Download Lecture 11: Cardiac Cycle

Lecture 11: Cardiac Cycle
Reading: OpenStax A&P Text Chapter 19
Cardiac cycle
The cardiac cycle includes all the events that take place during one complete heartbeat, including:
• Atrial and ventricular contraction (systole) and relaxation (diastole)
• Blood pressure
• Heart sounds (valve actions)
• “LUB” is heard when the AV valves snap shut.
• “DUP” is heard when the semilunar valves snap shut.
• ECG (EKG) events. The electrocardiogram (ECG) measures the TOTAL electrical activity in the heart. (This means that it
measures the SUM of ALL action potentials happening in the heart!) A normal ECG has the following key points:
• P wave: Atria are stimulated
• QRS complex: Ventricles are stimulated (Atria repolarize during this time, but you can’t see it (hidden))
• T wave: Ventricular repolarization
• Compare the ECG and the action potential…
Atrial and ventricular diastole
The heart is relaxed
1. Atria are filling with blood from veins, they expand…
2. AV Valves open
3. Ventricles fill passively
A. Volume of blood in ventricles increases, but pressure does not because the ventricular walls expand to acomodate the blood.
4. SA Node FIRES and AP passes through the internodal pathways and head toward the AV node.
A. The action potential passes through gap junctions between AR and contratile cells, and spreads throughout the contractile
cells in the atria
B. The P wave begins here
Atrial systole
5. FAST transmission of AP through atria stimulates atrial contraction
6. As atria slowly contract, blood is pushed into the ventricles
7. Volume of blood in ventricles increases, and so does pressure
8. Atrial contraction
A. Push blood into ventricles
B. (A little goes backwards into veins…apparently you can see this in the jugular vein if someone lies at a 30 degree angle??)
9. AV Node: Only way AP can reach the ventricles!
A. AP cannot pass to ventricles b/c of fibrous structure between atria and ventricles
B. There must be a pause here, while the ventricles fill with blood!
C. The AV Node cells (autorhythmic) fire their AP at 1/20 the rate of other autorhythmic cells!
10. The AP passes through the AV Bundle: Starting down the ventricular septum, then down the Rt and Lft bundle branches and into
the net of Purkinje fibers.
A. At this point, there is super – fast conduction of the AP through ventricles, ensuring all ventricular fibers contract together
11. The maximum volume of blood is now in the relaxed ventricle. This is the end diastolic volume (EDV).
Biol 7: Human Physiology Spring 16
80
CC-BY Wendy Riggs
Early ventricular systole
12. Early ventricular contraction begins at the apex of the heart .
A. Increased pressure in ventricles caused by ventricular contraction slam the AV valves closed (lub)
B. All valves are closed, creating increased pressure as the ventricles continue to contract (Isovolumetric contraction)
C. Atria start to repolarize and relax…which allows blood from the veins to flow into the atria again!
13. This is the beginning of the QRS complex
Ventricular systole and atrial diastole
14. Ventricular ejection results from high pressure (generated by the squeezing muscle), which forces opens the semilunar valves, so
blood rushes out.
A. This high pressure creates the driving force behind blood flow. It pushes the blood throughout the vessels
B. AV valves stay closed, because of the pressure. Semilunar valves stay open for the same reason.
15. At the end of ventricular systole, there is still a small amount of blood in the ventricles. This is the end systolic volume (ESV).
16. More QRS complex!
Ventricular diastole
17. The ventricles relax and the muscle tissue repolarizes.
A. Ventricular relaxation decreases the pressure in the ventricles, causeing the blood in the arteries to flow BACK into the
heart---thankfully the semilunar valves snap shut (DUP!) and prevent this.
B. At this very moment, for a brief time, all valves are closed and the volume of blood in the ventricle does not change.
C. However, it isn’t long before the pressure in the ventricle is so low that it is LOWER than the pressure of the blood in the
atria (because remember, the ventricular contraction caused blood to be pushed through the system, which means it must be
entering the atria!)
D. AV valves open, and passive filling begins again.
18. This is the T wave in the ECG.
Stroke volume
1. Stroke volume is the total volume of blood pumped by one ventricle during one contraction.
2. It is measured in mL/beat
3. Volume of blood before contraction - volume of blood after contraction = stroke volume
4. Stroke volume can CHANGE.
Cardiac output
A measure of heart function is cardiac output (CO), which can be calculated if you know the heart rate (beats/minute) and the stroke
volume (mL/beat).
Normally, CO on both sides of the heart is equal. But if there is damage to one side of the heart, then blood will POOL behind the
weaker side. This can cause problems in the body parts that aren’t being properly drained.
CO increases dramatically during exercise.
Managing heart rate OR stroke volume can both affect cardiac output, and as such, are strategies to maintain homeostasis.
Biol 7: Human Physiology Spring 16
81
CC-BY Wendy Riggs
Lab 11: ECG and Heart Sounds
(iWorx HH-2)
Introduction
Blood enters the arterial system from the ventricles of the heart in a pulsatile manner. However, when blood is leaving the arterial
system through the capillaries, it flows in a continuous manner. Between contractions, when the heart is relaxed and blood is not being
pumped into the arterial system, there is still enough pressure in the arterial system to move blood along the arteries. The pressure in
the arterial system exists because the elasticity of the arteries allow them to distend and recoil and function as a pressure reservoir.
When the ventricles contract, the pressure of the blood inside the ventricles increases to close the atrioventricular valves. Further
contraction increases the ventricular pressure until it exceeds the arterial pressure. At this point, when the arterial pressure is at its
lowest point during the cardiac cycle (called diastolic pressure) the semilunar valves are forced open, and blood flows into the artery.
Blood entering the arterial system inflates the arteries a little and increases blood pressure to a maximum, which is the systolic
pressure. In this lab you will record the ECG from a subject and listen to the characteristic “lub-dub” heart sounds. The “lub” sound
occurs during the early phase of ventricular contraction and is produced by closing of the atrioventricular valves, which prevents blood
flow into the atria. When the ventricles relax, the blood pressure drops below what is in the artery and the semilunar valves close,
producing the “dub” sound.
Equipment Required
• Laptop computer with LabScribe2 software and:
Set up the BOX (Marlena)
1. Do not turn the box on at any time during setup.
2. Place the BOX on the bench, close to the computer.
3. Use the USB cable to connect the computer to the USB port on the rear panel of the BOX.
4. Plug the power supply for the BOX into the electrical outlet and connect the other end to the labeled socket on the back
of the BOX. (Make sure the box is powered OFF.)
5. Plug the white tubing connector of the HSM-220 (stethoscope attachment) into the white Channel A1 input of the BOX.
6. Insert iWire-B3G connector on the end of the ECG cable into the isolated inputs of the iWire 1Channel on the front of the IXTA.
7. Insert the connectors on the red, black, and green electrode lead wires into the matching sockets of the ECG cable.
8. Plug the pulse plethysmograph (PT-104) into the Channel A5 input of the BOX.
SEE HANDOUT FOR COMPLETE INSTRUCTIONS
Biol 7: Human Physiology Spring 16
82
CC-BY Wendy Riggs
External Brain 11: Cardiac Cycle
Study Guide Questions
1. Know the path that blood follows around the body and know whether blood is oxygenated or NOT at various anatomical
locations.
2. What is the cardiac cycle?
3. Describe the intrinsic conduction system of the heart. Know the basic anatomy of the heart, so you can give a clear
description.
4. Suggest a possible mechanism for how different structures in the intrinsic conduction system can conduct action potentials at
different speeds.
5. What might be an evolutionary advantage of different structures in the intrinsic conduction system conducting action
potentials at different speeds?
6. Compare and contrast the SA node and the AV node.
7. How can the average human heart rate be 70 bpm?
8. Describe the stages of the cardiac cycle (to the degree discussed in lecture).
9. What causes the sounds heard during a heart beat?
10.What is the Wigger’s diagram? Be able to interpret information found on this diagram.
11. What is cardiac output? How is it measured?
12.Connect the stages of the cardiac cycle to an ECG reading.
13.Be able to assess the following parameters in an ECG tracing:
a. PR Interval = 0.12-0.20
b. QRS Interval = 0.04-0.12
c. QT Interval = Less than 0.44
d. Normal heart rate (sinus rhythm) = 60-100 beats per minute
14.Given any ECG tracing, be able to draw general conclusions about the situation.
Biol 7: Human Physiology Spring 16
83
CC-BY Wendy Riggs