Respiratory Physiology and Exercise

**11**

**Chapter 2**

**Abstract**

varied exercise modes.

muscle fatigue

**1. Introduction**

The Respiratory System during

Critical Distribution of Oxygen

*Ramón F. Rodriguez, Robert J. Aughey and François Billaut*

systemic blood flow is distributed to tissues relative to their metabolic oxygen demands. During sustained high-intensity exercise, competition for oxygen delivery arises between locomotor and respiratory muscles, and the heightened metabolic work of breathing, therefore, contributes to limited skeletal muscle oxygenation and contractility. Intriguingly, this does not appear to be the case for intermittent-sprint work. This chapter presents new evidence, based on inspiratory muscle mechanical loading and hypoxic gas breathing, to support that the respiratory system of healthy men is capable of accommodating the oxygen needs of both locomotor and respiratory muscles when work is interspersed with short recovery periods. Only when moderate hypoxemia is induced, substantial oxygen competition arises in favour of the respiratory muscles. These findings extend our understanding of the relationship between mechanical and metabolic limits of

In healthy individuals at rest and while performing moderate-intensity exercise,

**Keywords:** blood flow, hyperpnoea, metaboreflex, oxygen uptake, hyperventilation,

Blood flow to contracting skeletal muscles closely matches their metabolic rate [1, 2]. In humans, it has been robustly demonstrated that there is a positive linear relationship between the rate of oxygen uptake (VO2) in the quadriceps muscles and blood flow through the femoral artery [1], which ensures there is a match between oxygen (O2) supply and demand for the exercising muscles. Blood flow is directed to areas in need by adjusting vasoconstriction in the relatively inactive regions and vasodilatation in the active locomotor muscles [2–4]. During high-intensity and maximal exercise, the accompanying increase in cardiac output is almost exclusively devoted to the working skeletal muscle [5], whereas blood flow to the splanchnic, renal and inactive skeletal muscle tissue beds can fall by ≈70% from resting values [6, 7]. It is likely that multiple biological factors contribute to biological redundancy in the system [8]. However, there does appear to be a limit to systemic vasodilation, a procreative mechanism to maintain arterial blood pressure and ensure adequate

Respiratory Muscle Work and the

Intermittent-Sprint Work:
