Cardiac Output, Blood Pressure, and Flow

• Cardiac output (CO) is the volume of blood pumped by each ventricle in one minute.
• CO is calculated by multiplying heart rate (HR) by stroke volume (SV).
• Stroke volume changes during exercise to supply more blood to muscles, increasing both blood pumped out and heart rate.

Pressure Basics

• Pressure differences drive blood flow from high to low pressure areas.
• Resistance is how easy or hard it is to get to the place of high to low pressure
• Fluid flow (Q) relates to cardiac output.
• Increased blood flow through arteries increases pressure.

Blood Pressure Measurement

• Blood pressure measures the force pushing against vessel walls, typically as arterial pressure.
• Systolic BP reflects arterial contraction, around 120mmHg.
• Diastolic BP reflects arterial relaxation, around 80mmHg.
• Automatic blood pressure readings involve closing off the brachial artery.
• A stethoscope helps listen for "lub dub" sounds while cutting off blood flow with a cuff.
• Milligrams of mercury (mmHg) is the unit measuring the force needed to raise mercury in a tube.

Circulatory System Pressures

• The circulatory system operates under constant, varying pressures.
• Pressure is what forces blood to move.
• Systemic circulation carries oxygenated blood, while pulmonary carries deoxygenated.
• Higher pressure is needed to move fluid longer distances.
• Getting oxygenated blood to parts of the body takes more pressure compared to moving deoxygenated blood to the lungs.
• Pressure is high when blood exits the aorta, and low near capillaries.

Pressure-Volume Loop

• Applicable to the left ventricle, systolic and diastolic phases
• Stroke volume is how much blood is ejected
• Blood with resistance builds pressure inside a tube.
• A-B represents the end diastolic volume during the relaxation phase as the ventricle fills, valve is still open, minimal pressure changes.
• B-C Involves valves snapping shut, allowing pressure to build significantly.
• C-D is the opening of the semilunar valve, which pushes blood out of the left ventricle and ESV represents the blood amount we have pushed out during the end of the contractile phase.
• D-A is the relaxation phase.
• D marks when the aortic valve closes and the pressure drops.
• Pressure volume loops relate to ventricular contraction.
• A normal loop is desirable.

Components of the Pressure Volume Loop

• A to B involves the mitral valve opening and the ventricle filling with blood.
• B marks the end of the relaxation phase and the end diastolic volume.
• B to C involves the mitral valve closing.
• Contraction leads to even greater pressure, forcing the aortic valve to open.
• C to D is the period of ejection.
• Blood volume in the heart decreases as blood leaves, aortic valves open, pressure remains high.
• D marks when all the blood is leaving.
• There's a steep decline in pressure during isometric relaxation.

Stroke Volume Regulation

• Changes to the speed of beats can alter stroke volume, change amount pumped out.
• Increasing preload, increasing the degree of stretch of cardiac muscle increases blood in the ventricle, increases blood volume in the heart (can increase stroke volume).
• Increase afterload, increasing pressure, increasing the pressure threshold to be overcome for ejection to occur.
• Preload increases blood volume to fill the ventricle per beat.
• Afterload involves pressure needed for ventricles to eject blood (pressure inside the ventricles overcomes the pressure within the aorta.)
• Increase afterload makes it take more pressure to reach balance change to open the aortic valve.

Inotropic Change

• An example of positive inotropic change = Influx of calcium, increasing the contractility of the myosin and actin sliding over each other.

Frank-Starling Law

• The greater the heart myocardium stretch (preload) before systole, the stronger the ventricle contraction.
• Greater stroke volume is linked to increased blood influx. The more the blood influx, the stronger the contraction.
• Exercise produces positive stroke volume.
• The heart will normally complete the pressure volume loop.
• Positive inotropic effects (sympathetic) increase contractility, pumping faster and increasing blood and pressure and stroke volume and preload.
• Negative inotropic effects (parasympathetic) reduce contractility. The heart pumps less forcefully, lowering pressure and stroke volume and lowers the volume of blood pumped out.

Stroke Volume and Drugs

• Calcium channel blockers depress contractility.
• Blocking Calcium influx and myosin/actin interactions reduces the force of the contraction.
• Decrease contractility.

Chronotropic Effects

• Increase chronotropic effects will increase heart rate.
• Increased dromotropic effects will increase conduction of Aps.
• Increased inotropic effects will increase contractility.

Sympathetic Responses

• Norepinephrine and epinephrine increase.
• IF funny channels open and increase activity, allowing more calcium influx.
• Calcium floods into contractile cells.

Pacemaker Cells

• Increasing calcium channels and a longer refractory plateau create a stronger muscle contraction.

Parasympathetic Activity

• Longer time to reach threshold.
• Less action potentials means less contractility.
• Lower action potential speed.

Blood Pressure Regulation

• Baroreceptors detect stretch in blood vessels.
• A normal set point is determined by action potentials per minute.
• Deviations from the normal set point are detected by the brain, which signals changes.
• There is blood pressure regulation.

Heart Regulation

• The heart can self-regulate but needs some external "boss" as well.
• The autonomic nervous system regulates the heart.
• The cardiac center is in the medulla oblongata.
• Cardioacceleratory center: innervates SA and AV, sends messages to increase the contraction.
• Cardioinhibitory center: the vagus nerve decreases heart rate by inhibiting SA and AV nodes.

Autonomic Nervous System

• Sympathetic activation stems from emotional/physical stressors.
• Norepinephrine quickens pacemaker firing and boosts contractility.
• The parasympathetic nervous system opposes sympathetic actions.
• Acetylcholine hyperpolarizes pacemaker cells by opening K+ channels.
• The heart shows vagal tone at rest (parasympathetic).
• Increase contractility from increased calcium presence.
• Lowered contractility from decreased calcium presence.