Introduction
26 Regulation of Cardiac Function, Intracardial Autoregulation, and Cardiac Function Curves.
1. Intracardial Autoregulation: The Inherent Adaptive Mechanism
Heterometric autoregulation, also known as the Frank-Starling Mechanism, is a fundamental intrinsic property of the heart that allows it to adjust its stroke volume in response to changes in venous return. This ensures that the heart effectively pumps out the volume of blood it receives, maintaining a balance between cardiac output and venous return.
Key Points of Heterometric Autoregulation:
Venous Return and End-Diastolic Volume (EDV): The amount of blood returning to the heart through the veins (venous return) determines the EDV, which is the volume of blood in the ventricle at the end of diastole.
EDV and Preload: EDV is the primary factor influencing preload, the stretching force exerted on the ventricular muscle fibers at the end of diastole.
Preload and Force of Contraction: The Frank-Starling Law states that the greater the preload (stretch) on the ventricular muscle fibers, the stronger the force of contraction they generate. This relationship is based on the degree of overlap of thick and thin filaments within the sarcomeres at different muscle lengths.
Stroke Volume (SV) Regulation: The increased force of contraction resulting from increased preload leads to a greater SV, ensuring the heart ejects the volume of blood it receives.
The reason it is called length-dependent is because the length of the sarcomere’s in the heart muscles changes when for e.g. the ↑preload is increased then sarcomere’s stretch and become longer and this makes the contraction force stronger. So the contraction force is length-dependent because of this relation between length of sarcomere and force of contraction.
In essence, heterometric autoregulation allows the heart to adapt its output to match the incoming blood flow, maintaining a critical balance in the cardiovascular system.
2. Cardiac Function Curves: Graphical Representation of the Frank-Starling Relationship
Cardiac function curves visually depict the relationship between right atrial pressure (which reflects venous return) and cardiac output. These curves effectively demonstrate the Frank-Starling relationship.
Key Features of Cardiac Function Curves:
Steady-State Balance: The curves illustrate that in a steady state, the volume of blood ejected by the heart (cardiac output) is equal to the volume of blood it receives via venous return.