Additional Responses

During exercise, raising the heart rate is only part of the body's response. Several other changes happen together, so that respiring muscles get enough oxygen and glucose and waste products are removed.Stroke volume increases. More blood returns to the heart, so the ventricles fill more fully and are stretched more. Stretched cardiac muscle contracts more forcefully, and sympathetic impulses and adrenaline also strengthen contraction.Blood is redistributed. Arterioles supplying skeletal muscles dilate (vasodilation), because the smooth muscle in their walls relaxes. Arterioles supplying the gut and kidneys constrict (vasoconstriction). Blood flow to the brain is kept constant.Ventilation increases. Chemoreceptors detect the rise in carbon dioxide and send impulses to the ventilation centre in the medulla, which increases the rate and depth of breathing. This speeds up gas exchange in the alveoli.Blood glucose is maintained. Adrenaline and glucagon stimulate liver cells to convert glycogen to glucose.Extra heat is lost. Faster respiration releases more heat, so sweating increases and, as body temperature rises, skin arterioles dilate.Worked example: at rest, cardiac output is 5 dm³ min⁻¹ and 20% goes to skeletal muscle, which is 1 dm³ min⁻¹. During hard exercise, cardiac output rises to 20 dm³ min⁻¹ and 80% goes to skeletal muscle, which is 16 dm³ min⁻¹, a sixteen-fold increase.Together these responses match the supply of oxygen to the demand from the muscles.