[#2] Regulação do bombeamento cardíaco: PRÉ-CARGA, PÓS-CARGA E CONTRATILIDADE | MK Fisiologia

Summary

In this video, the concept of cardiac pumping regulation is explored, focusing on pre-load, after-load, and cardiac contractility. The video explains how these factors influence stroke volume and cardiac output. Pre-load refers to the pressure on the ventricular fibers before contraction, determined by end-diastolic volume. After-load is the pressure the ventricles must overcome during contraction, linked to aortic pressure. Cardiac contractility reflects the heart muscle's ability to contract and is influenced by calcium ion interactions and the autonomic nervous system, particularly the sympathetic division. The video highlights how these factors, including sympathetic stimulation, can significantly impact cardiac output.

Takeaways

• 😀 Cardiac output is the product of stroke volume and heart rate. To regulate cardiac output, both components must be managed.
• 😀 Pre-load refers to the pressure on the cardiac muscle fibers before contraction, influenced by the volume of blood returning to the heart (venous return).
• 😀 A higher venous return increases the end-diastolic volume (EDV), which increases pre-load and, consequently, cardiac output.
• 😀 After-load is the pressure the heart must overcome to eject blood during ventricular contraction. Higher after-load decreases stroke volume and cardiac output.
• 😀 After-load is primarily determined by systemic and pulmonary arterial pressure, and higher arterial pressure makes it harder for the heart to eject blood.
• 😀 Contractility refers to the heart's intrinsic ability to contract and pump blood. It is influenced by calcium ion responsiveness in cardiac muscle fibers.
• 😀 The sympathetic nervous system increases contractility by releasing norepinephrine and adrenaline, which increases stroke volume and cardiac output.
• 😀 Increased contractility leads to more blood being ejected per beat, raising stroke volume and improving cardiac output.
• 😀 Maximum sympathetic stimulation can increase cardiac output to around 25-30 liters per minute in trained individuals, much higher than the 5 liters per minute at rest.
• 😀 Both pre-load and after-load can affect stroke volume, but contractility can independently alter the volume of blood ejected by the ventricles.
• 😀 Regular stimulation of the sympathetic nervous system results in an increased heart rate and contractility, which together elevate cardiac output, especially during physical exertion.

Q & A

What is cardiac output, and how is it determined?

Cardiac output is the amount of blood pumped by the heart per minute. It is determined by the stroke volume (the volume of blood ejected per heartbeat) and the heart rate (the number of beats per minute).

What is stroke volume, and how can it be regulated?

Stroke volume is the volume of blood ejected by the ventricles with each heartbeat. It can be regulated by adjusting the end-diastolic volume (pre-load), the end-systolic volume (after-load), and the contractility of the heart muscle.

What does the term 'pre-load' refer to in cardiac physiology?

Pre-load is the pressure applied to the cardiac muscle fibers in the ventricles before the ventricular contraction (systole) begins. It is often associated with the end-diastolic volume, which is influenced by venous return to the heart.

How does venous return affect pre-load?

Venous return determines the volume of blood entering the heart, which influences the end-diastolic volume. A higher venous return results in a higher end-diastolic volume, thereby increasing the pre-load and potentially increasing stroke volume.

What is the relationship between after-load and stroke volume?

After-load is the pressure the ventricles must overcome to eject blood, primarily determined by arterial pressure. An increase in after-load (e.g., higher blood pressure) makes it harder for the ventricles to eject blood, reducing stroke volume.

How does increased aortic pressure affect the heart's function?

Increased aortic pressure raises the after-load, which forces the left ventricle to exert more force to overcome the higher pressure. This reduces the amount of blood ejected per beat, thus decreasing stroke volume and cardiac output.

What is contractility, and how does it impact stroke volume?

Contractility refers to the ability of the heart muscle to contract. It is independent of pre-load and after-load. Increased contractility results in more forceful contractions, leading to higher stroke volume and cardiac output.

What factors affect cardiac contractility?

Cardiac contractility is primarily influenced by the sympathetic nervous system. The release of adrenaline and noradrenaline enhances the force of contraction by increasing calcium influx into cardiac muscle cells, thus improving contractility.

How does the sympathetic nervous system influence the heart?

The sympathetic nervous system increases heart rate and contractility by releasing adrenaline and noradrenaline. These hormones bind to beta-adrenergic receptors on cardiac cells, increasing calcium influx, enhancing contraction strength, and reducing relaxation time.

How does hypertension impact cardiac function?

In individuals with hypertension, the left ventricle has to generate more force to overcome the elevated arterial pressure (after-load). This leads to ventricular hypertrophy as the heart muscle thickens to compensate, potentially reducing its efficiency over time.