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The Heart

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Biology

The heart is a ‘muscular bag’ filled with blood that contributes to the circulatory system by pumping blood around the body. It is about the size of an adult’s fist and weighs around 300g. In humans, the heart is in the chest, enclosed by the ribs and sternum (the breastbone, the bone located in the middle of the chest). Since the heart is filled with blood and constantly pumping, it is surrounded by an inelastic pericardial membrane, which helps prevent the heart from over-distending with blood.

The heart is made of specialised muscle cells called cardiac muscles cells. These cells can continuously contract and relax without fatigue. The movement of these cells does not require stimulation by the nervous system.

The dying off of heart muscles usually signals a heart attack, as the heart muscles do not receive enough oxygen to continue contracting and relaxing.

What does the heart look like from the outside?

Let’s explore the heart from the outside first.

The two sides of the heart

This muscular organ is divided into two parts: the left side and the right side.

The Heart [+] a model of a heart [+] StudySmarterFigure 1. A model of the heart. Source: Wagner Souza e Silva/Museum of Veterinary Anatomy FMVZ USP, CC BY-SA 4.0, via Wikimedia Commons.

You may wonder what purpose it serves for the heart to be divided into two. The left side receives oxygenated blood from the lungs and pumps it to the body, and the right side receives deoxygenated blood from the body and pumps it to the lungs. Blood has to pass through the lungs first to be oxygenated. There is a large drop in blood pressure after passing through the lungs, and the heart needs to pump this blood to increase its pressure. Hence, the heart is split into two separate pumps lying side by side instead of one pump.

Regardless, both sides of the heart contract and relax at the same time.

The Heart [+] the heart pumping blood both side sof the heart relax at the same time [+] StudySmarterFigure 2. The heart pumping blood. Notice that both sides relax at the same time. Source: josino, Public Domain, via Wikimedia Commons.

Blood vessels of the heart

The heart is connected to four blood vessels - two veins (vena cava and pulmonary vein) and two arteries (aorta and pulmonary artery).

The Heart [+] labelled diagram of a heart [+] StudySmarterFigure 3. The heart labelled. Source: Wapcaplet Yaddah, CC BY-SA 3.0, via Wikimedia Commons.

Veins carry blood to the heart. The two veins that connect to the heart include:

  • Vena cava - brings deoxygenated blood from both the lower and upper body

  • Pulmonary vein - brings oxygenated blood from the left and right lungs

In contrast, arteries carry blood away from the heart. The two veins that connect to the heart include:

  • Aorta - large and arching, also branches into several smaller arteries to transport oxygenated blood up to the head and the rest of the lower body

  • Pulmonary artery - branches into two to transport deoxygenated blood into the left and right lungs

Blood vessels supply oxygen and nutrients to the heart muscles; they need to contract and relax throughout the organism’s lifetime. Heart muscles have coronary arteries and cardiac veins. Coronary arteries branch off from the aorta and supply the heart muscles with oxygenated blood, whereas cardiac veins bring deoxygenated blood containing metabolic waste into the vena cava. On the other hand, cardiac veins bring deoxygenated blood containing metabolic waste into the vena cava.

Study tip: Remember that pulmonary arteries carry deoxygenated blood whereas pulmonary veins carry oxygenated blood, unlike all the other arteries and veins in the body. Many students get confused with pulmonary blood vessels in the exams.

Four chambers of the heart

The human heart comprises four chambers – two on the right side and another two on the left. The chambers of the heart are divided by a muscular structure called the septum. The septum stops the blood from both sides of the heart from mixing together (Figure 3).

The upper chambers that are smaller in size are the atria (singular atrium). The atria are connected to the veins, with the right atrium connected to the vena cava and the left atrium connected to the pulmonary vein. The atria have thin walls as they only pump blood into the small ventricles, whose lower pumping pressure prevents the ventricles from bursting.

The larger lower chambers are the ventricles. The ventricles are connected to the arteries, with the right ventricle connected to the pulmonary artery and the left ventricle connected to the aorta. The ventricles have thicker walls than the atria as they pump blood into organs further away from the heart. Furthermore, the walls of the left ventricle are thicker than the right. This is because the blood is pumped from the left ventricles to the extremities and needs to overcome the elastic recoil of arteries. As for the thinner walls of the right ventricles, the lungs that receive blood from the right ventricles are closer to the heart and smaller than the rest of the body, but the lungs’ capillaries are extremely delicate.

Study tip: It may be confusing to remember where veins and arteries connect. Just remember the acronym AV – arteries are connected to ventricles, whereas atria are connected to veins.

Heart valves

How the heart achieves a regular direction of blood flow across the different chambers?

This is due to structures called valves, which prevent the backflow of blood. The valves located between the atria and ventricles are called atrioventricular valves. The left and right sides of the heart each have an atrioventricular valve. The atrioventricular valve on the left side is called the bicuspid (or mitral) valve, whereas the atrioventricular valve on the right side is called the tricuspid valve. The atrioventricular valves have string-like, tendinous support? Atrioventricular valves have tendinous cords to prevent pressure from the heart from turning the valves inside out when the ventricles contract.

The Heart [+] heart valves [+] StudySmarterFigure 4. Heart valves. Source: OpenStax College, CC BY-SA 3.0, via Wikimedia Commons.

Moreover, there are valves located on the arteries connected to the heart. These valves are called semilunar valves, including aortic valves and pulmonary valves. The name ‘semilunar’ is derived from the shape as the valves look like two crescents. Both atrioventricular and semilunar valves have flaps that open and close. The key difference between bicuspid and tricuspid valves is the number of flaps these valves have. Bicuspid valves have two flaps, whereas tricuspid valves have three flaps. Semilunar valves, like the tricuspid valve, have three flaps.

Study tip: It might be difficult to remember what type of valve belongs to which side of the heart. It might help to remember that the right ventricle is thinner and needs three flaps to compensate.

How does the heart function?

The heart functions by a series of contractions and relaxations of the heart muscles. These move the blood across the different chambers and send it to respective organs via blood vessels. The valves also help regulate the unidirectionality of blood flow.

As such, the components of the heart all work together to form the cardiac cycle.

In brief, the heart functions to:

  • Pump oxygenated blood around the body.

  • Receive and pump deoxygenated blood to the lungs for oxygenation.

  • Transport hormones and other vital substances to different parts of the body.

  • Maintain a constant blood pressure across the different vessels.

The Heart - Key takeaways

  • The heart is a ‘muscular bag’ filled with blood that contributes to the circulatory system by pumping blood around the body. It is divided into the left and right sides and connected to four blood vessels - two veins (vena cava and pulmonary vein) and two arteries (aorta and pulmonary artery).
  • The heart is made up of four chambers - the right atrium and ventricle and the left atrium and ventricle. The veins connect to the atria, whereas the arteries connect to the ventricles.
  • Heart muscles have coronary arteries and cardiac veins. Coronary arteries branch off from the aorta and supply the heart muscles with oxygenated blood, whereas cardiac veins bring deoxygenated blood containing metabolic waste into the vena cava.
  • Valves are present in the heart to prevent backflow. Atrioventricular valves, between the atria and ventricles, include the tricuspid valve at the right side of the heart and the bicuspid valve at the left side of the heart.

The Heart

The chambers of the heart regularly contract and relax to transport both oxygenated and deoxygenated blood around the body.

The heart is a bag of muscle that pumps blood around the body.

These functions include:

  • Pump oxygenated blood around the body.

  • Receive and pump deoxygenated blood to the lungs for oxygenation.

  • Transport hormones and other vital substances to different parts of the body.

  • Maintain a constant blood pressure across the different vessels.

During a heart attack, the heart muscles die off due to a lack of oxygen supply to cardiac muscle tissues.

The four parts of the heart include the heart chambers, vessels, valves and septum.

Final The Heart Quiz

Question

The diagram of the heart on paper usually has the left side of the heart on the right-hand side and the right side of the heart on the left-hand side. (True/ False)


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Answer

True

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Question

How is the heart adapted to prevent over-distending with blood?


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Answer

It is surrounded by an inelastic pericardial membrane.

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Question

Explain the need for two sides of the heart.


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Answer

Each side of the heart is composed of separate pumps lying side by side. The left side receives oxygenated blood from the lungs and pumps it to the body and the right side receives deoxygenated blood from the body and pumps it to the lungs. Blood has to pass through the lungs first to be oxygenated before passing through the rest of the body. As there is a large drop in blood pressure after passing through the lungs, the heart needs to pump this blood to increase its pressure for it to pass through the rest of the body. Hence, the heart is split into two separate pumps lying side by side instead of one pump.


Show question

Question

Name the two veins that connect to the heart and outline their roles.


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Answer

  • Vena cava – brings deoxygenated blood from both the lower and upper body 
  • Pulmonary vein – brings oxygenated blood from the left and right lungs 

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Question

Name the two arteries that connect to the heart and outline their roles.


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Answer

  • Aorta – large and arching, also branches into several smaller arteries to transport oxygenated blood up to the head and the rest of the lower body
  • Pulmonary artery – branches into two to transport deoxygenated blood into the left and right lungs

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Question

Describe how the heart obtains its own blood supply.


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Answer

The heart obtains its own blood supply by having specialised arteries that branch from the aorta called coronary arteries. These arteries supply oxygenated blood to cardiac muscles to facilitate their necessary contraction and relaxation.

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Question

Name the structure of the heart that stops blood of the left and right sides of the heart from mixing together.


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Answer

Septum

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Question

Fill in the blanks

The upper chambers that are smaller in size are the ____ (singular _____). They are connected to the veins, with the _____ ______ connected to the vena cava and whereas the _____ _____  connected to the pulmonary vein. 

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Answer

The upper chambers that are smaller in size are the atria (singular atrium). The atria are connected to the veins, with the right atrium connected to the vena cava and whereas the left atrium connected to the pulmonary vein. 


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Question

 Explain why the atria have thin walls.


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Answer

The atria have thin walls as they only pump blood into the ventricles that are small structures, whose lower pressure of pumping prevents the ventricles from bursting.

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Question

Fill in the blanks

The lower chambers that are larger in size are the ______. The ventricles are connected to the _______, with the ____ ________ connected to the pulmonary artery whereas the ____ ______ is connected to the aorta.

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Answer

The lower chambers that are larger in size are the ventricles. The ventricles are connected to the arteries, with the right ventricle connected to the pulmonary artery whereas the left ventricle is connected to the aorta.

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Question

Explain why the walls of the left ventricles are thicker than the right ventricles.


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Answer

The walls of the left ventricles are thicker than the right ventricles because the blood is pumped from the left ventricles to the extremities. This blood also needs to overcome the elastic recoil of arteries. As for the thinner walls of the right ventricles, the lungs that receive blood from the right ventricles not only are closer to the heart and are smaller than the rest of the body, but the capillaries of the lungs are extremely delicate.

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Question

Valves serve to prevent backflow of blood. Valves in the arteries that connect to the heart are called semilunar valves. As for atrioventricular valves between the atria and ventricles, the right side of the heart has bicuspid valves whereas the left side of the heart has tricuspid valves. (True/ False)


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Answer

False - the right side of the heart has tricuspid valves whereas the left side of the heart has bicuspid valves

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Question

Outline the role of tendinous cords found connected to atrioventricular valves.

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Answer

To prevent pressure from the heart from turning the valves inside out when the ventricles contract.

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Question

Name the process given to the coordination of the different components of the heart in the organ’s series of contractions and relaxations.


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Answer

The cardiac cycle


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Question

Outline the four roles of the heart.


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Answer

  • Pumping oxygenated blood to the other body parts.
  • Pumping hormones and other vital substances to different parts of the body.
  • Receiving deoxygenated blood and carrying metabolic waste products from the body and pumping it to the lungs for oxygenation.
  • Maintaining blood pressure

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