Cardiovascular/Heart (ch.18) Pt 3

BIOL 234 Anatomy and Physiology Chapter 18 Cardiovascular System – The Heart

Notes (Part 3) - 18.6 Mechanical Events of Heart

SYSTOLE: period of HEART CONTRACTION

DIASTOLE: period of HEART RELAXATION

CARDIAC CYCLE: blood flow through heart during one complete heartbeat

• ATRIAL systole and diastole are followed by VENTRICULAR systole and diastole

• Cycle represents series of PRESSURE and BLOOD VOLUME changes

• Mechanical events follow electrical events seen on ECG

Four phases of the cardiac cycle - Following left side, starting with total relaxation

1. Ventricular filling: mid-to-late diastole

• Pressure is LOW; 80% of blood PASSIVELY flows from atria through open AV valves into ventricles from atria

• SL valves are CLOSED

• Atrial depolarization triggers atrial systole (P WAVE), atria contract, pushing remaining 20% of blood into ventricle

• END DIASTOLIC VOLUME (EDV): volume of blood in each ventricle at end of VENTRICULAR DIASTOLE

• Depolarization spreads to ventricles (QRS WAVE)

• Atria finish contracting and return to diastole while ventricles begin systole

2. Isovolumetric contraction

• Atrial Diastole (RELAX) ventricles begin to contract (SYSTOLE)

• Rising ventricular pressure causes closing of AV VALVES

  • FIRST HEART SOUND → “lub sound”

• Isovolumetric contraction phase is split-second period when ventricles are completely closed (ALL VALVES CLOSED)

• Volume remains CONSTANT

• Ventricles continue to contract

  • Pressure INCREASES

3. Phase 3 – Ventricular Ejection

• When ventricular pressure exceeds pressure in large arteries, SEMILUNAR VALVES are forced open

• AV valves remain CLOSED

• Pressure in aorta reaches about 120 mm Hg

• Amount of blood ejected is STROKE VOLUME

• END SYSTOLIC VOLUME (ESV): volume of blood remaining in each VENTRICLE after SYSTOLE
  
  

4. Isovolumetric relaxation

   1. Following ventricular repolarization (T WAVE) ventricles relax (DIASTOLE)

   2. Ventricular pressure drops causing backflow of blood from aorta and pulmonary trunk that triggers closing of SEMILUNAR VALVES

      1. SECOND HEART SOUND → “dub sound”

   3. AV valves are still closed

   4. Blood volume is constant

• Ventricles are completely closed chambers momentarily

• ATRIA receive blood from venous circuit

Mechanical Events of Heart

• Closure of aortic valve raises aortic pressure as backflow rebounds off closed valve cusps

  • Referred to as DICROTIC NOTCH

• Atria continue to fill during ventricular systole and when atrial pressure exceeds ventricular pressure, AV valves open; cycle begins again

  • Heart beats around 75 times per minute

  • Cardiac cycle lasts about 0.8 seconds:

    • Atrial systole lasts about 0.1 seconds

    • Ventricular systole lasts about 0.3 seconds

  • Quiescent period is COMPLETE heart relaxation that lasts about 0.4 seconds

• Blood moves through the heart by PRESSURE CHANGES

• Blood flows down a PRESSURE GRADIENT and moves through available openings

• Right and left side of the heart is the same EXCEPT for PRESSURE

  • Pulmonary artery = 24 /10 mm Hg

  • Aorta = 120 / 80 mm Hg