The Mammalian Heart

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Summary

  • The heart is a muscular organ in majority animals that pumps bloods via the blood vessels to form the circulatory system
  • Blood provides the human body with important nutrients and oxygen whilst also assisting in the removal of metabolic waste
  • The heart is situated off centre to the left side of the rib cage between the lungs in mammals
  • A mammalian heart is a double pump; where the blood passes through the heart twice in one cardiac cycle. This type of circulation is known as double circulation
  • Within the heart there are two main types of chambers; Upper atrium and lower ventricles where the human heart has two atria and two ventricles. Blood enters the atria and the blood leaves the ventricles
  • To prevent backflow of blood the heart also consists of heart valves; two semilunar valves and the atrioventricular valve.

Function of the heart

The function of the heart is to pump blood around the body and the mammalian heart consists of a double pump. As mammals we have a double circulatory system. This means that the blood passes the heart twice before it has completed a full lap of the body. There are two sides to the heart, the right side which deals with deoxygenated blood and then the left side of the heart deals with blood with oxygen also known as oxygenated blood.

The key blood vessels to remember:

  • Aorta which is the main artery in the body with its thick muscular wall and is linked to the left hand side of the heart
  • The vena cava is the major vein in the body where this returns deoxygenated blood to the right hand side of the heart.
  • Pulmonary artery is the artery which takes deoxygenated blood to the lungs
  • The pulmonary vein takes the oxygenated blood from the lungs and back to the heart.

An important way to remember which direction blood is moving in the body is:

Arteries Away
Veins Return

Structure of the mammalian heart

The External structure

In humans the heart is located between the lungs and situated in the middle compartment of the chest. The mammalian heart is shaped as a hollow and is surrounded by a protective sac known as the pericardium. The membrane is a double membrane where the space between the two membranes is filled with watery fluid that allows the heart beat to rhythm smoothly and prevent friction. What can be seen from looking at Figure 1 is the walls of the heart is covered with cardiac muscle.  Cardiac muscle is what allows the heart to pump blood to the rest of the body. There are two main types of chambers within the heart; the atrium and the ventricles. The atria are situated at the top of the ventricles and are thinner and smaller in size whilst below each atria is the ventricles which are significantly bigger and thicker than the atria. Also present on the hearts surface is a network of arteries known as coronary arteries (Figure 1). These supply the heart with oxygenated blood to ensure the heart continues to pump. Blockages of the coronary arteries can have profound effects where the body can suffer from angina, “pain in the heart” or suffer from a myocardial infarction also known as a “heart attack”. The right atrium has two types of blood vessels called superior vena cava and the inferior vena cava. Whereas, the left atrium has the four pulmonary veins. The pulmonary artery with its right and left branch is present at the upper central region of the heart. The major artery in the heart is known as the aorta. The ability to be able to distinguish the aorta from other blood vessels is it has three prominent blood vessels coming of it.

Figure 1: External structure of the Mammalian heart

Internal structure: The Chambers and Valves

Once the heart is cut open the internal structure of the heart can be seen in Figure 2. it is important to remember that when understanding the inside of the heart the left and right side are in labelled the wrong way round (Figure 2). Whilst doing a heat dissection another important feature that seen is the presence of adipose (“fat”) tissue. Since fat is a beneficial energy storage it is needed by the heart to effectively carry out its function and pump blood. The muscular septum is what is known to divide the heart internally into a left and a right side. Therefore, the heart is further divided into four chambers: 2 upper chambers (atria), the receiving chambers and 2 lower chambers (ventricles), the discharging chambers. The internal division between the chambers is caused by the endocardium, a smooth membranous lining. The left atrium is present and a whole space below the left ventricle is present. The left ventricle has an extremely thicker muscular wall as it needs to contract with great deal of force and pressure to pump blood around the rest of the body. Looking at Figure the right atrium and right ventricle are also present. The right ventricle has quite a large volume however, has a smaller muscular wall compared to the left ventricle as it only needs to pump blood to the lungs which isn’t a far distance. Hence it does not need to contract with a great deal of force.

Another important feature inside the heart is the valves and it consists of; two atrioventricular (AV) valves and two semilunar valves. The valves on the left hand side are called the left atrioventricular valves and its technical name is the bicuspid/mitral valve (Figure 2). Whilst the right handed side are called right atrioventricular valves also known as the tricuspid valve. These valves specially separate the atria from the ventricles and allow the direction of the blood to flow downwards when the atria and ventricles relax however, then close when the ventricles are contracting to prevent the backflow of blood back up to the atria. The semilunar valves are the gateway between the ventricles and arteries. More specifically the ventricles are separated from the aorta and the pulmonary artery by the semilunar valves (also known as the aortic and pulmonary valves). Again these valves prevent the blood from flowing backwards and in the wrong direction back within the heart and are listed for simplification below:

The tricuspid and bicuspid (mitral) valves lie between the atria and ventricle

  • Bicuspid/Mitral valve (Left AV valve)
  • Tricuspid valve (Right AV valve)

The aortic and pulmonary valves lie between the ventricles and their respective main blood vessels that are leaving the heart.

  • Aortic valve (Semilunar valve)
  • Pulmonary valve (Semilunar valve)

Overall blood wants to move from areas of high pressure to areas of low pressure. When a chamber within the heart contracts, there is a build-up of pressure within, as the volume is getting smaller and then pressure is going up.
When the pressure is built up, within the chamber that is contracting, then blood has a two options on which direction it wants to flow; it can either move to the next part of the heart or then flow backwards; since both of these options are at low pressure compared to the contracting chamber. Therefore, valves are what prevent the backflow of blood within the heart.
The structure of the atria and ventricles is closely related to their function. As already mentioned above the atria, receives the blood as it enters the heart and only pumps the blood to the ventricles. The wall of the atria is only needed to be thin to carry out its function. Whereas, ventricles have a significantly, thicker muscular wall as they need to pump the blood out of the heart and allow for the blood to reach to the respiring tissues within the body.

Below is a diagram of the heart (Internal):

Figure 2: Internal structure of the Mammalian heart

Flow of the blood

How does blood flow through the heart?. The simplified version to remember this is by looking at Figure 3. Typically deoxygenated blood enters the heart via the vena cava and this supplies deoxygenated blood from the body and flows straight into the right atrium where blood goes through the right AV valve and flow down to the right ventricle. Then the blood is going to move out through the semilunar (pulmonary) valve and into the pulmonary artery as we are going away from the heart and to the lungs. In the lungs the deoxygenated blood becomes oxygenated and now the blood returns to the heart via the pulmonary vein. From here the blood enters the left side of the heart, the left atrium. The blood passes through the left AV valve and into the left ventricle. From here the blood passes out the semilunar (aortic) value and go into the aorta. The aorta artery is what finally takes the oxygenated blood to the rest of the body. Whilst this is happening the blood loses its oxygen and becomes deoxygenated again to reach the vena cava and enter the right atrium. The cycle then continue and repeats itself again. This is occurring within you as you read this article.

Figure 3: Circulation of blood through the heart

References

[1]. https://commons.wikimedia.org/wiki/File:Circulation_of_Blood_Through_the_Heart.jpg#filehistory
[2]. https://my.clevelandclinic.org/health/articles/17057-your-heart–blood-vessels
[3]. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.3%3A_Mammalian_Heart_and_Blood_Vessels/40.3A%3A_Structures_of_the_Heart