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

 

(i.e. this question is going to be repeated)

 

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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