wolf.

The smallest animal hearts belong to the .006-inch long fairyflies. You need a microscope to see its heart, which is a tube running along its back. A new species of fairyfly found in Costa Rica is named Tinkerbella nana .

The tropical, freshwater zebrafish is a popular aquarium addition, but it’s the animal’s heart that deserves the most attention. It has amazing regenerative properties, quickly closing injuries and mending itself back to almost full function.

That’s why researchers study the zebrafish to uncover possible treatments for heart failure and other cardiac injuries. At the CVM, the zebrafish is helping us unlock some of the mysteries of the human immune system.

The zebrafish heart has amazing regenerative properties, quickly closing injuries and mending itself back to almost full function.

Human hearts, like those of all mammals, as well as birds, have four chambers. The heart’s “thump-thump” sound is the four valves opening and closing as they pump blood. But frog hearts have three chambers — two atria and one ventricle (you can actually see how it works in a glass frog ).

Actually, all reptiles have three-chambered hearts with one exception. Crocodilian hearts have four chambers, but unlike mammals they have an extra flap that can close to keep blood from going to the lungs. Researchers believe the blood can be sent to the stomach to aid digestion, which is just a smidge helpful when bones are often on the menu.

Many animals decrease their heart rate while diving into water. An emperor penguin’s heart rate dips 15 percent from its resting rate when diving and drops even more during long dives (in between dives it jumps rapidly, likely to replenish tissues with oxygen). A manatee heart rate cuts by half while on a long dive and seals decrease their heart rate from 50 to 80 percent while diving. By the way, seals eat squids , which, like octopuses , have three hearts.

A manatee cuts its heart rate by half while on a long dive.

There are also numerous animals with no hearts at all, including starfish, sea cucumbers and coral . Jellyfish can grow quite large, but they also don’t have hearts. Or brains. Or central nervous systems. It’s working for them, though. They’ve been around at least 500 million years.

Pairs of dragonflies and damselflies (damselflies are different than dragonflies and, yes, that’s what they’re called) form heart shapes when mating. The male grabs the female behind its head and the female … uh, how about we just show you a photo ?

Sources: The Central Florida Zoo National Park Service National Wildlife Federation Journal of Experimental Biology National Science Foundation


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