definition of heart
The heart is a muscular organ that pumps blood throughout the body. It is located in the middle cavity of the chest, between the lungs. In most people, the heart is located on the left side of the chest, below the breastbone.
The heart is made up of smooth muscle. It has four chambers that contract in a specific order, allowing the human heart to pump blood from the body to the lungs and vice versa with high efficiency. The heart also contains “pacemaker” cells that fire nerve impulses at regular intervals, causing the heart muscle to contract.
This animation shows the workings of this extraordinarily complex pump in action. As you read this article, try to scroll up and see if you can spot the chambers, valves and blood vessels we are discussing in action:
The heart is one of the most vital and delicate organs in the body. If it doesn't work properly, all the other organs – including the brain – start to die from lack of oxygen in just a few minutes. In 2009, the most common cause of death in the world was heart disease.
Most heart disease occurs as a result of age or lifestyle. Cholesterol can build up in the arteries as a person ages, and this is more likely in people who follow diets high in saturated fat and cholesterol. Rarely, though, heart disease can also occur due to a virus or bacteria that infects the heart or its protective tissues.
Scientists have had some success replicating the heart's pumping action with artificial pumps, but these pumps can be rejected by the body and break down over time.
The four-chambered heart found in mammals and birds is more efficient than the one-, two-, or three-chambered hearts found in some other animals. Warm-blooded animals are thought to need highly efficient circulation to meet their cells' high demand for oxygen. This is especially true of humans, whose massive brains require an almost constant supply of oxygen to function!
The heart pumps blood through our huge and complicated circulatory systems at high pressure. It's a truly impressive feat of engineering, as it must circulate about five liters of blood through 1,000 miles of blood vessels every minute! We'll talk more about how the heart accomplishes this remarkable task in the “Structure of the Heart” section below.
The heart's pumping action allows the movement of many substances between the body's organs, including nutrients, wastes, hormones, and other chemical messengers. However, arguably the most important substance circulating is oxygen.
Oxygen is necessary for animal cells to carry out cellular respiration. Without oxygen, cells cannot break down food to produce ATP, the cellular currency of energy. Soon, none of your energy-dependent processes will be able to function. Without its energy-dependent processes, a cell dies.
Neural tissues, including the brain, are particularly sensitive to oxygen deprivation. Neural tissues maintain a special cellular chemistry that must be constantly maintained by consuming lots and lots of energy. If ATP production stops, neural cells can begin to die within minutes.
For this reason, the body has taken many special measures to protect the heart. It is located under the strongest part of the ribcage and padded between the lungs. It is also surrounded by a protective membrane called the pericardium, which is filled with additional cushioning fluid.
The heart's unique design allows it to perform the incredible task of circulating blood through the human body. Here we'll review their essential components and how and why blood passes through them.
Heart Wall Layers
The heart has three layers of tissue, each with a slightly different purpose. These are:
- the epicardium. The epicardium is also sometimes considered part of the protective pericardial membrane surrounding the heart. Helps keep the heart lubricated and protected.
- the myocardium. The myocardium is the muscle of the heart. You may remember this because the root word “mio” comes from “muscle” while “cardio” comes from “heart”.
The myocardium is an incredibly strong muscle that makes up most of the heart. It is responsible for pumping blood throughout the body.
- the endocardium. The endocardium is a thin protective layer on the inside of the heart. It is made of smooth, slippery endothelial cells, which prevent blood from sticking to the inside of the heart and forming deadly blood clots.
Chambers of the Heart
The heart has four chambers, designed to pump blood from the body to the lungs and back with extremely high efficiency. Here we will see what the four chambers are and how they carry out their work:
- the right atrium. The right and left atria are the heart's smallest chambers and have thinner, less muscular walls. This is because they only receive blood from the veins – they don't have to pump it back through the entire circulatory system!
The right atrium just needs to receive blood from the body's veins and pump it into the left ventricle, where the real pumping action begins.
- the right ventricle. The ventricles are larger chambers with stronger, thicker walls. They are responsible for pumping blood to the organs at high pressures.
There are two ventricles because there are two circuits through which blood needs to be pumped – the pulmonary circuit, where blood receives oxygen from the lungs, and the body circuit, where oxygen-filled blood travels to the rest of the body.
Keeping these two circuits separate with two separate ventricles is much more efficient than simply pumping blood into the lungs and allowing it to flow from there to the rest of the body. With two ventricles, the heart can generate twice as much power and deliver oxygen to our cells much faster.
The right ventricle is the one attacked by the pulmonary circuit. It pumps blood through the pulmonary artery to the lungs, where the blood fills with oxygen at high pressure. The blood then returns to...
- the left atriumreceives oxygenated blood from the pulmonary veins. It pumps that blood into the left ventricle, which…
- the left ventriclepumps blood throughout the rest of the body.
After the blood circulates through the body and the oxygen is exchanged for carbon dioxide waste from the body's cells, the blood re-enters the right atrium and the process begins again.
In most people, this entire circulatory path takes only about a minute to complete!
You might be wondering how the heart ensures that blood flows in the right direction between these chambers and the blood vessels. You may also have heard “heart valves” mentioned in a medical context.
Heart valves are just that – biological valves that only allow blood to flow through the heart in one direction, ensuring that all the blood gets where it needs to go.
Here is a list of the most important heart valves and an explanation of why they are important:
- A tricuspid valve. The tricuspid valve is called the “atrioventricular” valve. As you might guess from the name, it ensures that blood only flows from the atrium to the ventricle – not the other way around.
These atrioventricular valves have to withstand very high pressures to ensure that no blood passes through, as the ventricle contracts with great force to squeeze out the blood.
The tricuspid valve is the valve that ensures that blood from the right ventricle goes into the pulmonary artery and reaches the lungs instead of being pushed back into the right atrium.
- A pulmonary valve. The pulmonary valve is called the semilunar valve. The semilunar valves are found in the arteries that leave the heart. Its role is to prevent blood from flowing from the arteries into the chambers of the heart.
This is important because the ventricles “suck” blood from the atria, expanding after they have expelled the blood into the arteries. Without the semilunar valves working properly, blood can back up into the ventricle instead of going to the rest of the body. This dramatically decreases the efficiency with which the heart can move oxygenated blood around the body.
The pulmonary valve sits between the pulmonary artery and the left ventricle, where it ensures that the blood pumped into the pulmonary artery continues to the lungs instead of returning to the heart.
- to mitral valve. The mitral valve is the other atrioventricular valve. This is between the left atrium and the left ventricle. Prevents blood from returning from the ventricle to the atrium, ensuring that blood is pumped to the rest of the body!
The mitral valve sits at the opening of the aorta, which is the largest blood vessel in the body. The aorta is the central artery from which all other arteries fill. Thicker than a garden hose, it extends from the heart to the pelvis, where it splits in two to become the femoral artery in each leg.
- to aortic valve. As you might have guessed, the aorta needs a semilunar valve, just like the pulmonary artery. The aortic valve prevents blood from flowing from the aorta into the left ventricle, as the left ventricle “sucks” oxygenated blood from the left atrium.
Many people have small irregularities in these valves, such as mitral valve prolapse, that make the heart less efficient or more prone to problems. People with minor valve problems can often lead normal, healthy lives.
However, the complete failure of any one of these valves can be catastrophic for the heart and blood flow. This is why people with conditions like mitral valve prolapse are often turned away from the military and other programs that involve conditions that can be very taxing on the heart.
The SA node is another very important part of the heart. It's a group of cells in the wall of the heart's right atrium – and it's what keeps the heart pumping!
Sinoatrial node cells produce small electrical impulses in a regular rhythm. These impulses are what drive the contractions of the four chambers of the heart.
Artificial pacemakers replicate the action of the SA node, delivering similar electrical impulses to people whose SA node is not working properly. However, healthy people have a natural pacemaker built into their heart!
1. Which of the following statements is NOT true regarding the human heart?
A.It has four chambers.
B.It has an integrated “pacemaker” called the sinoatrial node.
C.It is responsible for pumping oxygen-filled blood throughout the body.
D.It is the only organ in the body that you cannot live without.
Answer to question #1
Dis correct. Although the heart is a very important organ, there are many organs that you cannot live without!
2. Which of the following is NOT a heart layer?
Answer to question #2
Cis correct. The myometrium is a part of the uterus – not the heart. It may be helpful to remember that “cardio” is the Greek word for “heart”. This is also where we find words like “cardiologist” and “cardiovascular system”.
3. Why does the heart need valves?
A.To keep out pathogens and toxins that can damage the heart
B.To ensure that blood does not return to the wrong heart chamber
C.To ensure that the heart does not pump too much blood at once
D.None of the above
Answer to question #3
Bis correct. The heart's valves ensure that blood reaches its destination instead of flowing backwards and blocking out new blood!
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