Anatomy Of The Heart & Cardiac Cycle

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  1. Anatomy
    1. Heart - The CENTRAL component
      1. Anatomy of the heart -
        1. Cardiac skeleton -
          1. a tough sheet of fibrous CT separating the atria from the ventricles
          2. contains all of the valves
          3. serves as point of attachment for the cardiac muscle fibers
          4. serves to electrically isolate the atria from the ventricles
        2. Layers of the heart -
          1. endocardium - inner layer of simple squamous epithelium
            1. continuous with endothelium lining all blood vessels
          2. myocardium - middle layer = composed of striated cardiac muscle tissue
          3. epicardium - outer layer of tough fibrous CT
          4. the heart is contained within a tough sac called the pericardium
        3. Chambers of the heart
          1. Rt. & Lt. Atria - separated by interatrial septum
          2. Rt. & Lt. Ventricles - separated by interventricular septum
            1. wall of left ventricle thicker than right - generates higher pressures
          3. Both ventricles contain PAPILLARY MUSCLES - small mounds of muscle that connect to:
            1. Chordae tendineae, though threads of fibrous CT that serve as "guy wires" to position the A/V valve cusps.
      2. Pattern of circulation
        1. Anterior & Posterior Vena Cavae (no valves) to
          1. Anterior = Superior = Cranial, vena cava
          2. Posterior = Inferior = Caudal, vena cave
        2. Rt. Atrium via Rt. A/V valve (=tricuspid) to
        3. Rt. Ventricle via Pulmonary semilunar valve to
        4. Pulmonary Trunk to Pulmonary arteries to lungs & back via
        5. Pulmonary Veins (no valves) to
        6. left atrium via Lt. A/V valve (=bicuspid) to
        7. left ventricle via Aortic semilunar valve to
        8. Ascending Aorta to systemic circuit
      3. Two pumps operating in SERIES
        1. RIGHT SIDE: Rt. Atrium & Ventricle
          1. pumps from body to lungs
          2. called the PULMONARY CIRCUIT
          3. has lower pressures 15/5 torr (Systolic/Diastolic)
        2. LEFT SIDE: Left Atrium & Ventricle
          1. pumps from lungs to all other tissue of the body, incl. heart
          2. called the SYSTEMIC CIRCUIT
          3. has higher pressures 120/70 torr
    2. The blood vessels: the PERIPHERAL COMPONENT
      1. Arteries: carry blood away from the heart
        1. relatively high hydrostatic pressure
        2. therefore, have thick walls
      2. Arterioles: smaller, more numerous, site of much control
      3. Capillaries: walls of endothelium: site of all exchange
      4. Veins: carry blood back to the heart
        1. relatively low hydrostatic pressure
        2. therefore, thin walled
        3. serve as blood reservoir, contain ca. 60% of blood volume
    3. The Blood
      1. Comprises one component of the Extracellular Fluid
        1. the fluid compartment that is outside of the cells
        2. another component is Interstitial fluid, fluid around the cells
      2. It is a complex mixture of
        1. Plasma: 55% of blood volume
          1. ionic similar composition to Extracellular fluid
            1. low K+: 4 mM/L
            2. high Na+: 144 mM/L
          2. also contains many proteins, not found in Interstitial fluid
        2. blood cells: 45 % of blood volume (therefore, hematocrit = 45)
          1. Leukocytes: 7x106/ml of blood
          2. erythrocytes: 5x109/ml of blood
  2. Cardiac Cycle
    1. The numbers in this description correspond with those on the graph of the various events occurring during the cardiac cycle.
      1. The ventricle is filling with blood.
      2. The pressure is rising in both the atrium and ventricle, but the pressure is higher in the atrium. The pressure is rising because the muscle is being stretched as the volume of blood rises.
      3. The blood pressure in the aorta is dropping as the volume of blood it contains drops because the blood is flowing out through the "vascular tree".
      4. The "p" wave of the EKG is caused by a wave of depolarization passing over the atrium. This signal originated in the S.A. Node. Depolarization is the signal that initiated contraction by the atrial myocardium.
      5. An increase in the blood pressure in the atrium results from the contraction of the atrial myocardium.
      6. A slight increase in the rate of filling of the ventricle results from the contraction of the atrium. This "topping off" by the atrium accounts for as much as 30-40% of the final end diastolic volume at resting heart rate but much less at high heart rates.
      7. The "QRS complex" of the EKG signals depolarization of the ventricle and the beginning of ventricular contraction.
      8. Blood pressure in the ventricle rises rapidly due to ventricular contraction. This causes ventricular pressure to rise above atrial pressure.
      9. The first heart sound "lub" is associated with closing of the mitral (left atrioventricular) valve. Closing of the valve is caused by the reversal of the pressure gradient between the atrium and the ventricle.
      10. The blood pressure in the ventricle continues to rise due to contraction of the ventricular myocardium.
      11. No blood is ejected from the ventricle until the pressure in the ventricle rises above the pressure in the aorta. This is known as the phase of isovolumetric (or isometric) contraction; it lasts about 0.07 seconds.
      12. Aortic pressure reaches it minimum (about 70 mm Hg), the value reported as Diastolic blood pressure (notice that it occurs during Systole).
      13. Ventricular pressure exceeds aortic pressure, forcing the aortic semi-lunar valve open.
      14. The volume of blood in the ventricle drops as blood is ejected into the aorta. The first 0.1 seconds of the ejection phase account for 80-85% of the total stroke volume. This is known as the rapid ejection phase of ventricular emptying.
      15. The blood pressure in the ventricle and aorta rise due to the continued contraction of the ventricular myocardium and the rapid increase in the volume of blood in the aorta. This distends the aorta.
      16. Eventually, the force exerted by the ventricle reaches a maximum and aortic pressure peaks at what is reported as Systolic pressure (averaging about 120 mm Hg in a healthy resting adult).
      17. Ventricular pressure drops below aortic pressure due to the relaxation of the ventricular myocardium.
      18. The second heart sound ("dub") is associated with closing of the aortic semilunar valve.
      19. The "T" wave of the EKG occurs has started slightly before this time, produced by repolarization of the ventricular myocardium.
      20. Blood pressure in the ventricle drops due to relaxation of the ventricular myocardium, but ventricular pressure is still higher than atrial pressure.
      21. There is no change in the volume of blood in the ventricle because the mitral valve is still closed; this is the phase known as isovolumetric (or isometric) relaxation, and it lasts about 0.07 seconds.
      22. Ventricular pressure drops below atrial pressure and the mitral valve opens.
      23. Blood begins to flow from the atrium into the ventricle. Pressure. Ventricular pressure continues to drop because the myocardium is relaxing faster than the ventricle is filled by blood from the atrium. The first 0.06 seconds after the valve opens constitute the Rapid Ventricular filling phase. At the end of this phase, ventricular pressure begins to rise as the ventricle starts becoming distended. A large fraction of ventricular filling occurs during the rapid filling phase. This is particularly important at high heart rates.

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