Measuring metabolic rate
The sum of all metabolic reactions in the body can be measured as a function of oxygen consumption, carbon dioxide production and heat production. Oxygen is needed in aerobic respiration, carbon dioxide is a byproduct of respiration, while heat is lost energy by many reactions.
The higher these variables, the higher the metabolic rate is. Due to the big range of metabolic rates between organisms at rest versus active, for comparison purposes the resting metabolic rate is obtained.
There is a positive correlation between the volume of oxygen used per hour and the body mass of animals. Unsurprising!
Adaptations in organisms with different metabolic rates
Given that larger animals have accordingly larger metabolic rates, more efficient oxygen delivery is needed to sustain them. Therefore, adaptations in heart chambers, circulation and lungs can be seen in amphibians, reptiles, mammals, birds and fish (excepting lungs in fish).
Amphibians such as frogs and salamanders have two distinct circulatory systems between the juveniles and the adults. The young tadpoles carry out circulation without specialised organs in a single circulation similar to that of fish, using gills. Adults develop lungs and have a heart with one ventricle and two atria. The blood is pumped to the lungs via the pulmonary artery, while body-wide pumping of blood takes place at the same time.
The heart’s anatomy prevents overlapping of the two pathways. Not having ribs, breathing is achieved by gulping air then raising the jaw and tongue to create positive air pressure that enables oxygen uptake in the lungs, as the air is pushed inside. Some air ends up being pushed out through the nostrils at the same time. Indeed, this is that throat-expanding, repetitive movement frogs do which can be accompanied by loud noises.
The very wet skin also contributes to circulation as gas exchange takes place across it with the air. It is richly vascularised for this purpose.
Reptiles such as turtles and lizards have three-chambered hearts and circulation which includes mixed blood i.e. oxygenated and deoxygenated. Crocodiles have four-chambered hearts resembling those of mammals and birds. They, however, have two aortas. Having two aortas enables circulation that can bypass the pulmonary circulation.
How much of the blood is mixed can change based on the species as well as activity level. It may be particularly useful as a contributor to good thermoregulation, and in species that spend long times in deep water, by maximising use of the oxygenated blood.
The heart has two aortas, and despite only having three chambers, can act as if it had four chambers in some species e.g. pythons. This is enabled by a muscle ridge in the ventricle that fully separates it into two sections during ventricular systole.
Fish have a single circulatory system, while mammals have a double circulatory system. This refers to the mammalian double circulation of blood to the lungs for oxygenation, and separately the circulation of oxygenated blood around the body.
Fish only carry out single circulation of blood around the body, as blood gets oxygenated directly through gills which are en route to blood vessels leading to the heart.
Mammals have a double circulation system which involves:
1. Deoxygenated blood being pumped by the heart to the lungs for more oxygen
2. Oxygenated blood being pumped by the heart to the rest of the body
Having both a pulmonary circuit and a systemic circuit optimises the time taken for blood to oxygenate fully, as it’s slower to move through the lungs (giving more time for diffusion to take place), as well as a better blood flow rate to all the tissues of the body. Oxygenated blood is pumped fast and under high pressure to the rest of the body, while deoxygenated blood can take its time to reload with oxygen from the lungs.
Birds have a heart and circulation comparable to mammals. Their air ventilation process, however, is quite distinct. Upon inhaling, most of the air does not go in the lungs directly. Only about a quarter does this, while the rest is directed into one of the many bird air sacs, specifically the posterior air sacs which are also connected to the spaces in the bird’s bones. Upon exhaling, this extra air is pushed into the lungs, resulting in a constant supply of oxygenated air.
The used up air collects in the anterior air sacs and gets expelled during expiration.
Living under low oxygen
There are environmental niches where atmospheric oxygen is lower than elsewhere. Notably, these are high-altitude areas and the deep sea. Animals evolve adaptations to these low oxygen niches to be able to continue staying active and surviving. Supplying themselves enough oxygen to support this via aerobic respiration takes place through a variety of means.
Some fish supplement their usual oxygen intake via their gills by gulping air from the atmosphere above water and storing it in a special organ called air-breathing organ.
Behavioural changes also contribute to maximising oxygen use e.g. spending more time in shallower water, being less active and moving around seasonally.
Rising to the top layer of water which is in contact with the air and ventilating their gills there is also a method used to improve oxygen uptake, compared to simply increasing ventilation where there is a lower concentration of dissolved oxygen in the water. This practice is called aquatic surface respiration.
Seals that hunt deeper in the ocean can hold their breath for more than an hour. Their brain also contains special proteins, globins, that protect their brain from low oxygen by storing it.
Haemoglobin changes are also associated with different oxygen concentrations in the environment. Each red blood cell is packed with haemoglobin molecules that bind and release oxygen for cellular respiration. The balance of high affinity for oxygen versus low affinity for oxygen corresponds to the environmental partial (21% of air) pressure of oxygen.
A high affinity for oxygen means that as soon as oxygen is available, it will be bound by haemoglobin even at very low concentrations. The problem is that haemoglobin needs to actually release the oxygen in cells if it is to be used, which also corresponds to higher need at lower oxygen concentration (and higher CO2 concentration, as the cell undergoes respiration).
If we take, say, 80% saturation, the corresponding partial pressure of oxygen is about 35 mm Hg for high altitude deer mice, and about 50 mm Hg for low altitude deer mice. What does this mean? It means that high altitude deer mice’s haemoglobin binds oxygen at a lower partial pressure than that of low altitude deer mice. Essentially, their haemoglobin is able to make better use of less oxygen. Why?
Well, let’s think, what do we know of these mice? Some live at high altitudes, and others live at low altitudes. What do we know about altitudes that is relevant? We know that there is less oxygen at higher altitudes in the atmosphere. Hence the haemoglobin of high altitude deer mice has evolved to bind oxygen at lower partial pressures – where there is less of it available. This confers a clear advantage in terms of survival.
Maximum oxygen uptake and fitness in humans
Maximum oxygen uptake does what it says on the tin and is denoted by VO2 max which refers to the maximum volume of oxygen.
It represents the amount of oxygen used up through breathing during incremental exercise that exerts the body to that maximal point. For example, a patient or athlete can be hooked up to a respirometer that records ventilation (breathing), oxygen concentration and carbon dioxide concentration in the air inhaled and exhaled, while using a treadmill.
The data can be expressed as volume of oxygen per time i.e. litres of oxygen per minute, L/min, or more adjusted for weight in volume of oxygen per weight per time i.e. litres of oxygen per kilogram per minute.
For example, the highest recorded VO2 max was 97.5 mL/(kg*min) in cyclist Oskar Svendsen.
Maximum oxygen uptake is a measure of cardiovascular and aerobic function used in the world of sport as well as in healthcare. For some patients who would benefit from not undertaking the interval exercise method of measuring VO2 max, alternative methods are used. These do not get to measure the actual maximum itself, and are therefore sum-maximal tests.