A&P Ch 20 Part 2
Parasympathetic control of heart rate
Acetycholine slows down the heart (parasympathetic)
70% of filling of the ventricles happens passively and
30% happens actively
Preload
-Frank-Starling Law -In heart muscle, more stretching means a more powerful contraction (to a to a point): *More blood (ventricular preload) is the amount of stretch caused by blood entering the ventricles *Preload is volume of ventricles during filling
Mean Arterial Pressure (MAP)
-Average blood pressure in aorta *MAP = CO x PR -PR is total peripheral resistance against which blood must be pumped
Cardiac Output (CO)
-Cardiac Output is the volume of blood ejected from the left or right ventricles into the aorta or pulmonary trunk each minute. *CO = SV x HR *SV: Stroke volume (blood pumped during each heart beat) *HR: Heart rate (number of times heart beats per minute) In an adult male, normal CO = 70ml/min x 75 BPM = 5.25 L/min (More than the entire blood volume each minute)
Period of Isovilumetric Relaxation
-Completion of T wave results in ventricular repolarization and relaxation. -Ventricular pressure falls very rapidly. -Pulmonary trunk/aorta pressure is higher than ventricular pressure. -Elastic recoil of the arteries causes blood to flow back toward the relaxed ventricles: the semilunar valves close, which is the beginning of ventricular diastole. -Note that the AV valves are also closed.
Period of Isovolumetric Contraction
-Contraction, but both valves are closed so no volume change -Begins at the completion of the QRS complex. -Ventricular muscles start to contract, increasing the pressure inside the ventricles. This causes the AV valves to close, which is the beginning of ventricular systole. -The semilunar valves were closed in the previous diastole and remain closed during this event. -120-130 mL of blood are in the ventricles, left from the last diastole when the atria emptied into the ventricles. This is referred to as the end diastolic volume.
Active Ventricular Filling
-Depolarization of the SA node generates action potentials that spread over the atria (P wave) and the atria contract. This completes ventricular filling. -At rest, contraction of atria not necessary for heart function. -During exercise, atrial contraction necessary for function as heart pumps 300-400%.
Cations of extrinsic control of heart rate
-Elevated Na+ increases heart rate -Elevated Ca2+ increases heart rate
Sympathetic extrinsic control of heart rate
-Epinephrine (hormone) increases contraction force and can cause vasoconstriction or vasodilation -Norepinephrine increase speed and contractility of the heart (sympathetic)
Cardiac reserve
-Maximum Cardiac Output minus Cardiac Output at rest -4-5 X resting value is average -Severe heart disease patients have little or no reserve -At rest, stroke volume is 50-60% of the end diastolic volume (only about half the blood is ejected at rest) -Near 100% stroke volume is possible in healthy hearts
Afterload
-Pressure caused by resistance of the aorta or pulmonary artery to blood flowing into it *Higher afterload leaves blood in the ventricle after contraction! *Caused by aortic valve issues (stenosis - narrowing) and high blood pressure
Events during cardiac cycle
-Pressure in the aorta maintains an elevated diastolic pressure (close to the BP we measure) -Diastolic pressure in the Ventricle drops to almost zero A/V Valves closed -Semilumar valves close
Period of ejection
-Pressure in the ventricle has increased to the point where it is greater than the pressure in the pulmonary trunk/aorta. This pushes the cusps of the semilunar valves against the walls of the vessels, opening the valve. -Blood is ejected from the ventricles. -The pressures in the two ventricles are different: (120 mm Hg in the left ventricle, 25 mm Hg in the right ventricle). Remember: blood in the left ventricle must be pumped to the whole body; blood in the right ventricle is pumped to the lungs. -After the first initial spurt, pressure starts to drop. -At the end of the period of ejection, 50-60 mL remain: end-systolic volume (down from 120-130 mL end-diastole - roughly half).
Passive Ventricular Filling
-While the ventricles were in systole, the atria were filling with blood. -Atrial pressure rises above ventricular pressure and the AV valves open. -Blood flows into the relaxed ventricles, accounting for most of the ventricular filling (70%).
Extrinsic control of heart rate
Control originating from outside the heart
Intrinsic control
Normally, the heart regulates itself without hormone or nervous system input
Diastole
Relaxation of heart chambers
Cardiac Cycle
Repetitive contraction (systole) and relaxation (diastole) of heart chambers
Systole
Repetitive contraction of heart chambers
In the cardiac cycle, contraction follows
depolarization
The heart must increase or decrease
based on a metabolic need (exercise or resting)
Isovolumetric periods are very
brief (no blood moving because all valves are closed)
Different drugs affect preload and afterload
differently
Increased temperature causes the SA node to depolarize more rapidly causing heart rate to
increase.
Below MAP of 60mmHg in the aorta,
organs become ischemic (not enough oxygen perfused through them)
The More resistance,
the less blood released.
The more blood,
the more forceful contraction.
