Cardiovascular System s2

CARDIOVASCULAR SYSTEM THE HEART Behavioral Objectives

1. Describe the cardiovascular system: its components, circulatory routes, and distribution of blood. The cardiovascular system is the system composed of the heart, blood, and blood vessels. The heart is the pump that provides the pressure to move blood through the blood vessels. Many different types of materials are carried in the blood and will be discussed in greater detail in the unit on the blood.

2. Describe the size and location of the heart. The heart is approximately the size of an individual’s fist and weighs less than one pound. It is a hollow muscular organ located in an area called the mediastinum (between the lungs), lying anterior to the vertebral column and posterior to the sternum. Its approximate position is between ribs 2-6. The apex lies on the diaphragm.

3. Describe the layers of the pericardial sac? The incision would be made through the fibrous layer of the pericardial sac and then the parietal pericardium of the pericardial sac. It would then pass through a cavity called the pericardial cavity that contains pericardial fluid. Directly adhering to the heart is a thin layer called the visceral pericardium (also called the epicardium).

4. The heart is divided into four chambers: a right and left atrium and a right and left ventricle. Understand the general structure and function of the atria and the structure and function of the anatomical features associated with the atria. The atria are small hollow chambers located on the superior aspect of the heart. They receive blood as it returns to the heart from the body. The right atrium receives deoxygenated blood from three large vessels; the superior vena cavae (drains the head and upper regions of the body), the inferior vena cavae (drains the lower regions of the body) and the coronary sinus (drains the heart). The left atrium receives oxygenated blood as it returns from the lungs via the pulmonary veins.

Associated structures of the atria include pectinate muscles - ridges of muscles on the internal aspect of the atria predominantly in the auricles of the atria, and the fossa ovalis - a remnant structure of the foramen ovale that allowed blood to pass from the right atrium into the left atrium during fetal circulation.

5. Understand the general structure and function of the ventricles and the structure and function of the anatomical features associated with the ventricles. The ventricles are large hollow chambers located on the inferior aspect of the heart. They receive blood from the atria after passing through the atrioventricular valves. The function of the ventricles is to pump the blood out of the heart. The right ventricle pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary trunk (which becomes the pulmonary arteries) and leads to the lungs. The left ventricle pumps oxygenated blood through the aortic semilunar valve into the aorta (which branches into many vessels and delivers oxygenated blood throughout the body).

Associated structures of the ventricles include the trabeculae carnae - large ridges of muscle on the internal aspect of the ventricles (analogous to the pectinate in the atria), and the papillary muscles - cone-like muscles that attach to the chordae tendinae. The chordae tendinae ligamentous strands attach papillary muscles to the atrioventricular valves.

6. There are two major types of valves associated with the heart. The atrioventricular valves (tricuspid and bicuspid) are located between the atrium and ventricles, and the semilunar valves (pulmonary and aortic) are located between the chambers of the heart and the vessels leading out of the heart. Describe the structure of each of the atrioventricular valves, where they are located, structures associated with them, and how they function. The atrioventricular (AV) valves are located between the atrium and the ventricle on each side of the heart. The AV valve between the right atrium and right ventricle is called the tricuspid valve because it has three cusps (or flaps) that make up the valve. The AV valve between the left atrium and left ventricle is the bicuspid valve (also called the mitral valve) because it has two cusps (or flaps) that make up the valve. These valves are positioned between the atrium and the ventricle to prevent the backflow of blood from the ventricle into the atrium when the ventricle is contracting. However, due to the force in the ventricle as it contracts, additional support is needed to keep the AV valves from flapping back into the atrium (thus allowing blood to flow backwards). Additional support is provided to these valves through the papillary muscles and the chordae tendinae. The papillary muscles are the cone-shaped muscles on the internal structure of the ventricle. They are attached to the chordae tendinae that are attached to the AV valve. As the pressure builds inside of the ventricle, pressure is exerted on the AV valve and the chordae tendinae becomes taut, preventing the AV valves from flapping back into the atrium.

7. Describe the structure of each of the semilunar valves, where they are located, and how they function. Semilunar valves are located between the ventricle and the vessel they pump blood into. The valve between the right ventricle and the pulmonary trunk is the pulmonary semilunar valve. The valve between the left ventricle and the aorta is the aortic semilunar valve. They both have two cusps that are crescent-shaped. These valves prevent the blood from back flowing from the vessel back into the ventricle. They do not need additional supporting structures because the vessels do not contract and thus do not exert a backward force on them.

8. As blood moves through the heart it results in two distinctive sounds. Describe these sounds and understand what causes them. Be familiar with unusual heart sounds (heart murmurs) and what causes them. Normal heart sounds Lubb-longer more resonating sound caused by the closure of the AV valves Dupp - short, sharp sound caused by the closure of the semilunar valves

Unusual heart sounds Incompetent valve - swishing sound heard after the valve has closed due to the incomplete closure of the valve, allowing blood to backflow. Stenotic valve - higher pitched sound heard when the valve is open due to the incomplete opening of the valve and blood being forced through a smaller opening, which creates a whistling sound.

9. The purpose of the heart is to pump blood through the blood vessels. Know the major vessels attached to the heart. Aorta - receives oxygenated blood from the left ventricle Pulmonary trunk - receives deoxygenated blood from the right ventricle Superior vena cavae - drains deoxygenated blood from above the heart into the right atrium Inferior vena cavae - drains deoxygenated blood from below the heart into the right atrium. Pulmonary veins - drain oxygenated blood from the lungs into the left atrium.

10. Blood circulation is divided into two major circuits; the pulmonary circuit and the systemic circuit. Be able to trace the blood through these two circuits, listing all of the vessels, chambers, and valves. The heart is divided into two halves; the right half and the left half. The right half is associated with the pulmonary circuit and the left half is associated with the systemic circuit.

The pulmonary circuit involves the pumping of deoxygenated blood from the right side to the lungs until it enters the left side of the heart. Its circuit is as follows: the superior vena cavae, the inferior vena cavae, and the coronary sinus return deoxygenated blood from the body. The blood enters the right atrium, passes through the tricuspid valve, enters the right ventricle, and is pumped into the pulmonary trunk through the pulmonary semilunar valve. The pulmonary trunk splits into the pulmonary arteries that lead to the lungs. As the blood passes through the lungs it is oxygenated (and releases carbon dioxide). It is returned to the left atrium through the pulmonary veins.

The systemic circuit involves pumping oxygenated blood throughout the body. Its circuit is as follows: the left atrium receives oxygenated blood from the lungs. The blood passes through the bicuspid valve and into the left ventricle. The left ventricle pumps it into the aorta through the aortic semilunar valve. The many branches that come off of the aorta lead to different areas of the body, providing them with the oxygen and nutrients that the tissues need, and picking up carbon dioxide and waste. 11. After blood is pumped into the aorta, it passes into the coronary arteries, which are the first branches off of the aorta. These arteries deliver blood to the heart tissue itself. This is referred to as coronary circulation. Trace blood to the heart tissue (coronary arteries) and back from the heart tissue (coronary veins). Coronary circulation involves providing oxygenated blood (and nutrients) to the myocardium and other components of the heart wall, and then returning deoxygenated blood back to the heart chambers after being provided to the heart tissue. The major artery that feeds the heart is the coronary artery, and the major vein that drains the heart is the coronary sinus. The major branching patterns are described below. The coronary artery, after branching off of the aorta, forms two major branches: the left coronary artery and the right coronary artery. The left coronary artery is located between the atria and branches immediately into the anterior interventricular artery (located in the anterior interventricular groove) and the circumflex (which continues around the back of the heart in the atrioventricular groove). The right coronary artery is located between the right atrium and right ventricle and branches into a small branch called the marginal branch (which serves the right ventricle) and the posterior interventricular artery (which continues onto the posterior aspect of the heart in the posterior interventricular groove). The great cardiac vein drains the anterior surface of the heart and the middle cardiac vein drains the posterior surface of the heart. Both of these major veins drain into the coronary sinus. The coronary sinus drains the blood back into the right atrium of the heart.

12. Explain the effect of the heart contracting and its ability to receive oxygen and nutrients. What effect does a fast heart rate have on the ability of the heart to receive blood for the myocardium? What implications does this have for an individual who is a potential heart attack victim? Whenever the heart is contracting it compresses the coronary vessels, temporarily stopping the blood supply to the heart. The time that the heart contracts is relatively constant (0.3 seconds). Whenever heart rate increases, interruption of blood flow to the heart tissue becomes more frequent. For example, when an individual’s heart rate goes from 60 to 80 beats per minute, then the blood supply is occluded an additional 20 times per minute. The more time the heart is contracted, the more the vessels are closed off, and the less blood is delivered to the heart tissue. This can reach a point where the blood supply is insufficient to maintain the tissue. This is especially true if the ability to deliver blood is impeded by partially occluded vessels due to atherosclerosis.

13. Define ischemia and infarction and explain the significance of each. Ischemia is defined as a reduced blood supply. It is a warning sign of vessel blockage, impaired blood delivery to tissue, and potential problems in future. Infarction is tissue death due to an inadequate blood supply. Cardiac tissue can not replicate, so when it dies it is replaced with scar tissue. This decreases the ability of the heart to pump blood through the body, resulting in a decreased ability to deliver oxygen, shortness of breath, and fatigue. If too much heart tissue dies, the heart is unable to supply an adequate amount of blood to the body and the individual dies from the heart attack.

14. In order for heart tissue to contract, three major steps must occur: 1. Stimulation of nodes and the spread of the electrical stimulus; 2. stimulation of the muscle cell membrane; and 3. contraction of cardiac muscle proteins. Understand what happens at each of these stages. Stimulation of the nodes: The nodes of the heart (the sinoatrial or SA node and the atrioventricular or AV node) are specialized, noncontractile, nerve-like tissues that are self-exciting (they do not need an outside stimulus to be excited, such as a neuron). The most important node is the sinoatrial node, which is considered to be the heart’s pacemaker. There are a number of conditions that allow self-stimulation to occur: 1. Sodium and calcium are found outside these cells; potassium inside these cells. 2. The resting membrane potential is approximately -90 mV 3. The cells that make-up the nodes contain K+ leak channels that allow K+ to leak out and Na+ leak channels that allow Na+ to leak in. 4. The K+ leak channels are slow, and as a result Na+ leaks in faster than K+ leaks out (more positive in than positive out). The inside becomes more and more positive until it hits the threshold. 5. When it hits the threshold, calcium voltage-gated channels open and calcium rushes in causing depolarization of these cells which initiates the stimulation. Spread of the electrical stimulus: These nodes are connected to “strands” of modified nerve-like cardiac cells, aligned in parallel, that provide a “highway” for conducting the impulse through the heart. These “strands”, as well as gap junctions, allows the passage of the electrical impulse across the atria, stimulating the atria to contract. The electrical impulse is delayed at the AV node before traveling down the Bundle of His. The Bundle of His forms right and left branches and becomes Purkinje fibers. The Purkinje fibers stimulate the ventricles from the bottom, and this stimulation is spread from the bottom to the top of the ventricle via gap junctions. The ventricles contract in sync.

Stimulation of the muscle cell membrane and contraction of the myofibrils: The sarcolemma of the cardiac muscle cells is stimulated and the impulse spreads across the sarcolemma. It spreads inside the cardiac muscle cell, via the t-tubules, to stimulate the SR to release calcium. Calcium binds to the troponin, and the troponin pulls back the tropomysin, exposing the active sites of the actin. The myosin heads attach to the active sites of the actin, pulling the actin past it, and the sarcomere shortens.

15. Heart tissue has the intrinsic ability to stimulate the cardiac muscle to contract (that’s why if you take the heart out of the body it will continue to beat). This stimulation is initiated in specialized heart cells collectively called nodes. Define the general structure of the nodes and know their locations. The SA node is located in the superior right atrium near the entry point of the superior vena cavae. The AV node is located at the base of the right atrium at the right posterior portion of the interatrial septum.

(see question 14 above for their structure)

16. Be familiar with factors that can influence the rate of stimulation of the heart. Although the nodes establish an intrinsic beat of the heart (rate of 100 beats per minute), this rate can, and is, modified by controls outside of the heart.

Neural stimulation: Sympathetic nervous system - increases the rate Parasympathetic nervous system - decreases the rate (at rest the PNS slows the rate to the typical resting rate of approximately 60-70 bpm) Hormones: Epinephrine, norepinephrine, and thyroid - increase heart rate Ions: Increased K+ or Increased Na+ - decrease heart rate Increased Ca2+ - increases heart rate Others: Age - newborns have a higher heart rate – approx. 120 bpm Gender - females generally have a smaller heart and higher heart rate Physical fitness - heart rate slows with training Increased body temperature - increases heart rate

17. Conduction fibers are connected to the nodes to assist in transmitting the electrical stimulation through the heart. Describe these fibers and know their locations.

See question 14 above

18. Trace the sequence of electrical stimuli in the heart. Note that the atria are stimulated before the ventricles. What is the reason for this?

See question 14 above

20. Explain what occurs at each step involved in electrical stimulation of cardiac muscle and explain its purpose. Depolarization - the graph is rising, depicting a change in electrical value inside the cardiac cells from -90 mV to +30 mV. This is due to voltage-gated Na+ channels being opened and Na+ coming down its concentration gradient into the cardiac muscle cells. Early repolarization - the graph line begins to drop, depicting a change in electrical value inside the cardiac cells from + 30 mV. This is due to voltage-gated K+ channels opening and K+ coming down its concentration gradient out of the cardiac cells. Plateau - the graph line levels off, depicting no change in the electrical value. This is due to K+ channels remaining open (K+ comes out of cell) and Ca2+ voltage-gated channels opening and Ca2+ coming into the cell. Repolarization - the graph line decreases back to the original value of -90 mV. This is due to K+ channels remaining open and Ca2+ channels closing.

Draw graph here from textbook:

21. Although similar to what occurs with skeletal muscle, the electrical activity of cardiac muscle has one significant difference. Describe this difference and explain its significance. Refractory Period The refractory period of cardiac muscle is significant because it prevents the heart from seizing up. If the heart were to seize up, clenched fist, it could no longer pump blood. Refractory means unresponsive or stubborn. In physiology it refers to the period of time when a muscle or nerve cell is unresponsive to stimulation. There are two refractory periods: during the absolute refractory period a cell will not respond regardless of how strong the stimulus; during the relative refractory period a cell will respond if the stimulus is supra threshold (stronger than a threshold stimulus). Cardiac muscle vs. Skeletal muscle The absolute refractory period for muscle cells lasts approximately as long as the muscle action potential. As soon as the muscle action potential is completed, a supra threshold stimulus can trigger another action potential, which causes a muscle response (contraction). One significant difference between cardiac muscle cells and skeletal muscle cells is the length of their action potentials, and therefore the length of their absolute refractory periods. Cardiac muscle action potentials last about 25 milliseconds (0.25 second). The time for contraction and relaxation of cardiac muscle is about 300 milliseconds. The cardiac muscle action potential is almost as long as its period of contraction and relaxation. This means that the cardiac muscle cell cannot start to contract a second time until it has nearly completed its relaxation period. This is extremely important, because it is during the period of ventricular relaxation that the heat fills with blood. If it doesn’t have time to properly fill, it will have less blood to pump, decreasing the cardiac output. Skeletal muscle action potentials last only 1 millisecond. The time for relaxation and contraction of a skeletal muscle cell is about 150 milliseconds. This means that a skeletal muscle cell can be stimulated to contract again before it has had time to relax. If the frequency of stimuli is great enough the skeletal muscle cell will go into a state of sustained contraction (tetanus), as when a weight is being held.

22. Included is a review of the contraction of muscle cells. Refamiliarize yourself with these events. 1. sarcolemma and T-tubule spread impulse inside 2. stimulates SR to release calcium (calcium also in extracellular space) 3. calcium binds to troponin 4. troponin pulls back tropomysin 5. active site of actin exposed and myosin heads attach 6. contraction occurs 23. An ECG (EKG) is a graph of the electrical activity of the heart. It is a powerful tool in being able to detect heart abnormalities such as a myocardial infarction, an enlarged atrium or ventricle, or malfunctioning nodes. Be able to draw a representative ECG, label the components, and explain what is occurring at each wave. P wave - atrial depolarization - cardiac cells of the atria are depolarizing and Na+ is rushing in QRS wave - ventricular depolarization* - cardiac cells of the ventricles are depolarizing T wave - ventricular repolarization - cardiac cells of the ventricles are repolarizing and K+ is rushing out.

*atrial repolarization - (cardiac cells of the atria are repolarized) - K+ rushing out still occurs. However, this electrical event occurs at the same time as the ventricular repolarization. Because it is a smaller electrical event, it is masked by the QRS wave.

24. Earlier we discussed the general flow pattern through the heart. In order to understand the physiological changes that are occurring this process can be divided into stages called the cardiac cycle. Be able to describe the pressure changes, whether the atria and ventricles are relaxed or contracted, and whether or not the atrioventricular and semilunar valves are opened or closed during these different stages. The cardiac cycle is the term applied to the events that occur in the heart as the blood is moved through it due to pressure differences. Although a continuous process, it can be divided into four major stages. In each stage, be able to recognize pressure differences, the contracted state of the chambers (atria and ventricles), and the position of the valves (AV and semilunar).

Beginning - Blood is returning from the body (via the superior and inferior vena cavae and from the heart via the coronary sinus). The atria and the ventricles are both relaxed and movement of the blood from the atrium into the ventricle keeps the AV valve open. There is no force on the semilunar valve, so it remains closed.

Atrial contraction - As the atrium contracts it pumps the remainder of blood from the atrium into the ventricle. There are no other changes.

Ventricular contraction - The atria relaxes and the ventricles contract, increasing the pressure within the ventricle. The result is two-fold. The AV valve closes (because the pressure in the ventricle is greater than the pressure in the atrium), and the semilunar valve opens (because the pressure in the ventricle is greater than the pressure in the vessel). This forces the blood to move from the ventricle into the vessel.

Ventricular relaxation – The ventricles return to their beginning positions. The ventricles relax, causing the pressure inside of them to decrease. The blood in the vessels falls back and is caught by the semilunar valves and they close. The blood that was returning from the body builds up in the atria forcing the AV valves open.

25. One of the best measurements of a heart’s effectiveness is its cardiac output. Define cardiac output and define the two variables that influence it. Cardiac output is the amount of blood ejected from the left ventricle (or right ventricle) into the aorta (pulmonary) during a given period of time.

Cardiac output is determined by stroke volume (volume per stroke) and heart rate (number of beats per minute). SV x HR = CO 26. Stroke volume is directly related to cardiac output. If stroke volume is increased, cardiac output is increased. There are a number of variables that will influence stroke volume: end-diastolic volume, neural stimulation, hormonal stimulation, and afterload (opposition to blood being pumped out of the heart). Define end-diastolic volume and describe its effect on stroke volume.

End-diastolic volume is the volume of blood remaining in the ventricle at rest (right before contraction). The end-diastolic volume is determined by the amount of time the heart has had to fill (the slower the heart rate, the longer the time to fill) and the pressure of the veins that is forcing the blood into the heart (a factor of breathing and movement - the faster the breathing and the greater the amount of movement of the body, the greater the pressure on the veins to return blood into the heart). The end-diastolic volume influences stroke volume due to the Frank- Starling Law, which states that the greater the stretch of the cardiac muscle (due to the greater amount of blood in the chamber), the greater the force of contraction on the blood to move it out of the heart. This is analogous to a stretched rubber band. The more it is stretched, the greater the force by which it will snap back. The force of contraction (and thus stroke volume) can also be increased independent of end-diastolic volume. This can occur by the sympathetic nervous system releasing norepinephrine and the adrenal medulla (which is stimulated by the SNS) releasing epinephrine.

27. Define afterload and explain how it effects stroke volume. Stroke volume is opposed by the pressure in the vessel that it is pumping into (aorta or pulmonary trunk). Afterload is defined as the pressure that must be exceeded before ejection of blood from the ventricles can begin. Factors that can altar afterload include atherosclerosis and high blood pressure. These result in a decrease in stroke volume (if heart rate did not compensate, cardiac output would decrease).

28. The second major factor affecting cardiac output is heart rate. Both neural stimulation and chemical stimulation influence heart rate. Explain how neural stimulation influences heart rate. Include the receptors (and the stimuli they detect) that will alter heart rate, the segment of the brain that will receive the information (control center), and the effect of sympathetic and parasympathetic stimulation (effectors). Although the heart has an intrinsic heart rate established by the SA node of the heart, this rate is regulated through neural stimulation by both sympathetic and parasympathetic controls.

At rest At rest, the cardiac inhibitory center (located in the medulla) sends neural impulses via the vagus nerve (a parasympathetic cranial nerve) to the heart, slowing the heart rate down from its intrinsic rate of 100 beats/min to approximately 60-70 beats/minute.

Conditions that will result in increasing the heart rate: There are times when the body needs to increase its rate of circulation. It can do so by increasing the heart rate. The detection of this increased need can come from two different sources: stimulation of the baroreceptors in the atrium and the proprioceptors in the joints.

Baroreceptors located in the atrium detect an increase in stretch (these baroreceptors are located before the pump), indicating increased blood moving into the heart. In order for the heart to pump this increased load it must increase its rate of pumping. When the right atrium is stimulated, this sensory information is sent to the cardiac acceleratory center in the medulla. The medulla increases the sympathetic output to the heart, and the heart rate increases (as well as the force of contraction).

Proprioceptors are stimulated when the joints are moved in the body. If there is increased movement, the oxygen demands of the body are increased, and an increased blood supply is needed. Stimulation of the proprioceptors results in sensory input being sent to the cardiac acceleratory center in the medulla. The medulla increases the sympathetic output to the heart, and the heart rate increases (as well as the force of contraction). Conditions that will result in decreasing the heart rate: The detection of the need to slow the heart rate down comes from stimulation of the baroreceptors in the aorta and carotid (these baroreceptors are located after the pump). If they are stretching it indicates that the blood is being moved from the heart at a greater rate, potentially damaging these vessels. When the baroreceptors in the aorta or carotid are stimulated, this sensory information is sent to the cardiac inhibitory center in the medulla. The medulla decreases the sympathetic output and increases the parasympathetic output to the heart, and heart rate decreases (as well as the force of contraction).

29. Different chemicals also influence heart rate. List the different chemicals that alter heart rate and describe their effects. Include hormones and ions. Hormones - epinephrine, norepinephrine, and thyroid - increase heart rate Ions - Increased K+ or Increased Na+ - decrease heart rate

30. Exercise has a proven effect on longevity. One of the reasons for this is the change that occurs to the cardiovascular system. Exercise alters all three of the components of the cardiovascular system including blood composition, blood vessels, and the heart. Describe the effects of exercise on the cardiovascular system for each of these components.

Blood composition - A sustained exercise program increases the level of high-density lipoproteins (HDL). These structures remove excess lipids from the bloodstream and return them to the liver.

Blood vessels - A sustained aerobic exercise program stimulates the growth of new capillaries, especially to skeletal muscle and cardiac muscle. As a result, more blood can be delivered in any given period of time, resulting in more oxygen being delivered and more aerobic production of energy. This allows for longer periods of sustained exercise that can be performed.

Heart - A sustained aerobic exercise program causes hypertrophy of the heart (the heart becomes a bigger and stronger muscle) and as a result the stroke volume increases. Because cardiac output is dependent on stroke volume and heart rate, and the stroke volume has increased, in order to maintain the same cardiac output heart rate decreases. The advantage to a slower heart rate is that it does not have to work as hard and the blood supply to the heart is occluded less often.