• Heart is composed of
• -3 major types of cardiac muslce
• 1. atrial muscle
• 2. ventricular muscle
• 3. excitatory and conductive muscle fibers(more of conductive function)
Cardiac muscle as a syncytium
• -presence of intercalated disc-
• --cell membrane separating individual muscle fibers
• ---offers easy diffusion of ions resulting in rapid spread of action potential to cardiac muscles---gap junction
• 2 syncitiums: atrial and ventricular
Action potential in cardiac muscle
• Resting membrane potential of
– -85 to -95mv in ventricular muscle
– -90 to -100mv in conductive fibers
– Shows spike potential followed by a plateau
– --due to fast channel and slow channel
• Contraction of cardiac muscle
• -excitation contraction coupling mechanism
• Extra calcium from T tubules (not only form SR)
• Regulation of heart pumping
• -at rest –pumps only 4 to 6 liters/min
• -severe exercise – pumps 4 to 7 times
• Regulation by
• 1. Intrinsic cardiac regulation
• 2. autonomic nervous system.
• Intrinsic regulation
• Frank-Starling mechanism
• --the more venous return the more blood is pump out to the systemic circulation
• --extra amount of blood in the ventricles stretch the muscle to its optimum interdigitation
• Sympathetic and parasympathetic regulation
• -sympathetic –excites the heart
• -parasympathetic(vagal) stimulation –depressed
• The normal electrocardiogram
• - reflection of cardiac impulse occuring thru the heart
• -electrical currents spread into the tissues surrounding the heart and some to the surface of the body
• Depolarization and repolarization waves
• --P wave is caused by electrical potentials generated as the atria depolarizes
• --QRS complex is caused by ventricular depolarization
• --T wave is caused by potentials generated as the ventricles recover from the state of depolarization(repolarization)
• Cardiac arrhythmias
• Due to
• 1. abnormal rhythmicity of the pacemaker
• 2. shift of the pacemaker from the sinus node to other parts of the heart
• 3. blocks at different points in the transmission of impulse through the heart
• 4. abnormal pathways of impulse transmission through the heart.
• 5. spontaneous generation of abnormal impulses in almost any part of the heart
Abnormal rhythms
• Tachycardia- faster than 100 beats/min
• --increased body temperature
• --stimulation of the heart by sympathetic nerves
• Bradycardia – less than 60 beats/min
• ---athletes
• --vagal stimulation
Abnormal rhythm resulting from impulse conduction block
• -atrioventricular block
• ----conditions causing decrease or total block of impulse conduction in the bundle of his
Premature contraction
• -contraction of the heart prior to the time that normal contraction would have been expected.
• ---caused by ectopic beat
• --premature atrial or ventricular contraction
Most serious of all cardiac arrhythmias
• -ventricular fibrillation--fatal
• --impulse have gone berserk resulting in uncoordinated, asynchronous and ineffective heart muscle contraction
• consequence--pumping action is impaired
• Causes
• -sudden electrical shock
• -ischemia of the heart
Electroshock of the ventricles
• -strong electrical current passed through the ventricles can stop fibrillation by throwing all ventricular muscle to refractory stage …all impulse stopped
• -after which the heart will begin to beat again from the SA node
Muscle blood flow and cardiac output during exercise
• Very strenuous exercise is the most stressful condition that the normal circulatory system faces
• -increase of more than 20 fold in blood flow to the muscle
• -1 liter/min to as great as 20 liters/min
• - increase in cardiac output to 5-6 times normal
Intermittent blood flow during muscle contraction
• -blood flow increases and decreases during contraction- relaxation and contraction phases
• -opening of muscle capillaries during exercise—during rest only 20-25% of muscle capillaries have flowing blood
• during strenuous exercise all capillaries open up—faster diffusion of oxygen from the capillaries to the muscle fiber
Control of blood flow through the skeletal muscles
1. Local effects in the muscles acting directly on the arterioles to cause vasodilatation
• -decrease in oxygen concentration
• -release of vasodilator substances
• -potassium ions,
• acetylcholine,
• adenosine triphosphate,
• lactic acid, and carbon dioxide
2.nervous control
• -presence of sympathetic vasoconstrictor nerves
• -release of norepinephrine
• -additional secretion of norepiniphrine and epinephrine from adrenal medullae
Circulatory readjustments during exercise
• 1. mass discharge of the sympathetic nervous system
• 2.increase in arterial pressure
• 3. increase cardiac output
The coronary circulation
• 1/3 of all deaths in the affluent society result from coronary artery disease
• Left coronary artery-
• --anterior and lateral portions of the left ventricle
• Right coronary artery-
• --most of the right ventricle and post. Part of left ventricle
Ischemic heart disease
• -atherosclerosis
• -most common site- first few centimeters of the coronary arteries
• -myocardial infarction
• --follows immediately after acute occlusion
• --area of muscle that has low or zero flow cant sustain muscle contraction
• Ischemic heart diseases
• -single most common cause of death in western culture
• -may cause sudden death due to acute coronary occlusion
• -or slow death over periods of weeks to years as a result of progressive weakening of heart pumping process
Causes of death following acute coronary occlusion
• 1. decrease cardiac output
• 2. damming of blood in the pulmonary circulation—resulting in edema
• 3.fibrillation of the heart
• 4. rupture of the heart
• Angina pectoris
• -cause is unknown but could be due to lactic acid, histamine kinins or proteolytic enzymes(not removed due to lost of circulation)
• Pain is felt in the chest, sternum, left arm and shoulder
• --trigger by exercise or emotional experience
Treatment for angina pectoris
• --vasodilators
• --beta blockers-blocks beta receptors which then prevents sympathetic stimulation of the heart that occurs during exercise and emotional episodes
• --surgical treatment
• ---aortic-coronary by-pass
• ---coronary angioplasty
Peripheral circulation
• The function of circulation is
• - to service the needs of the tissues
• -transport nutrients
• -waste products away,
• -to conduct hormones
• -maintain appropriate environment in all tissue fluids(maintain the internal ‘mellieu’)
Functional parts of circulation
• Arteries- transport blood under high pressure
• Arterioles – last, smallest branch of the arterial system, act as control valves through which blood is released to the capillaries
• Capillaries- for exchange of fluid –thus very thin and permeable to small molecular substances
• Venules- collects blood from capillaries
• -- reservoir of blood- low pressure and thin walls- keeps 84% of blood volume
Microcirculation and lymphatic system
• Where most purposeful function of he circulation occurs,
• -transport of nutrients to the tissues
• -removal of cellular excreta
Structure of the microcirculation
• Each nutrient artery entering an organ branches six to eight times before the arteries become small enough to be called arterioles
• Arterioles branch 2 to 5 times- capillaries
• Diameter of arterioles – 20 micrometer
• Diameter of capillaries-5 -9 micrometer
Pores in the capillary membrane
• - intercellular cleft- thin slit between endothelial cells in the capillaries
• - plasmalemmal vesicles- transport substance across endothelial cells
Flow of blood in the capillaries
• vasomotion
• -intermittent flow due to metaarteriole and precapillary sphincters –regulated by oxygen concentration
Exchange of nutrients between blood and interstitial fluid
• Diffusion
• -diffusion of lipid-soluble substances through the capillary membrane
• -diffusion of water-soluble substances through the capillary membrane
• Effect of molecular size on passage through the pores
• Effect of concentration difference on net rate of diffusion through the capillary membrane
The interstitium and the interstitial fluid
• -spaces between cells
• -collagen fibers and proteoglycan filamints
• - gel in the interstitium
• - rivulets
Distribution of fluid volume between the plasma and the interstitial fluid
• Four primary forces that determine fluid movement through the capillary membrane
• 1. capillary pressure – force fluid outward
• 2. Interstitial fluid pressure- force fluid inward
• 3.plasma colloid osmotic pressure- forced fluid inward
• 4. interstitial fluid colloid osmotic pressure -forces fluid outward
The lymphatic system
• Accessory route
• Carry proteins and large particulate matter
• Removal of protein from the interstitial space is essential without it can result in death
• Lymphatic channel of the body
• Lymph from the lower part of the body flows up the thoracic duct and empties into the venous system at the juncture of the left internal jugular vein and subclavian vein
• Lymph form left side of the head the left arm and parts of the chest region also enters the thoracic duct
• Lymph from the right side of the neck and head, from right arm and thorax- right subclavian vein
• Most fluid filtering from the arterial capillaries flows among the cells and is finally reabsorbed back into the venous ends of microcirculation.
• One tenth of the fluid enters the lymphatic capillaries
• High molecular weights like protein cant be reabsorbed into the venous capillaries but thru lymphatic capillaries
Structure of the lymphatic capillaries
• - overlapping endothelial cell provide valve for passage.
Cardiac failure
• -decrease ability of heart to pump blood
• --causes
• --decrease contractility of the heart
• --decrease coronary blood flow
• --damage to the heart valves
• --external pressure
Acute effects of moderate cardiac failure
• --reduced cardiac output
• --damming of blood in the veins- increased in systemic venous pressure
• -from 5 liters/min to less than 2/min
• At <2/min- can sustain life but associated with fainting
• Acute cardiac failure is compensated by sympathetic reflexes
• --baroreceptor reflex
• --chemoreceptor reflex
• --cns ischemic response
• Response
• --strengthens heart contraction
• --increase venous return by increasing vasomotor tone in the veins
• Sympathetic reflex is good for acute phase only
Chronic stage heart failure
• -characterized by
• 1. retention of fluid by the kidneys
• 2. progressive recovery of the heart
• In general renal output remains reduced as long as cardiac output is significantly less than normal
• Beneficial effects of moderate fluid retention in cardiac failure
• --increase tendency for venous return
• ---- increase in mean systemic pressure
• -----distends the veins reduces resistance to venous flow
Decompensated heart failure
• --if severely damaged –cannot be compensated
• --continue to retain fluid – severe edema and eventually lead to death
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