Anatomy: Parts of the Heart
Figure 1. Basic Anatomy of Human Heart
(Jiang, Pajic, and Mangharam, 2011) |
Important Structures of the Heart:
1) Tricuspid (Right) 2) Mitral (Left) - Semilunar valves: 1) Pulmonary 2) Aortic |
Composition: What is the heart made of
![Picture](/uploads/4/3/0/1/43011017/6150860.jpg?250)
The heart is composed of cardiac muscle tissue, collectively known as the myocardium. This tissue is primarily composed of contractile cardiac muscle cells, known as cardiac myocytes, which appear striated, due to the presence of organized sarcomeres. These sarcomeres are necessary for the contraction of the heart muscle, which plays a key role in the pumping of oxygenated blood out of the heart and out to the rest of the body.
In addition, intercalated disks are seen throughout the myocardium. These are cell junctions composed of desmosomes and gap junctions, and they help form a network of cells in the heart that communicate effectively to spread the contractile force and electrical impulse throughout the heart. This cell-to-cell communication allows cells in each part of the heart to work together to properly pump blood through the heart (Silverthorn, 2013).
The remaining small portion of cells in the myocardium is the autorhythmic cells, which are responsible for producing the autonomous heartbeat of the heart. These cells are special in that they require no stimulation from the nervous system, but rather, they use the action of ion channels to allow for the continuous generation of action potentials (Silverthorn, 2013).
In addition, intercalated disks are seen throughout the myocardium. These are cell junctions composed of desmosomes and gap junctions, and they help form a network of cells in the heart that communicate effectively to spread the contractile force and electrical impulse throughout the heart. This cell-to-cell communication allows cells in each part of the heart to work together to properly pump blood through the heart (Silverthorn, 2013).
The remaining small portion of cells in the myocardium is the autorhythmic cells, which are responsible for producing the autonomous heartbeat of the heart. These cells are special in that they require no stimulation from the nervous system, but rather, they use the action of ion channels to allow for the continuous generation of action potentials (Silverthorn, 2013).
Physiology: How Does The Normal Heart Function
Heart Cycle:
The human heart cycles through a constant contraction and relaxation phases to perform its function in pumping blood. The contraction phase is referred to as systole, and the relaxation phase is known as diastole. Specifically, systole consists of the contraction of the ventricles, causing blood on the right side of the heart to be passed into the pulmonary circulation and blood on the left side of the heart to be propelled out into the systemic circulation. The closure of the valves during systole causes the familiar “lub-dub” sound heard by a beating heart. The “lub” is the first closure of the atrioventricular valves, increasing pressure in the ventricles. Then, the “dub” sound is the closing of the semilunar valves after blood of high pressure has already been passed through them.
Following systole, diastole occurs and causes the ventricles to be refilled with blood coming from the atria due to a pressure gradient. During this longer phase, both the ventricles and atria are relaxed and allowing blood to flow passively into the ventricles. After this point, the remaining blood in the atria is pushed into the ventricles by contraction of the atrial wall. Finally, the blood-filled ventricles return to the systole phase and restart the heart cycle (Silverthorn, 2013).
Following systole, diastole occurs and causes the ventricles to be refilled with blood coming from the atria due to a pressure gradient. During this longer phase, both the ventricles and atria are relaxed and allowing blood to flow passively into the ventricles. After this point, the remaining blood in the atria is pushed into the ventricles by contraction of the atrial wall. Finally, the blood-filled ventricles return to the systole phase and restart the heart cycle (Silverthorn, 2013).
Action potential of cardiac contractile cells:
![Picture](/uploads/4/3/0/1/43011017/7447482_orig.jpg)
The contractile cells of the heart have a characteristic action potential consisting of a plateau phase, creating a longer duration for the contraction of the heart muscle. Each phase of the action potential is marked by a change in the state of specific ion channels (Fig. 3). The plateau phase is primarily attributed to calcium influx into the cell and a decrease in potassium efflux, causing the membrane potential to stay more positive.
This action potential cycle is crucial to the function of the heart because the heart must go through continuous phases of contraction and relaxation. By having a longer action potential of approximately 200 msec, the contractile cells in the heart end their contraction phase and are almost completely relaxed by the time the refractory period is complete. This means that a normal heartbeat can be sustained without causing action potential summation, which could lead to no refilling of the ventricles and could cause major cardiac issues (Silverthorn, 2013).
This action potential cycle is crucial to the function of the heart because the heart must go through continuous phases of contraction and relaxation. By having a longer action potential of approximately 200 msec, the contractile cells in the heart end their contraction phase and are almost completely relaxed by the time the refractory period is complete. This means that a normal heartbeat can be sustained without causing action potential summation, which could lead to no refilling of the ventricles and could cause major cardiac issues (Silverthorn, 2013).
Action potential of Autorhythmic cells:
![Picture](/uploads/4/3/0/1/43011017/9298691_orig.jpg)
The autorhythmic cells in the heart are special for their ability to continuously produce action potentials on their own. Rather than having a normal stable resting membrane potential, these cells are constantly depolarizing slightly starting from -60 mV. This is caused by the work of ion channels in the membrane of the cell, known as If channels. Essentially, these channels allow sodium and potassium to pass through the membrane, so sodium enters the cell and potassium exits the cell. At this unstable “pacemaker potential,” the If channels allow more sodium influx rather than potassium efflux, thereby constantly increasing the membrane potential up to a threshold of -40 mV.
When the threshold is reached, the cells initiate an action potential where depolarization occurs through calcium influx. The repolarization phase is then caused by potassium efflux, returning to the unstable pacemaker potential which will trigger the next action potential (Silverthorn, 2013).
When the threshold is reached, the cells initiate an action potential where depolarization occurs through calcium influx. The repolarization phase is then caused by potassium efflux, returning to the unstable pacemaker potential which will trigger the next action potential (Silverthorn, 2013).
Electrical conduction:
The conduction of the heart follows a specific path throughout the heart muscle. It is generated in the SA node, follows through the internodal tracts, and comes to the AV node. Here, the electrical impulse is delayed to allow for the ventricles to be filled with blood from the atria before they are contracted to pump the blood out. Then the impulse passes through the Bundle of His and separates into the right and left bundle branches, which further divide into Purkinje fibers. As observed in the animation, the impulse travels from the top of the heart to the bottom and then back up. This unique pathway is used to contract the heart from the bottom towards the top to push blood up and out of the heart and pump it into the lungs or the rest of the body (Silverthorn, 2013).
References:
Jiang, Z., Pajic, M., & Mangharam, R. (2011). Model-Based Closed-Loop Testing of Implantable Pacemakers.
Silverthorn, D. U. (2013). Human Physiology: An Integrated Approach. Glenview, IL: Pearson Education, Inc.
Silverthorn, D. U. (2013). Human Physiology: An Integrated Approach. Glenview, IL: Pearson Education, Inc.