Evans D L
Vet Clin North Am Equine Pract. 1985 Dec;1(3):513-31. doi: 10.1016/s0749-0739(17)30748-4.
The cardiovascular system provides the link between pulmonary ventilation and oxygen usage at the cellular level. During exercise, efficient delivery of oxygen to working skeletal and cardiac muscles is vital for maintenance of ATP production by aerobic mechanisms. The equine cardiovascular response to increased demand for oxygen delivery during exercise contributes largely to the over 35-fold increases in oxygen uptake that occur during submaximal exercise. Cardiac output during exercise increases greatly owing to the relatively high heart rates that are achieved during exercise. Heart rate increases proportionately with workload until heart rates close to maximal are attained. It is remarkable that exercise heart rates six to seven times resting values are not associated with a fall in stroke volume, which is maintained by splenic contraction, increased venous return, and increased myocardial contractibility. Despite the great changes in cardiac output, increases in blood pressure during exercise are maintained within relatively smaller limits, as both pulmonary and systemic vascular resistance to blood flow is reduced. Redistribution of blood flow to the working muscles during exercise also contributes greatly to the efficient delivery of oxygen to sites of greatest need. Higher work rates and oxygen uptake at submaximal heart rates after training imply an adaptation due to training that enables more efficient oxygen delivery to working muscle. Such an adaptation could be in either blood flow or arteriovenous oxygen content difference. Cardiac output during submaximal exercise does not increase after training, but studies using high-speed treadmills and measurement of cardiac output at maximal heart rates may reveal improvements in maximal oxygen uptake due to increased stroke volumes, as occurs in humans. Improvements in hemoglobin concentrations in blood during exercise after training are recognized, but at maximal exercise, hypoxemia may reduce arterial oxygen content. More effective redistribution of cardiac output to muscles by increased capillarization and more efficient oxygen diffusion to cells may also be an important means of increasing oxygen uptake after training.
心血管系统在细胞水平上建立了肺通气与氧气利用之间的联系。运动期间，将氧气有效输送到工作中的骨骼肌和心肌对于通过有氧机制维持ATP生成至关重要。马在运动期间对增加的氧气输送需求的心血管反应，在很大程度上促成了次最大运动期间氧气摄取量超过35倍的增加。运动期间的心输出量会大幅增加，这是因为运动期间能达到相对较高的心率。心率与工作量成比例增加，直至接近最大心率。值得注意的是，运动心率达到静息值的六到七倍时，并未伴随每搏输出量下降，这是通过脾收缩、静脉回流增加和心肌收缩力增强得以维持的。尽管心输出量变化很大，但运动期间血压的升高仍保持在相对较小的范围内，因为肺和全身血管对血流的阻力均降低。运动期间血流重新分配到工作肌肉，这也极大地有助于将氧气有效输送到最需要的部位。训练后在次最大心率下能达到更高的工作率和氧气摄取量，这意味着训练带来了适应性变化，使氧气能更有效地输送到工作肌肉。这种适应性变化可能体现在血流或动静脉氧含量差方面。训练后次最大运动期间的心输出量不会增加，但使用高速跑步机并在最大心率下测量心输出量的研究可能会揭示，由于每搏输出量增加，最大摄氧量有所改善，就像在人类中发生的那样。训练后运动期间血液中血红蛋白浓度的改善是公认的，但在最大运动时，低氧血症可能会降低动脉血氧含量。通过增加毛细血管密度使心输出量更有效地重新分配到肌肉，以及使氧气更有效地扩散到细胞，这也可能是训练后增加氧气摄取的重要方式。
