OXYGEN UPTAKE AND TRANSPORT

VO₂ and VO_2max

A few points from last week’s Blog as a recap:

1. Aerobic exercise means exercise fueled by oxygen.
2. The  rate of oxygen usage is directly associated with the intensity of exercise.
3. The rate of oxygen use is usually expressed as millilitres of oxygen per minute per kilogram of body weight (ml/min/kg) and is abbreviated as VO₂
4_. VO₂ at rest is about 3.5 ml of oxygen per minute per kilogram of body weight (ml/min/kg). This known as one MET (metabolic equivalent).
5. Intensity of exercise can be expressed as VO₂ or as number of METs.
6. As an example, pottering about uses between one and three METs (3.5 to 10.5 ml/min/kg), mild to moderate exercise uses about three to six METs (10.5 to 21ml/min/kg) and vigorous exercise uses more than six METs.
7. Exercise to exhaustion, maximum exercise, is defined as maximal oxygen uptake or VO_2max.
8. VO_2max is the best measure of aerobic fitness, also known as cardiorespiratory fitness.

Reaching VO_2max

Exercise to VO_2max can only be attained using the large muscle groups of the legs. Because of their smaller bulk, maximum arm exercise will only achieve about two thirds of maximum leg exercise. Also, once maximum oxygen uptake has been reached with leg exercise, bringing other muscles, like the arm muscles, into action will not increase oxygen uptake further. The limiting factor is not muscular effort but the ability of the lungs and heart to supply oxygen to the muscles.

Oxygen transport during exercise

Going from the resting state to exercising, increased oxygen uptake and transport are achieved by several changes:

Ventilation or breathing

Faster and deeper breathing brings more oxygen into contact with the capillary blood in the air sacs in the lungs. For people without lung disease the saturation of oxygen in the blood does not fall with exercise, indicating that breathing and the lungs are not the limiting factor for exercise or for VO_2max.

Cardiac output or the amount of blood pumped out by the heart

At rest the normal heart rate (HR) is about 70 beats per minute. With each heart beat about 70 to 80ml is pumped into the circulation – this is the stroke volume (SV). So the volume pumped out each minute is about 70 times 75 which comes to roughly 5.5 litres per minute – this is the resting cardiac output (CO).

With increasing exercise the CO rises steadily with oxygen uptake to the point of VO_2max at which it levels off – at about 20 to 25 litres per minute. This four to five-fold increase in Cardiac Output is mediated by a more than doubling of heart rate to 180 to 200 per minute and a less than doubling of stroke volume to about 130ml ie the heart beats more rapidly and pumps out more blood with each stroke. There is a straight line relationship between increasing heart rate and increasing oxygen uptake.

The fit individual can perform a heavier workload then the unfit at any given heart rate. Since getting fit by exercise training does not change the maximum heart rate, the greater exercise level which can be achieved by the fit individual is mediated by an increase in stroke volume combined with more efficient oxygen use by the muscles.

In adults, maximum heart rate reduces gradually with age and is approximately 220 minus age. So a 20 year old exercising maximally should reach a HR of about 200 while for a 70 year old the rate would be around 150.

Extraction of blood by the muscles

The arteries supplying the muscles carry a full load of oxygen – the haemoglobin molecules are fully saturated with oxygen. The amount of oxygen extracted from the blood by the muscles depends on how hard the muscles are working. At rest the muscles extract about 5.5ml oxygen for every 100ml of blood flow. This rises to about 16ml oxygen during maximal exercise. This is achieved partly by an increased rate of oxygen uptake by each muscle fibre and partly by an increased blood flow to the muscles. Working muscles may use up to 18 times as much oxygen as they do at rest.

Combined effect on oxygen uptake

For a young person of average fitness, the increase of VO₂ from rest to maximum exercise is about 12 fold – mediated by a fourfold rise in cardiac output (HR up by 2.7 times and SV up by 1.4 times) and a threefold rise arterio-venous oxygen difference.

Variations in Physical Fitness/VO_2max

VO_2max varies with age, sex and habitual physical activity. As time goes by, maximal heart rate and stroke volume both decline as do muscle bulk and strength so that the fall in VO_2max each year is between 0.5 and 1 ml/min/kg body weight. Although the older you are the less fit you become, the variation between individuals is much greater than the variation with age – mainly because of the effect of habitual activity. Women, who have smaller frames and smaller hearts than men but more fat, have about a 20% lower VO_2max.