Cardiac muscle is striated and uniquely structured to function in a muscle that is used approximately 70 times a minute, 24 hours a day, and 365 days a year for an average of 80 years. Cardiac muscle is branched, not linear like skeletal muscle. Cardiac muscle has alternating thick (myosin) and thin (actin) protein filaments which are the sliding filaments. These are the two primary proteins that build the cardiac fiber which is called a myofibril.
When viewed under the microscope, also seen are the darker lines perpendicular to the muscle fiber which divide the cardiac muscle. These are the intercalated disks. These structures enable transmission of muscle contraction signals. The intercalated disk allows the rapid propagation of the muscle action potential which enables the myocardium to contract together and allows for ventricular synchrony.
The heart must work continuously. That is why it has an abundance of mitochondria in the myocardium. In fact, approximately 50% of the myocardium is mitochondria. These structures are the cellular power generators for the cells. They enable constant aerobic respiration which supplies energy to the myocardial cell for constant function.
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What are intercalated discs and what do they do?
The intercalated disks enable transmission of muscle contraction signals. The disks allow for the rapid propagation of the muscle action potential which enables the myocardium to contract together and allows for ventricular synchrony.
Why does cardiac muscle have to be both elastic and strong?
The human heart must be elastic as well as strong in order to meet the needs of the human body. The elasticity of the atria and ventricles is important in order to accommodate the preload that is essential to trigger the stretch of the myofibrils that is inherent in contractility and vital for cardiac output or the ejection of blood. When an individual participates in exercise and has a faster heart rate, the chambers must dilate (or stretch) to accommodate the returning volumes in order to meet the greater need for oxygen in the exercising muscles. As the heart returns to normal resting rates, these dilated chambers return to their normal size. Chronic chamber dilation (or cardiomyopathy) can be detrimental to the function of the heart and result in decreased pumping, loss of forward flow as well as heart failure.
Contractility of the actin and myosin fiber is an inherent property of cardiac muscle. But strength of contractility is important to overcome afterload. Afterload is defined as the force that is needed to overcome aortic pressure for cardiac ejection of stroke volume. Afterload is increased in the presence of sustained diastolic hypertension. Over a period of years this continual increase in the work of the heart will lead to hypertrophy of the ventricular muscle and not only will strength of contraction be lost but increased oxygen consumption will be detrimental to the myocardium and may result in angina or infarction.
D. Which of the three layers of the heart did the tissue used to make your slide originate from? The slide was made from the myocardium or muscle tissue.
Activity 2: The Pulmonary and Systemic Circuits
Draw a diagram showing pulmonary and systemic circulation. Color the deoxygenated side blue and the oxygenated side red. Use a regular pencil to draw arrows to indicate blood flow. ** REFER TO THE ATTACHED DIAGRAM AT THE END OF THE LESSON
Trace the flow of blood through the pulmonary and systemic circuits. Begin in the right atrium and end in the superior/inferior vena cava. Be sure to list every vessel, heart chamber, and heart valve the blood flows through.
Blood flow through the heart begins with the returning deoxygenated systemic bolus into the right atrium. Passively it flows through the low pressure atrioventricular tricuspid valve into the right ventricle. As pressures equalize, the tricuspid valve closes. In the right ventricle, this bolus is pressurized isometrically and then ejected through the low pressure semilunar pulmonic valve into the right and left pulmonary arteries. The bolus then travels through the pulmonary arteries into the pulmonary arterioles and capillaries until it is in single file in the capillary at the alveolar-pulmonary basement membrane. At this point the red blood cell transverses the capillary lengthwise in order to present a greater surface area for oxygen and carbon dioxide exchange.
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Now the oxygenated red blood cell travels through the pulmonary capillary bed into the pulmonary venules and into the pulmonary veins. Through the pulmonary veins, of which there can be 3 to 5, the blood enters into the left atrium ( there are no valves in the pulmonary veins, unlike other veins, and the bolus of blood is pushed forward by the continual movement of blood from the right atrium and ventricle ) . Again passively the bolus of blood flows through the higher pressure mitral valve into the left ventricle. As the pressures again equalize, the mitral valve closes. As the bolus of blood, now known as left ventricular end diastolic volume or more correctly preload, is pressurized again isometrically, the pressure of the aorta (afterload) is exceeded and ejection occurs though the semilunar high pressure aortic valve and into the aorta.
The oxygenated blood now travels through the aorta for dispersal to all the organs and tissues of the body for oxygenation and removal of the cellular waste product, carbon dioxide. From the aorta, blood travels into the smaller arteries, arterioles and eventually the various capillary beds. After the O2 and CO2 exchange occurs at the cellular level, the deoxygenated blood travels from the capillary beds into the venules, veins and eventually into the vena cava. The vena cava returns the deoxygenated blood into the right atrium via the inferior and superior vena cava vessels. And the process continues as long as the heart beats.
Explain what you learned from the online human heart dissection.
I must truthfully state that I did not gain any new information from this exercise. I have been an RN for almost 40 years and my primary area of practice is critical care. One of my areas of expertise is cardiology. I have been certified in critical care (CCRN) for 36 years and I am also cardiac medicine certified (CMC) by my national organization. However, it was interesting and a very nice review.
Activity 3: Sheep Heart Dissection/Fetal Pig Comparison
Compare the structure of the fetal pig and sheep heart. How are they similar? How are they different?
Except for the differences in size, I did not note many differences between the sheep heart and the fetal pig heart. They are similar in that their construction is a 2 chambered atria and ventricle and there is similar vasculature. I did note that the fetal pig heart showed similar muscle mass of both the right and left ventricles. The left ventricle had not increased in myocardial muscle mass yet as the heart had not needed to be the sole source of power for perfusion. After birth, I believe that the left ventricle will increase in mass as seen in both the sheep heart and the human heart.
Why is the heart referred to as a double pump?
The human heart is referred to as a double pump because of the differences needed to handle systemic return and peripheral forward flow. The right heart is a lower pressure system and blood enters passively, driven by the left ventricular ejection force. This blood enters into the lower pressure pulmonary system, where pulmonary arterial systolic pressure is comparable to right ventricular systolic pressure.
The left side of the heart is a much higher pressure system. As the blood leaves the left ventricle, it is at a much higher pressure due to the need to overcome the afterload of the aorta. (Systemic vascular resistance.) Because of this increased need, the left myocardium is considerably thicker than the right ventricle. Additionally, because of this increased muscle mass, during the sustained contraction phase (consistent with the ST segment in the electrical tracing), there is much higher oxygen consumption in the left ventricle. This predisposes the left ventricle to more cardiac issues such as angina, infarction, myopathies, hypertrophies and sudden death due to electrical disturbances.
There are four valves in the heart. Name each valve, list its location and give its function.
The four valves of the heart are the tricuspid, the pulmonic, the mitral and the aortic valve.
The tricuspid valve is located between the right atria and right ventricle. It is a low pressure atrioventricular valve and its job is to prevent back flow or regurgitation of blood into the right ventricle during systole or contraction
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The pulmonic valve is a semilunar cusped valve located in the ostium of the trunk of the pulmonary artery which takes off from the right ventricle. When the right ventricle contracts and blood is pushed into the pulmonary artery, the pulmonic valve closes as the pressures are equalized. The cusp like structure helps to push blood forward. With the valve closed there should be no regurgitation from the pulmonary artery into the right ventricle, this time during diastole as the dilation of the ventricle creates a vacuum to enhance diastolic filling from the atrium.
The mitral valve is a bicuspid or 2 leaflet atrioventricular valve. It is located between the left atrium and the left ventricle. As the left heart is a higher pressure system, the mitral valve must be able to withstand greater pressures. During the contraction of the left ventricle, closure of the mitral valve prevents regurgitation of blood from the ventricle into the left atrium. The mitral valve, as well as the tricuspid valve, is attached to the ventricular walls by chordae tendinea and papillary muscles. During infarctions of the left anterior ventricular wall, these structures can become ischemic and lead to mitral valve dysfunction, further complicating the infarction.
The aortic valve is located at the root of the aorta at its junction with the left ventricle. It is a high pressure, cusped, semilunar valve that must withstand tremendous pressures during left ventricular ejection. Again, if the valve becomes incompetent, blood can flow back into the left ventricle during systole (ejection). This can lead to ventricular overload and pulmonary vascular congestion as well as forward perfusion issues. Both ostia of the coronary arteries are located at the aortic valve near the sinuses of Valsalva. If ventricular ejection is decreased either through poor contractility or incompetent aortic valve, coronary artery perfusion can be affected.
Compare the left and right sides of the dissected heart. What differences do you see?
When the dissected human heart is laid flat, both atria and ventricles can be seen. In this view, it is very obvious how much more muscular the left ventricle and even the left atrium is, when compared to the right side of the heat. The interventricular septum bows slightly into the right ventricle. The left ventricular chamber is larger in size and the left atrium is also larger
In the right and left ventricles you can see the papillary muscles and chordae tendinea that attach the tricuspid and mitral valves to the interventricular wall. In the right ventricle there are trabeculae, light finger like projections that are not readily noted in the left ventricle.
Compare and contrast the functions of the atria and the ventricles.
Both atria are receiving chambers, with the left larger and more muscular than the right. The right atrium receives systemic, deoxygenated blood from the superior and the inferior vena cava and deoxygenated cardiac blood from the coronary sinus. The left atrium receives oxygenated blood from the pulmonary circuit via the multiple pulmonary veins in the left lateral wall. The right atrium is also the home of the heartâ€™s electrical system. The sino-atrial node (SA node) is located in the roof of the right atrium and is the primary pacemaker of the heart maintaining an inherent rhythm and rate of between 60 and 100 beats per minute. In the floor of the right atrium, near the junction of the atrium and the interventricular muscular septum is the atrioventricular node. The AV node or junction is a backup pacemaker of the heart, designed to come in as an escape mechanism if the SA node fails as can happen in cardiac disease or sometimes drug induced mechanisms. The junction is designed to only run the heart when it does not receive the SA nodal signal and so its intrinsic rate is 40 to 60. There is an automatic pause built in between the electrical signal of the SA node and the firing of the AV node and this is known as the AV interval. This slight pause is sufficient to allow the final filling of the ventricles before systole. The waveform generated with SA nodal firing causes the right atrium to contract and push the remaining blood into the right ventricle. This contribution to ventricular filling is known as atrial kick and can be extremely important to individuals with limited cardiac reserve.
There are conduction pathways running through the right atrium and over to the left atrium but the atrium does not have any electrical nodes similar to the right atrium. In abnormal rhythms such as atrial fibrillation and atrial flutter, there are sometimes aberrant pathways in the left atrium that contribute to these dysrhythmias. They are occasionally treated with ablation therapy and scarring of the atrial wall.
Both ventricles are pumping chambers but the left is the more massive and more vital of the two. The right chamber receives the preload initially and if it is diseased or injured, its dysfunction can severely impact the left sided filling and cardiac output. The left ventricle is 2 to 3 times more muscular that the right and even thought the ventricles work in harmony, the leftâ€™s job is ejection and perfusion to the organs and tissues. The predominance of the coronary artery system is located on the left ventricle and the majority of coronary artery disease involves the left ventricle.
Where is the myocardium located?
The myocardium is the inner muscular layer of the heart, located between the epicardium and the endocardium. The epicardium is the outermost layer and is formed by the pericardium folding back onto itself at the aorta. Inflammation of this lining can cause pericarditis. The endocardium is the epithelial lining of the inside of the heart and includes all the structures internally. The endocardium also has an endocrine function, secreting endocardin which helps to sustain contraction. Infarctions can involve the endocardium and previously were known as subendocardial infarctions but are now call non-stemis. (Non ST segment elevation myocardial infarctions.) Additionally, bacteria from peripherally inserted central lines or IV drug abuse or even dental disease can cause infections of the endocardium called endocarditis.
The myocardium is the muscle mass that is involved in stretch and contraction. When there is a disruption of oxygen to an area of myocardium, it is referred to as a myocardial infarction (Stemi) Death of the tissue with subsequent ventricular dysfunction can result if oxygenation is not restored.
How does the heart supply blood to its own cells?
The heart needs a continuous and rich source of oxygenated blood. None of the blood that is pumped through the left chambers oxygenates the heart. The hearts source of oxygen comes from the coronary artery system. These vessels lay on the epicardium of the heart and through collaterals and perforators supply the myocardium with oxygen rich blood.
The left coronary artery arises at the left Sinus of Valsalva on the aortic valve. It travels a short distance across the left atrium behind the pulmonary artery, which protects it from any external trauma, caudally towards the left ventricle and is known as the left main. Any significant disruption of flow at this level through stenosis or thrombus can cause sudden cardiac death. As the left main emerges from behind the pulmonary artery, it becomes known as the left anterior descending coronary artery (LAD). This artery travels from the atrium to the apex of the left ventricle, giving off significant branches and lies in the interventricular sulcus directly overlying the ventricular septum. When you look at the human heart and notice the heavy streak of fat running across the anterior wall of the left ventricle, you are looking at the protective covering of fat for the LAD as it lies in the interventricular sulcus. When the left main changes to the LAD, there is another branch given off that runs circumferentially between the left ventricle and the left atrium. This is known as the Circumflex and also lies in a groove known as the atrioventricular groove and is protected also by a fat pad. On occasion there is a 3rd branching that arises at this area known as the Ramus or Intermediate branch.
Arising from the LAD are diagonals that cross the anterior surface of the left ventricle from the IV sulcus to the left lateral aspect of the left ventricle. The LAD also gives rise to a group of vessels known as septal perforators that perforate the anterior aspect of the muscular septum. The LAD crosses over the apex and turns upward for a short distance on the inferior aspect of the left ventricle.
The Circumflex travels around the left ventricle and terminates on the inferior aspect of the left ventricle not far from the right ventricle in branches referred to as the muscular branches. As the Circumflex travels around the heart, there are branches that arise and travel caudally and laterally across the left ventricle. These 3 branches are referred to as the obtuse marginals.
The Right Coronary Artery (RCA) arises of the right Sinus of Valsalva on the right side of the aorta. The RCA lies in the same atrioventricular sulcus that the Circumflex lies in as it travels between the right atrium and the right ventricle. When the RCA reaches the division between the anterior aspect of the right ventricle and the inferior aspect of the left ventricle, there is a large branch that comes off the RCA and travels laterally towards the LAD. This is referred to as the Acute Marginal branch of the RCA and feeds the right ventricle.
Near the ostium of the RCA, there is a small lateral branch that is important for collateral feed to the LAD, known as the conus branch. As the RCA traverses the right ventricle branches known as muscular branches supply additional blood to the right ventricle.
Having turned and been protected behind the inferior vena cava for a short distance, the RCA is now traveling on the inferior wall of the left ventricle. When it reaches the point where it would be over the interventricular septum, it makes a 90 degree turn and heads towards the apex of the left ventricle. At this time it is now referred to as the Posterior Descending Coronary Artery of the Right Coronary Artery (PDA). There is a mirror sulcus here called the posterior IV sulcus to protect the PDA. Just like on the LAD, the PDA gives off septal perforators that feed the posterior portion of the interventricular septum. The electrical system of the heart is feed predominately by the RCA as 60% of the time the SA Nodal artery arises off the PDA near this 90% turn and is a perforator type of artery. The AV Nodal artery, as a perforator, arises from the PDA directly at this angle, which is known as the Crux.
Collaterals exist from birth and are widespread across the anterior surface of the left ventricle as well as the inferior surface. As we age ,they begin to develop and gain importance for cardiac blood flow when the other major coronary vessels develop atherosclerotic disease and stenosis.
Coronary blood flow is on the supply and demand system. When another organ has an increased need for oxygen, (for instance, the colon) the heart rate increases to meet that demand. In the heart, however, the supply is meet internally through the perforators and smaller capillaries that run a small distance into the myocardium. The best filling of the perforators and capillaries is during diastole or rest. During systole, they are compacted by the muscles in which they lie. As the coronaries receive flow both during systole and passively during diastole, it is to the hearts advantage to not be tachycardia.