- The amount of carbon dioxide produced as a byproduct of aerobic metabolism varies with the material being used as a fuel source.
- The respiratory quotient (RQ) is the ratio of carbon dioxide produced per unit of oxygen consumed by the body. The fuel used by the body is a determinant of the RQ. For carbohydrate, it is 1.0; for fats, it is 0.7; and for protein, it is 0.8.
- Oxygen consumption is a measure of the metabolic activity of the body.
- The anaerobic threshold (AT) is the level of oxygen consumption at which the body begins to rely on anaerobic metabolism to meet the energy needs during exercise to such an extent that lactic acid accumulates in the blood.
- As soon as the AT has been reached, the body must compensate for the associated metabolic acidosis that occurs by increasing ventilation above and beyond what is needed to eliminate carbon dioxide produced from aerobic metabolism.
- The AT is determined by the capacity of the respiratory and cardiovascular systems to deliver oxygen to the muscles and by the ability of the muscles to use oxygen to support aerobic metabolism.
- Ventilation increases during exercise in three phases: the neurological phase, metabolic phase, and compensatory phase.
- Ventilation during exercise increases through a combination of enlarged tidal volume and increased respiratory frequency. Healthy individuals will double their tidal volume during moderate to severe exercise. Further increases in tidal volume are energy inefficient because of the reduced compliance of the respiratory system at higher lung volumes.
- The dead space to tidal volume ratio decreases during exercise in healthy persons because of the increase in tidal volume and because there is more even perfusion of the lung (hence, less alveolar dead space) as cardiac output increases.
- The alveolar–arterial oxygen gradient widens slightly during heavy exercise in normal individuals.
- The heart has an intrinsic pacemaker that establishes the rate at which contraction occurs. The pacemaker is under the control of the autonomic nervous system.
- Cardiac output is the volume of blood pumped by the heart each minute.
- The force of contraction of the heart muscle is determined, in part, by the volume of the ventricle at the end of diastole. The stretch of the cardiac muscle cells corresponding to left ventricular end-diastolic volume is the preload of the ventricle. The amount of blood ejected with each contraction of the ventricle is the stroke volume (SV).
- Contractility, the force with which the ventricle contracts, is affected by the autonomic nervous system.
- Systemic arteries (i.e., vessels serving all of the body except the lungs) dilate in the presence of hypoxia in the tissues. This is in contrast to pulmonary vessels, which constrict when exposed to low levels of PO2 in the alveolus.
- The accumulation of acid in the muscle causes the oxygen–hemoglobin dissociation curve to shift to the right, facilitating the release of oxygen from hemoglobin and subsequent diffusion of oxygen to the tissue.
- Oxygen delivery, the amount of oxygen transported to the tissues each minute, is the product of cardiac output and the arterial oxygen content of the blood.
- The amount of oxygen used by the tissues to support metabolic processes is reflected in the difference in the oxygen content of arterial and venous blood, the A-VO2 difference.
- The Fick equation allows you to calculate the cardiac output if you know the oxygen consumption and the A-VO2 difference.
- The heart increases cardiac output by a combination of increased SV and HR. In healthy persons, SV doubles during moderate to heavy exercise, and HR may increase two- to threefold.
- Stroke volume increases during exercise because of increased filling of the ventricle (caused by increased venous return leading to greater preload) and increased contractility (caused by activation of the sympathetic nervous system).
- The Starling curve illustrates the relationship between end-diastolic volume of the ventricle and the SV.
- During exercise, selective constriction of blood vessels serving the internal organs and dilation of arterioles in muscles results in the redistribution of an increased fraction of cardiac output to the muscles.
- During exercise, maximal sustainable ventilation is approximately 40 times the FEV1.
- Healthy individuals are limited in the amount of exercise they can perform by the cardiovascular system (the ability of the heart to deliver oxygen) and by the ability of the muscles to use oxygen for aerobic metabolism. A person’s level of fitness reflects both of these factors.