CHAPTER 14.docx

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

CHAPTER 14 CARDIOVASCULAR PHYSIOLOGY CARDIAC MUSCLE AND THE HEART Heart: Is a muscle that contracts continually resting only in the milliseconds-long pause between beats Requires continuous supply of nutrients and oxygen. THE HEART HAS 4 CHAMBERS Is a muscular organ About the size of a fist BASE Lies in the center of the thoracic cavity. Is an inverted cone. Apex: is pointed. Heart angles to the left side of the body. Base: is broad. Lies just behind the breastbone or sternum. *base is on the top and the apex is at the bottom Apex With in the thoracic cavity, the heart lies on the ventral side sandwiched between the two lungs The apex rests on the diaphragm. The heart is encased in a tough membranous sac: pericardium A thin layer of clear pericardial fluid inside the pericardium lubricates the external surface of the heart as it beats with in the sac. Inflammation of the pericardium may reduce this lubrication to the point that the heart rubs against the pericardium creating a sound known as friction rub. Heart is mostly composed of cardiac muscle or myocardium Is covered by thin outer and inner layers of epithelium and connective tissue. The bulk of the heart is the thick muscular walls of the ventricles, the two lower chambers The thinner-walled atria lie above the ventricles. The major blood vessels all emerge from the base of the heart The aorta and the pulmonary trunk direct blood away from the heart to tissues and lungs respectively The venae cava and the pulmonary veins return blood to heart Anterior view: the pulmonary veins are hidden behind the other major blood vessels. Shallow grooves run across the surface of the ventricles. - The shallow grooves contain the coronary arteries and coronary vein which supply blood to the heart muscle. Cross-Sectional view: The left and right sides of the heart are separated by the interventricular septum so that blood on one side doesnot mix with the blood on the other side, Even though the blood flow is separate, the two sides contract in a coordinated fashion, First the atria contract together, then the ventricles contract together. Blood flows from veins in to the atria and through one-way valves, into the ventricles The ventricles are the pumping chambers Blood leaves via the pulmonary trunk from the right ventricle and via the aorta from the left ventricle. A second set of valves guard the exits of the ventricles so that blood cannot flow back into the heart once it has been ejected. The blood enters each ventricle at the top of the chamber but also leaves at the top. -this is because during development, the tubular embryonic heart twists back on it self. This twisting results in arteries close to the top of the ventricles.--> functionally, this mean that the ventricle smust contract from the bottom up so that blood is squeezed out of the top. Bottom-to-top contraction. Four fibrous connective tissue rings surround the 4 heart valves, an arrangement that pulls the apex and the base of the heart together when the ventricles contract. Also, the fibrous connective tissue acts as the electrical insulator, blocking most of the transmission of electrical signals between the atria and the ventricles. Heart valves ensure one-way flow in the heart Blood flows through the heart in one direction Two sets if heart valves ensure this one-way flow 1) Atroventricular valves: Between the atria and the ventricles 2) Semilunar valves between the ventricles and the arteries The two sets of valves vary in structure but they have the same function: preventing the backward flow of blood The AV valevs is formed from thin flaps of tissue joined at the base to a connective tissue ring. The flaps are slightly thickened at the edge and connect on the ventricular side to the collagenous tendons: the chordate tendineae Most of the chordate fasten to the edges of the valve flaps The opposite ends of the chordate are tethered to moundlike extensions of ventricular muscles known as the Papillary muscles These muscles provide stability for the chordae but they cannot actively open and close the AV valevs. The valves move passively when flowing blood pushes on them. When a valve contracts, blood pushes against the bottom side of its AV valve and forces it upward into a closed position. The two AV valves are not identical. The valve that seperates the right atrium and the right ventricle has three flaps and is called the tricuspid valve. R-S-T- right side has tricuspid The valve between the left atrium and the left ventricle has only two flaps and is called the bicuspid valve. The bicuspid valve is also called the mitral valve. The semilunar valves separate the ventricles from the major arteries . The aortic valve is between the left ventricle and the aorta The pulmonary valve is between the right ventricle and the pulmonary trunk Each semilunar valve has three cuplike leaflets that snap close when blood attempts to flow back into the ventricles. Because of their shape, the semilunar valves do not need connective tissue tendon as AV valves do. CARDIAC MUSCLE CELLS CONTRACT WITHOUT INNERVATION The bulk of the heart is composed of cardiac muscle cells or myocardium. Pacemaker cells: Most cardiac muscle is contractile but about 1 % of it is are specialized to generate action potentials spontaneously. Are responsible for heart being able to contract without any outside signal. The heart can contract without a connection to other parts of the body because the signal for contraction is myogenic: originating within the heart muscle itself. The signal for myocardial contraction comes not from the nervous system but from specialized myocardial cells called autorhythmic cells The autorhythmic cells are also called pacemakers because they set the rate of the heartbeat.Myocardial autorhythmic cells are anatomically distinct from contractile cells: autorhythmic cells are smaller and contain fewer contractile fibers and because they donot have organized sacromeres, autorhythmic cells donot contribute to the contractile force of the heart. Contractile cells are typical striated muscle contractile fibers are organized into sarcomeres cardiac muscle cells differ from skeletal muscle and shares some properties with smooth muscle: 1) Cardiac muscle fibers are much smaller than skeletal muscle and usually have a single nucleus per fiber 2) Cardiac muscle cells branch and join neighboring cells end-to-end to create a complex network . the intercalated disks are cell junctions consisting of interdigitated membranes. Intercalated disks have two components: desmosomes and gap junctions. 3) Gap junctions in the intercalated disks electrically disconnect cardiac muscle cells to one anotherthey allow waves of depolarization to spread rapidly from cell to cell so that all the heart muscle cells contract almost simultaneouslyresemble smooth muscle. Desmosomes are strong connections that tie adjacent cells together allowing force created in one cell to be transferred to adjacent cell. 4) The t-tubules of myocardial cells are larger than those of skeletal muscle and they branch inside the myocardial cells. 5) Myocardial sarcoplasmic reticulum is smaller than that of skeletal muscle. 6) Mitochondria occupy about one-third the cell volume if a cardiac muscle fiber During increased activity, the heart uses almost all the oxygen brought to it by coronary arteries. Therefore, the only way to get more more oxygen to exercising heart muscle is to increase the blood flow. Reduced myocardial blood flow fro
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