Chapter 15 Cardiovascular System Describe the general structure, function, and location of the heart

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Chapter 15

Cardiovascular System

  1. Describe the general structure, function, and location of the heart.

The heart is a hollow, cone-shaped, muscular pump located in the thorax. The average adult heart is about nine centimeters long and seven centimeters wide. It functions to pump deoxygenated blood to the lungs and pump oxygenated blood to the body.

  1. Describe the pericardium.

The pericardium consists of an outer fibrous bag know as the fibrous pericardium that surrounds a more delicate, double-layered sac. The inner layer of the sac, the visceral pericardium covers the heart. At the base of the heart, the visceral layer turns back upon itself to become the parietal pericardium, which forms the inner lining of the fibrous pericardium.

  1. Compare the layers of the cardiac wall.

The epicardium (visceral pericardium) functions as a protective layer. This layer consists of connective tissue covered by epithelium. Its deeper portion often contains adipose tissue.

The myocardium is relatively thick and consists largely of cardiac muscle tissue. The muscle fibers are arranged in planes, separated by connective tissues, which are richly supplied with blood capillaries, lymph capillaries, and nerve fibers.

The endocardium consists of epithelium and connective tissue containing many elastic and collagenous fibers. The connective tissues house the Purkinje fibers, which function with the conduction system of the heart. This layer is continuous with the inner linings of the blood vessels attached to the heart.

  1. Identify and describe the locations of the chambers and valves of the heart.

The upper chambers are called atria. These have relatively thin walls and receive blood returning to the heart. The lower chambers are called ventricles. They receive blood from the atria and force blood out of the heart into the arteries.

There are two atrioventricular valves (A-V valves). The tricuspid valve is located between the right atrium and the right ventricle. The bicuspid (mitral) valve is located between the left atrium and the left ventricle. There are two semilunar valves. The pulmonary semilunar valve is found between the right ventricle and the pulmonary artery. The aortic semilunar valve is found between the left ventricle and the aorta. All four valves function to prevent backflow between the respective chambers and vessels.

  1. Describe the skeleton of the heart, and explain its function.

Rings of dense fibrous connective tissue surround the proximal ends of the pulmonary trunk and the aorta. This provides attachments from heart valves and muscle fibers. These fibrous rings, together with other masses of dense fibrous tissue in the upper portion of the interventricular septum, constitute the skeleton of the heart.

  1. Trace the path of blood through the heart.

The deoxygenated blood enters the right atrium from the superior and inferior vena cava and the coronary sinus. It passes through the tricuspid valve to the right ventricle. It passes through the pulmonary semilunar valve into the pulmonary trunk. The pulmonary trunk further divides into the right and left pulmonary arteries. These arteries carry the deoxygenated blood to the lungs. After the gas exchange is complete, the oxygenated blood is returned to the left atrium from the right and left pulmonary veins (two on each side for a total of four). It flows through the bicuspid (mitral) valve to the left ventricle. It then is pumped through the aortic semilunar valve into the aortic arch and beyond.

  1. Trace the path of blood through the coronary circulation.

The blood is supplied to the tissues of the heart by the first two branches of the aorta called the right and left coronary arteries. The openings to these vessels lie just beyond the aortic semilunar valve. It then goes through the capillary system of the heart. Branches of the cardiac veins that roughly parallel the coronary arteries drain the blood that has passed through the capillaries. These veins empty into the coronary sinus that is located on the posterior side of the heart emptying into the right atrium.

  1. Describe a cardiac cycle.

A cardiac cycle is one complete heartbeat. This consists of atrial systole, which is the atria contracting while ventricular diastole (ventricular relaxation) occurs. The second part of the cycle is when atrial diastole (atria relaxing) occurs while ventricular systole (ventricles contracting) occurs. This entire process is one complete cardiac cycle.

  1. Describe the pressure changes that occur in the atria and ventricles during a cardiac cycle.

When the atria are relaxed, the pressure begins to rise as they fill with blood. As the atria contract, the pressure rises suddenly, forcing the remaining blood into the ventricles. Pressure is low in the ventricles when they are filling but begins to rise as they fill with blood. It rises sharply as the ventricles contract forcing the blood out into the appropriate vessels.

  1. Explain the origin of heart sounds.

The first heart sound (lubb) occurs during ventricular contraction, when the tricuspid and bicuspid valves are closing. The second heart sound (dupp) occurs during ventricular relaxation, when the pulmonary and aortic semilunar valves are closing.

  1. Describe the arrangement of the cardiac muscle fibers.

Cardiac muscle fibers are arranged in branching networks to pass impulses through the heart so it contracts as a unit.

  1. Distinguish between the roles of the S-A node and the A-V node.

The S-A (sinoatrial) node is a small mass of specialized muscle tissue just beneath the epicardium. The fibers are self-exciting so they initiate impulses that spread to the myocardium and stimulate the muscle fibers to contract. The S-A node initiates one impulse seventy to eighty times per minute. Because it generates the rhythmic contractions of the heart, it is often called the pacemaker.

The A-V (atrioventricular) node is located in the inferior portion of the septum and just beneath the endocardium, providing the only normal conduction pathway between the atrial and ventricular syncytia.

  1. Explain how the cardiac conduction system controls the cardiac cycle.

Once the impulse leaves the S-A node (sinoatrial node), it passes into the atrial syncytium and the atria contract almost simultaneously. From there, the impulse travels along a system continuous with the atrial muscle fibers and into the A-V node (atrioventricular node) which is the only normal route to the ventricular syncytia. Because of the small diameter of nerve fibers from here, the impulse is slowed enough for the atria to empty and the ventricles to fill with blood. From here, the impulse passes through the A-V node and into the A-V bundle (atrioventricular bundle or bundle of His), which is comprised of larger diameter nerves. The impulse speeds up and splits into two branches (right and left) on the way to the Purkinje fibers. These fibers are even larger, and cause the impulse to move even faster. The Purkinje fibers branch into the papillary muscles and downward to the apex of the heart. There, they curve upward and branch along the entire walls of the ventricles until they become continuous with the cardiac muscle fibers. The whorl design of the myocardium causes the ventricles to contract in a "wringing" fashion that squeezes the blood out and into the arteries.

  1. Describe and explain a normal ECG pattern.

A normal ECG pattern consists of a P wave, a QRS complex, and a T wave. The P wave is caused by a depolarization of the atrial fibers just before they contract. The QRS complex is caused by the depolarization of ventricular fibers just prior to contraction of the ventricular walls. The T wave is caused by the relatively slow repolarization of the ventricular muscle fibers. The QRS complex obscures atrial repolarization.

  1. Discuss how the nervous system regulates the cardiac cycle.

When the body needs the heart to slow down, the medulla oblongata sends impulses down the parasympathetic tracts to the S-A and A-V nodes. The release of acetylcholine causes the heart to decrease in activity. The parasympathetic nerves seem to be the main control of the heart. So, if the impulses increase, the heart will slow down. If the impulses decrease, the heart will speed up.

The sympathetic nervous system also has tracts that innervate the S-A and A-V nodes, along with other areas within the atria and ventricles. These nerves secrete norepinephrine, which causes the heart to speed up and increase the force of its contractions. The cardiac center of the medulla controls the impulses from the parasympathetic and sympathetic nerves by interpreting information from the pressoreceptors (baroreceptors) in the aortic sinus and aortic arch, and the carotid sinuses in the carotid arteries.

  1. Describe two factors other than the nervous system that affect the cardiac cycle.

Temperature change and certain ions influence the cardiac cycle. If the body temperature rises, the heart rate will increase, as with a fever. If the body temperature decreases, the heart rate will slow accordingly. K+ and Ca++ are the most important ions that influence the cardiac cycle. Potassium affects the electrical potential of the cell membrane, thereby altering its ability to reach the threshold for impulse conduction. Calcium affects the cardiac muscle’s ability to contract as the sarcoplasmic reticulum stores less and relies on extracellular calcium for contraction to occur.

  1. Distinguish between an artery and an arteriole.

Arteries are strong, elastic vessels that are adapted for carrying the blood away from the heart under high pressure. An arteriole is a smaller, finer branch of an artery.

  1. Explain control of vasoconstriction and vasodilation.

Vasoconstriction occurs when the smooth muscle in the wall of the vessels is stimulated by the vasomotor fibers of the sympathetic branches of the autonomic nervous system. This causes contraction to occur, thereby decreasing the diameter of the vessel. If the vasomotor impulses are inhibited, the muscle relaxes and the diameter of the vessel increases. This is known as vasodilation.

  1. Describe the structure and function of a capillary.

Capillaries are the smallest blood vessels and connect the smallest arterioles to the smallest venules. They consist of a single layer of squamous epithelial cells that are continuous with the endothelium of the larger vessels. A capillary provides the semipermeable membranes through which substances in the blood are exchanged for substances in the tissue fluid surrounding body cells.

  1. Describe the function of the blood-brain barrier.

The function of the blood-brain barrier is to shield delicate brain tissue from toxins in the bloodstream and from biochemical fluctuations that could overwhelm the brain.

  1. Explain control of blood flow through a capillary.

This is regulated mainly by the smooth muscles that encircle the capillary entrances. These muscles form precapillary sphincters, which may close a capillary by contracting or opening it by relaxing. The control of the precapillary sphincters is not clear.

  1. Explain how diffusion functions in the exchanges of substances between blood plasma and tissue fluid.

Blood entering the capillaries of the tissues has higher concentrations of molecules and ions than in the tissue fluid itself. Diffusion is the process of moving from areas of higher concentrations to lower concentrations. The nutrients and oxygen tend to move into the tissues because the concentrations of these substances are higher in the blood. The wastes, such as carbon dioxide, are in higher concentrations in the tissues so these diffuse into the blood plasma.

  1. Explain why water and dissolved substances leave the arteriolar end of a capillary and enter the venular end.

The blood pressure at the arteriolar end is greater than in the capillary itself. This allows for filtration of substances to occur at this site. The blood pressure decreases as the blood moves through a capillary, making the outward filtration force less than the osmotic pressure at the venular end. Consequently, there is a net movement of water and dissolved materials, into the venular end of the capillary by osmosis.

  1. Describe the effect of histamine on a capillary.

Histamine will increase capillary permeability.

  1. Distinguish between a venule and a vein.

A venule is a microscopic vessel that continues from the capillaries and merges with other venules to form veins. Veins are the counterpart to the arterial system that carries blood back to the heart.

  1. Explain how veins function as blood reservoirs.

The valves prevent a backflow of blood. Their smooth muscle layer is much less developed, allowing more blood to remain in the vein. In times of hemorrhage accompanied by the drop in arterial blood pressure, the muscular walls are stimulated reflexly by sympathetic nerve impulses. The resulting venous constriction pushes out the extra blood, which raises the blood pressure.

  1. Distinguish between systolic and diastolic blood pressures.

The maximum pressure achieved during ventricular contraction is called the systolic pressure. The diastolic pressure is the lowest pressure that remains in the arteries during ventricular diastole.

  1. Name several factors that influence the blood pressure, and explain how each produces its effect.

  1. The amount of blood that enters the arterial system with each ventricular contraction. This is known as heart action.

  2. The amount of blood cells and plasma volume in the cardiovascular system, which is known as blood volume.

  3. The amount of peripheral resistance within the walls of the blood vessels.

  4. The viscosity (the ease with which molecules in a fluid slide past one another).

  1. Describe the control of blood pressure.

Two important mechanisms for blood pressure control involve the regulation of cardiac output and the peripheral resistance. Starling's law of the heart ensures that the volume of blood discharged from the heart is equal to the volume entering its chambers. Pressoreceptors trigger the neural regulation of the heart rate. Chemicals, such as epinephrine, emotions, physical exercise, and increased body temperature can also play a role in regulation of heart rate, thereby influencing the cardiac output. Peripheral resistance is regulated primarily by the changes in the diameters of arterioles. The vasomotor center of the medulla oblongata has neural control of the smooth muscle in the arteriole wall. Chemical substances, including carbon dioxide, oxygen, and hydrogen ions, also influence peripheral resistance by affecting the smooth muscle in the walls of arterioles and the actions of precapillary sphincters.

  1. List the major factors that promote the flow of venous blood.

  1. Skeletal muscle contractions

  2. Respiratory movements

  3. Venoconstriction

  1. Define central venous pressure.

The pressure that is within the right atrium of the heart is known as the central venous pressure.

  1. Distinguish between the pulmonary and systemic circuits of the cardiovascular system.

The pulmonary circuit of the cardiovascular system consists of those vessels that carry the blood from the heart to the lungs and back to the heart. The systemic circuits of the cardiovascular system are responsible for carrying the blood from the heart to all other parts of the body and back again.

  1. Trace the path of the blood through the pulmonary circuit.

The deoxygenated blood leaves the right ventricle into the pulmonary trunk, which divides into the right and left pulmonary arteries. These branches penetrate the right and the left lungs, respectively. These further divide into arterioles that continue into the capillary networks associated with the walls of the alveoli, where the gas exchanges occur between the blood and the air. From these pulmonary capillaries, the blood enters the venules that eventually merge to form veins. Four pulmonary veins, two from each lung, carry the oxygenated blood back to the left atrium.

  1. Explain why the alveoli normally do not fill with fluid.

The epithelial cells of the alveolar membrane are tightly joined so that most ions fail to enter the alveoli. This helps to maintain a relatively high osmotic pressure in the interstitial fluid. Osmosis will then move any water that gets into the alveoli back into the interstitial space. This mechanism prevents excess water from entering the alveoli and helps keep the alveoli from filling with fluid.

  1. Describe the aorta, and name its principal branches.

The aorta is the largest artery in the body extending upward from the left ventricle. It arches over the heart to the left and descends just in front of the vertebral column.

Principal branches of the ascending aorta include the aortic sinus, which gives rise to the right and left coronary arteries. The aortic arch gives rise to the brachiocephalic artery, the left common carotid artery, and the left subclavian artery. The thoracic aorta gives rise to the brachial artery, the pericardial artery, the esophageal artery, the mediastinal artery, and the posterior intercostal artery. The abdominal aorta gives rise to the celiac artery, the phrenic artery, the superior mesenteric artery, the suprarenal artery, the renal artery, the gonadal artery, the inferior mesenteric artery, the lumbar artery, the middle sacral artery, and the common iliac arteries.

  1. Describe the relationship between the major venous pathways and the major arterial pathways.

Major veins typically parallel the courses taken by major named arteries. This, with some exceptions allows the vein to be named from the major artery next to it.

  1. List and describe the changes occurring in the cariovascular system as a result of aging.

Some degreee of cholesterol deposition in blood vessels may be a normal part of aging, but accumulation is great enough to lead to overt disease.

Fibrous connective tissue and adopose tissue enlarge the heart by filling in when the number and size of cardiac muscle cells fall.

Blood pressure increases with age, while resting heart rate decreases with age.

Moderate exercise correlates to lowered risk of heart disease in older people.

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