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

 

[1]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

·         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

->> ↑Arterio-venous flow -> ↑VR -> ↑CO
->> ↓Afterload -> ↓SVR -> ↑CO

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

 

 

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