Clase 19 Fisiología Cardíaca 4 - Regulación del bombeo cardíaco (IG:@doctor.paiva)

Regulation of Cardiac Pumping

Overview of Cardiac Regulation

• Introduction to the 19th class on physiology, focusing on cardiac pumping regulation.

• Discussion of heart function at rest (4-6 liters/minute) and during exercise (up to 16,250 liters/minute).

• Explanation of intrinsic cardiac regulation via the Frank-Starling mechanism and autonomic nervous system control.

Frank-Starling Mechanism

• The Frank-Starling mechanism allows the heart to adjust its pumping based on venous return; more blood leads to stronger contractions.

• Each tissue regulates its own blood flow, influencing overall venous return to the heart.

• Example illustrating that increased venous return results in greater cardiac output.

Cardiac Distension and Contraction

• Greater distension of cardiac muscle during filling correlates with stronger contractions during systole.

• Comparison between two hearts: one ejecting 50 ml and another 200 ml due to different levels of distension.

• The relationship between muscle stretch and optimal overlap of actin and myosin filaments for effective contraction.

Physiological Limits of Frank-Starling

• Importance of physiological limits; excessive dilation can weaken heart function, as seen in congestive heart failure.

• Overstretching leads to ineffective contraction due to misalignment of actin and myosin filaments.

Autonomic Nervous System Control

• Overview of sympathetic (increases heart rate) vs. parasympathetic (decreases heart rate) influences on cardiac function.

• Description of vagus nerve's role in parasympathetic innervation versus sympathetic chain effects.

Effects of Sympathetic Stimulation

• Normal resting heart rate ranges from 60 to 90 beats per minute; sympathetic stimulation can raise it up to 200 bpm or more.

• Intense sympathetic stimulation doubles muscular contraction strength, potentially tripling cardiac output.

Effects of Parasympathetic and Sympathetic Stimulation on Cardiac Function

Overview of Cardiac Response to Parasympathetic Stimulation

• The vagus nerve (neumogastric) has minimal impact on ventricular contraction strength due to low innervation, primarily affecting heart rate.

• Intense parasympathetic stimulation can lead to a significant drop in heart rate, potentially causing the heart to stop temporarily; however, an intrinsic defense mechanism called "vagal escape" maintains a minimum heart rate of 20-40 beats per minute.

• With strong parasympathetic stimulation, muscular contraction force decreases by 20-30%, and cardiac output may reduce by up to 50%.

Mechanisms Behind Heart Rate and Contraction Force

• High parasympathetic innervation in the atria leads to a more pronounced decrease in heart rate compared to the slight reduction in ventricular contraction strength due to lower innervation.

• The sinoatrial node's location in the atria means that increased parasympathetic activity significantly lowers heart rate while having minimal effects on ventricular contractions.

Graphical Representation of Cardiac Output

• A graph illustrates how cardiac output decreases with increased right atrial pressure during parasympathetic stimulation, highlighting the relationship between frequency and cardiac output.

Intrinsic Regulation vs. Nervous System Influence

• In absence of both sympathetic and parasympathetic stimulation, intrinsic mechanisms regulate cardiac function up to a certain limit based on blood volume returning to the heart.

• To increase cardiac output beyond this limit under physiological conditions, sympathetic nervous system activation enhances both contraction strength and heart rate.

Maximum Cardiac Output Under Sympathetic Stimulation

• Maximum sympathetic stimulation can elevate cardiac output close to 15 liters per minute when all adrenergic receptors are activated by drugs or other stimuli.

Impact of Temperature on Heart Function

• Increased body temperature raises heart rate significantly; for every degree Celsius rise, there is an approximate increase of 10–15 beats per minute due to enhanced membrane permeability for sodium ions.

• Conversely, lower temperatures (16–21 degrees Celsius range) result in decreased heart rates as physiological responses slow down.