Mechanical Activity of the Heart

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Questions and Answers

What role do Ca ions play in the cardiac muscle contraction mechanism?

They decrease the heart rate during contraction.

They bind with troponin C in the myofibrils. (correct)

They trigger the release of energy from the mitochondria.

They inhibit the release of neurotransmitters.

What happens to Ca ions at the end of the plateau phase of the cardiac action potential?

Calcium ions are pumped back into the sarcoplasmic reticulum. (correct)

Calcium ions bind to myosin to increase contraction strength.

Calcium ions remain in the sarcoplasm to sustain contraction.

Calcium ions are released into the bloodstream.

Which phase of the cardiac cycle is characterized by the heart chambers contracting?

Refractory period

Diastolic phase

Filling phase

Systolic phase (correct)

What mechanism allows the heart to increase the strength of contraction based on the amount of blood filling the heart?

Frank-Starling mechanism Signup and view all the answers

How does the influx of Ca ions through L-type calcium channels affect cardiac muscle contraction during the plateau phase?

It triggers the release of Ca from the sarcoplasmic reticulum. Signup and view all the answers

What is the role of troponin C in muscle contraction?

It binds to calcium ions to facilitate muscle contraction. Signup and view all the answers

Which of the following factors influences the cardiac output?

Stroke volume and heart rate Signup and view all the answers

What pump is primarily responsible for transporting Ca back into the sarcoplasmic reticulum?

Ca-ATPase pump (SERCA) Signup and view all the answers

What does preload represent in the context of cardiac muscle contraction?

The load placed on muscle fibers before contraction Signup and view all the answers

Which of the following is not a characteristic of the refractory period in cardiac muscle?

It allows for a new action potential to trigger contraction. Signup and view all the answers

How is cardiac output determined?

By both stroke volume and heart rate Signup and view all the answers

According to the Frank-Starling law, what happens as the end diastolic volume (EDV) increases?

Stroke volume increases Signup and view all the answers

What is the primary mechanism for removing Ca ions from the cell?

Na-Ca exchanger Signup and view all the answers

What does preload primarily depend on?

Ventricular filling Signup and view all the answers

During the cardiac cycle, which phase is characterized by repetitive contraction?

Systole Signup and view all the answers

Which statement about sarcomeres in cardiac muscle is correct?

The strength of contraction is graded based on Ca2+ levels. Signup and view all the answers

Which of the following describes afterload?

The resistance the ventricle faces during contraction Signup and view all the answers

What is characterized by an upward shift in the diastolic pressure-volume curve?

Diastolic dysfunction Signup and view all the answers

Which factor primarily influences venous return?

Skeletal muscle pump Signup and view all the answers

How is cardiac reserve affected in athletes compared to older individuals?

Increased in athletes, decreased with age Signup and view all the answers

In heart failure due to systolic dysfunction, which of the following occurs?

Increased end-systolic volume Signup and view all the answers

Which mechanism is NOT considered a compensatory mechanism for heart function?

Increased afterload Signup and view all the answers

What results from impaired ventricular contractility in heart failure?

Systolic dysfunction Signup and view all the answers

What happens to end-diastolic volume in the case of diastolic dysfunction?

Decreases Signup and view all the answers

Study Notes

### Mechanical Activity of the Heart

Cardiac muscle excitation, contraction and relaxation occurs through a series of events:
- During the plateau phase of cardiac worker cells' electric activity, calcium ions influx through L-type calcium channels
- Triggered calcium is released from the sarcoplasmic reticulum
- Calcium ions bind with troponin C in the myofibrils
Cardiac muscle relaxation:
- Calcium influx is halted at the end of the plateau
- Calcium ions in the sarcoplasm are pumped back into the sarcoplasmic reticulum and the T tubule-extracellular fluid space via a Ca-ATPase pump (SERCA). Phospholamban plays a role here.
- Calcium ions are also removed by Na-Ca exchanger.
Strength of contraction in cardiac muscle is not an 'all or none' response like skeletal muscle.
- Contraction strength is graded and depends on the calcium levels in the cytosol
- Strongest contraction is achieved when stretched 80-100% of its maximum (physiological range)

The Cardiac Cycle

The cardiac cycle is a sequence of events that occur when the heart beats.
It involves changes in chamber volume and pressure, valve opening and closing, heart sounds, and blood movement through the circulatory system.

Wigger's Diagram

This diagram illustrates the relationship between electrical activity (ECG), pressure and volume changes, heart sounds, and the cardiac cycle.

Preload

Represents the load on the muscle fibers before contraction.
Frank-Starling law states that stroke volume increases as EDV (ending diastolic volume) increases - stretching leads to more force.
EDV is determined by venous return.
Venous return is affected by the skeletal muscle pump and the respiratory pump

Cardiac Output

The amount of blood pumped by the heart in 1 minute.
Controlled to maintain proper blood flow to tissues.
Determined by stroke volume and heart rate

Factors Affecting Stroke Volume

Contractility: The intrinsic ability of cardiac muscle to develop force for a given length. Affected by inotropic agents.
Preload: The muscle length prior to contraction, dependent on ventricular filling (EDV).
Afterload: The tension (arterial pressure) against which the ventricle must contract. Depends on vessel wall diameter and elasticity.

Cardiac Output Distribution & Cardiac Reserve

Cardiac reserve: The extra volume that the heart can pump beyond normal resting conditions.
- 3-4 times cardiac output.
- Increases in athletes.
- Decreases with age and pathological conditions.

Heart Failure

Pressure-Volume Loop: This loop shows the relationship between pressure and volume changes in the heart during a cardiac cycle. It can be used to diagnose and understand heart failure.
Pathophysiology of Heart Failure:
- Impaired ventricular contractility
- Increased afterload
- Impaired ventricular relaxation and filling.
Systolic Dysfunction: Heart failure resulting from an abnormality of ventricular emptying (due to impaired contractility or excessive afterload).
Diastolic Dysfunction: Heart failure caused by abnormalities of diastolic relaxation or ventricular filling.

Compensatory Mechanisms in Heart Failure

Frank-Starling Mechanism: The heart attempts to compensate by increasing stroke volume in response to increased EDV.
Neurohormonal Response: The body releases hormones (like norepinephrine and angiotensin II) to increase heart rate and contractility, but can lead to further complications.

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Description

Explore the intricate mechanisms of cardiac muscle activity through excitation, contraction, and relaxation. Understand the roles of calcium ions, troponin C, and various pumps in the heart's function. This quiz delves into the unique characteristics of cardiac muscle contractions and their regulation.