2020A08 Describe the process by which increased tissue demand for oxygen
 leads to an increase in cardiac output.

 

 

List:

·         Summary

·         Tissue activity

·         Respiratory muscle pump

·         Skeletal muscle pump

·         Sympathetic nervous system

·         Intracardiac reflexes

 

Summary: cardiac output

Definition

·           Volume of blood ejected from LV into aorta per unit time

Determinants

·           Coupling factorsCardiac factorsHeart rate

·           Contractility

·           Preload

·           Afterload

Principles

·           Most important: tissue activity venous return cardiac output

·           i.e. negative feedback loop

·           i.e. cardiac output (CO) is a slave to venous return (VR)

 

Matching of supply to demand:

1.Tissue activity

Metabolic autoregulation:

·           ↑Tissue metabolic activity

·           ↑VO2, ↑VCO2, ↑H+, K+, adenosine, lactate

·           Local vasodilatation

->> ↑Rate of flow from arterial to venous circulation -> ↑VR
->> ↓Afterload -> ↑CO (minor effect)

Heterometric autoregulation: Frank-Starling relationship

·           ↑VR to RV

·           ↑VR to LV

·           ↑Preload

·           ↑Sarcomere stretch

·           ↑Ca2+ sensitisation

·           ↑Force of contraction

->> ↑Cardiac output

Equilibration:

·           ↑CO

·           ↓CVP

·           ↓Preload

->> New equilibrium with ↑CO, ↑VR and slight ↑CVP

Representation:

       

2.Respiratory muscle pump

·           ↑Tissue activity

·           ↑VO2

·           ↑VCO2 (Krebs cycle)

·           ↑PaCO2

·           ↑Activation of peripheral and central chemoreceptors

·           ↑Frequency and amplitude of breathing

·           ↑-ve pressure during inspiration -> ↑VR to RV

·           ↑+ve pressure during expiration -> ↑VR to LV

·           ↑CO

->> VR during spont vent exceeds VR during apnoea

->> important during exercise

3.Skeletal muscle pump

·           ↑Muscle isometric contraction

·           ↑Compression of capacitance vessels

·           ↑VR

·           ↑CO

->> important during exercise

4.Sympathetic nervous system

·           ↑Skeletal muscle activity

·           Multiple reflex pathways (central, vascular, skeletal)

·           ↓↓SVR, ↑HR, ↑contractility, constrict capacitance vessels -> ↑VR

·           ↑CO

->> important during exercise

5.Cardiac reflexes

Bainbridge effect:

·           ↑VR

·           ↑Atrial stretch

·           ↑HR

·           ↑CO

Bowditch effect:

·           ↑HR

·           ↑ICF [Ca2+]

·           ↑Contractility

·           ↑CO

 

 

Feedback welcome at ketaminenightmares@gmail.com