L5 Human Physiology

🧠 Concepts Introduced

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Learning Objectives

1. Identify the major components of cardiovascular system and their functions.
2. Describe major structures of the heart and path of blood flow through the heart.
3. Describe how the phases of electrocardiogram (ECG) relate to the events of the cardiac cycle.

Cardiovascular system

The Heart

• Muscular pump that drives flow of blood to various organs.

Blood vessels

• Conduit through which the blood flows.

Blood

• Fluid that circulates around the body.
• Carrying materials to and from the cells.

Blood flow through cardiovascular system

Arteries take blood awav from heart. Veins return blood to the heart. Series flow: blood travels through systemic and pulmonary circuits in series Portal system: two capillary beds connected in series.

Heart

Anatomy of Heart

Located in thoracic/chest cavity between the lungs.

Pericardium - a double serous membrane:

• Visceral pericardium - next to heart
• Parietal pericardium - outside layer

Serous fluid fills the space between the layers of pericardium.

Heart Wall

Composed of three distinct layer

Epicardium

• Connective tissue layer.
• Same as visceral pericardium.

Myocardium Cardiac muscle.

Endocardium

• Endothelium.
• Contains the Purkinje fibers

Structure of the heart

Heart chambers and valves

Four Chambers

• Two atria receive blood returning to the heart from vasculature.
• Two ventricles receive blood from atria and push the blood out of heart.

Heart valves

• Promotes unidirectional blood flow.
• Atrioventricular (AV valves) - between atria & ventricles.
• Semilunar valves - between ventricles & arteries.

Electrical conduction in the heart

1. SA node depolarizes.

2. Electrical activity goes rapidly to AV node via internodal pathways.

3. Depolarization spreads more slowly across atria. Conduction slows through AV node.

4. Depolarization moves rapidly through ventricular conducting system to the apex of the heart.

5. Depolarization wave spreads upward from the apex.

Control of heart rate

Both SA node and AV node - capable of generating spontaneous action potentials.

SA node usually sets the pace of heartbeat

• fires more frequently than AV node (70 impulses /min compared to 50 impulses/min).

If SA node fails to fire or if impulses from it are blocked, the AV node can take over.

Electrocardiogram (ECG)

Represents electrical activity of all cells of the heart during cardiac cycle. Recorded from surface of body (extracellular).

Einthoven's triangle

• Imaginary triangle that surrounds the heart with its corners at right and left arms and left leg
• Correspond to places where electrodes are placed

Provides information on:

heart rate and rhythm, conduction velocity, and heart condition.

Three major waves: P wave, QRS complex, and T wave.

Waves correspond to events of cardiac cycle.

Electrodiogram (ECG) waves

P wave Atrial depolarization

QRS complex Ventricular depolarization and atrial repolarization

T wave Ventricular repolarization

Correlation between an ECG amd electrical events in the heart

Correspondence between electrical events in ECG and depolarizing (purple) and repolarizing (peach) regions of the heart.

Normal & Abnormal electrodiograms

1. The Spark (Depolarization): Rushes from the inside to the outside. Because this positive wave is rushing downward toward the outside surface (closer to your foot electrode), the graph shoots UP (forming the giant R wave).
2. The Pause (ST Segment): The entire heart wall is fully turned on. Everything is flat at the baseline.
3. The Reset (Repolarization): The outside cells' short timers run out first. They start resetting. The inside cells are still holding onto their charge because of their long timers. Therefore, the reset process spreads backward, moving from the outside layer back to the inside layer.

Because the wave of relaxation (repolarization) is moving from the outside to the inside, it is traveling up and away from your foot electrode.

Referring back to our rule: A repolarization wave (-) moving away from the electrode creates an upward (+) curve. This is why a normal, healthy T wave curves upward in the exact same direction as the R wave

Questions to ask when analyzing ECG tracings:

1. What is the rate? Is it within the normal range of 60-100 beats per minute?

2. Is the rhythm regular?

3. Are all normal waves present in recognizable form?

4. Is there one QRS complex for each P wave? If yes, is the P-R segment constant in length?

5. If there is not one QRS complex for each P wave, count the heart rate using the P waves, then count it according to the R waves. Are the rates the same? Which wave would agree with the pulse felt at the wrist?

Phases of cardiac cycle

Pressure - volume curve

Stroke volume = EDV - ESV (70 mL, at rest)

Cardiac output & control

Cardiac output (CO) = volume of blood pumped by one ventricle per minute. CO = HR X SV (heart rate x stroke volume). Average CO = 72 beats / min x 70 mL/ beat =5 L/ min at rest. Average blood volume = 5 L.

SV is directly correlated with CO, the greater the SV the greater the CO. SV represents the difference in the amount of blood between: EDV and ESV

Control of stroke volume

Stroke volume (SV)

• Volume of blood pumped per ventricle per contraction.
• Directly related to force generated by cardiac muscle during contraction.

Primary factors affecting stroke volume:

• Ventricular contractility (force of contraction).
• Preload (relate to EDV, degree of muscle stretch).
• Afterload (pressure needed to eject blood out).

Ventricular contractility

Contractility

• Intrinsic ability of muscle fiber to contract at any given fiber length.
• Increase in contractility will increase stroke volume.

lonotropic agent

• Chemical that affects contractility → ionotropic effects.
• Positive ionotropic effects- e.g. catacholamines (sympathetic stimulation) and drugs enhance contractility

End-diastolic volume (EDV)

Volume of blood at the beginning of contraction (EDV) - determines the length of muscle.

Preload - degree of myocardial stretch before contraction begins.

Relationship between EDV and SV

• Frank-Starling law of the Heart

Frank-Starling law of the heart

Heart automatically adjusts its output to match EDV.

Increased EDV stretches muscle fibers:

• Fibers closer to optimum length.
• Optimum length = greater strength of contraction.
• Increased SV.

An increase in EDV causes stroke volume to increase