Mammals, from an Etruscan shrew (0.0018 kg) to the blue whale (180,000 kg), span a 100 million range of body mass. All mammals are formed of the same kinds of cells, tissues and organs and to similar overall body plan.
In this feature, we look into the design of the oxygen transport system, fundamental to sustaining life, in terrestrial mammals.
All living beings need a constant supply of energy (capacity to do work) to survive. Even when you are resting, your body uses energy to perform essential life-sustaining functions - the rate of energy used is called the base metabolic rate (BMR). Energy is produced by the mitochondria in each cell by using oxygen to break down nutrients (mainly glucose but also fat & amino acids) into ATP, CO2 & water. Details of the energy production in cells are available in my article.
This feature is concerned with the transport of oxygen from the atmosphere to the mitochondria in each cell of the body. Additionally, this article also discusses how the oxygen transport system scales in mammals - from the tiny 30 gm mouse to a 6000 kg elephant (a range of 200,000 in body mass). The oxygen transport system used by all mammals may be described, in the simplest terms, as follows:
Oxygen from the atmosphere is taken up by the lungs, where in the alveoli (Latin - little sacs) it diffuses to the red blood cells (RBC) and attaches to the haemoglobin. The blood circulation system (maintained by the pumping heart) takes the oxygen-saturated blood to individual cells where the oxygen diffuses to the mitochondria to initiate the cellular energy production by breaking down glucose. The waste products of CO2 and water are transported from the cell by the blood for disposal.
BMR & Animal Size: The metabolic energy is converted into heat that must be disposed of, mostly from the skin, to maintain a constant body temperature. While the body mass M increases as the cube of the linear dimensions (L^3), the surface area S only goes up as its square (L^2). This suggests that in larger animals, the ability to dispose heat only increases as M^2/3. The rate of heat production at rest (BMR), generally measured as the rate of consumption of oxygen, should only increase as 2/3 power of the mass or as M^0.67. This is close to M^0.75 variation observed in measurements; however, due to the difficulty in measuring BMR, there is some uncertainty in the exponent and this is a matter of considerable debate. The situation is shown in slide 1 (some data for birds is also shown).
Slide 1:
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Mammals are warm blooded animals and maintain a near constant core body temperature at all times. The body rectal temperature shows no discernible trend with body mass and is close to 38±1.5C (Slide 3). 
It might be worth pointing out that being warm-blooded (endotherms) gives the mammals a definite evolutionary advantage. Because they generate their own heat, warm-blooded mammals can potentially inhabit all geographical areas independent of the climate. They also produce steady supply of energy and can sustain complex energy demanding organs like the brain.
Details of mammalian lung structure and function are described here (1).
Breathing through lungs works like tidal waves - we inhale oxygen-rich air into lungs and exhale oxygen-depleted air out. By studying how lungs transfer oxygen to the blood for a range of mammals, we can draw some interesting conclusions. In Appendix 1, I list some of the respiratory variables that have been measured and provide the results summarized in slide 5.
Slide 3:
It might be worth pointing out that being warm-blooded (endotherms) gives the mammals a definite evolutionary advantage. Because they generate their own heat, warm-blooded mammals can potentially inhabit all geographical areas independent of the climate. They also produce steady supply of energy and can sustain complex energy demanding organs like the brain.
Note:-- Seals and whales are aquatic mammals with mean body temperatures of 37.8 and 36C respectively - similar to terrestrial mammals. But because they live in water that more efficiently removes heat from their skin, seals and whales have a BMR that is about twice as high as other mammals.
Warm blooded mammals maintain their body temperature within quite tight limits - to do so they must continually generate energy by burning glucose in the mitochondria. Oxygen must also be supplied continuously through breathing.
We next study the empirical evidence as to how the size of the important elements of the oxygen transport system vary with body mass. We look at these in the following:
The Lungs: We breath air (21% oxygen) with our lungs. Lungs must have adequate capacity to supply sufficient oxygen to meet the requirements to maintain BMR. We had noticed in slide 2 that a 0.1kg mammal has a BMR* that is 10 times higher than a 1000 kg mammal. However, the empirical data (slide 4) shows that the lung volume relative to the body mass (or body volume - all mammals have a density equal to 1 unit) is nearly the same for all mammals. The question then arises as to how do small mammals supply sufficient oxygen to maintain their base energy requirements?
Slide 4:
Details of mammalian lung structure and function are described here (1).
Slide 5:
The total volume of blood in mammals is 60 to 70 mL per kg body mass (M). Blood volume is a size independent parameter - blood in mammals makes up a constant fraction of body mass.
Blood consists of 45% red blood cells (RBC) and 55% blood plasma. Over the vast range of body sizes, mammalian blood has the same oxygen carrying capacity - equal to 170 mL oxygen per litre of blood.
In the design of the blood and the circulatory system, it is desirable that maximum amount of oxygen is extracted from the air in the lungs (Hb must have a high affinity to oxygen in the lungs), but at the same time, oxygen must be easily released from Hb in the cell environment (requires Hb to have a low affinity to oxygen in the cells).
******************* Red blood cells are erythrocytes (erythros is 'red' and kytos is 'hollow vessel or cell' in Greek). There are 20 to 30 trillion RBCs in the body and make up to 70-80% of all cells. In mammals, there are 5 trillion RBCs per mL of blood. RBC lifespan is about 120 days, and more than 2 million are created per second in the bone marrow (erythropoiesis) *******************
RBC size in all mammals varies over a fairly narrow range of 5 to 8 µm without any correlation to body size. RBC are round, biconcave discs, easily deformed and pass through pulmonary capillaries that are about 8 µm diameter (7.48 µm in humans) and have a wall thickness of 0.5 to 1 µm - the short path allows for efficient diffusion of oxygen and CO2 through the capillary walls. RBC (they have flexible shapes) travel through the capillaries in single-file. The alveoli blood capillaries (see Appendix 1) cover approximately 70% of the alveolar surface area (in human lungs, total alveolar surface area is 70 to 80 m^2 - roughly the size of a tennis court!!).
Oxygen Uptake and Delivery: Ideally, blood at the alveoli should pick up maximum amount of oxygen from inhaled air, carry and deliver it to the tissue cells. At the same time, blood collects waste CO2 from tissue cells and dumps it into the lungs where it is exhaled into the atmosphere. For a detailed and very readable description of this process, click here and here. I describe the main features in Appendix 2 (The discussion in appendix 2 may be missed without loss of continuity).
The Mammalian Heart: The heart is the pump that circulates blood through the blood vessels. Slide 6 shows the structure and function of the heart. It is a four-chamber pump with the right ventricle pumping deoxygenated blood to the lungs where it picks up oxygen (and discards CO2). Oxygenated blood from the lungs is circulated to all cells of the body by the pumping action of the left ventricle muscles. The mean arterial pressure (MAP) in mammals is around 100 mmHg (slide 7).
Slide 6:
The function of the heart is not only to supply oxygen and nutrients (dissolved in the blood) to all body parts, but also to produce mean arterial pressure (MAP) that is adequate to maintain glomerular filtration in the kidneys. Glomeruli are tiny blood vessels in the kidneys and require an internal pressure of about 50 to 60 mmHg to push fluid and waste out of the blood into the urine.
In larger mammals, in many cases, the heart must maintain a higher pressure to ensure uninterrupted supply of blood to distant body parts (for example, to supply sufficient blood to the brain in giraffes, a MAP of 220 mmHg is required).
Slide 7:
Slide 8:
Heart Rate and Body Mass in Mammals: It was indicated above that smaller mammals use relatively larger amounts of energy from the surface and have correspondingly larger energy requirements than their bigger cousins. Also, as the lung and heart sizes scale linearly with body mass, smaller mammals generate energy by having a higher heart rate - while an elephant heart beats 30 times a minute, a tiny shrew's heart might beat more than 1000 times per minute.
Slide 9:
Additionally, efficient energy generation is achieved by respiring oxygen in the mitochondria. The empirical evidence suggests that larger mammals have a lower density of mitochondria (where oxygen is used to produce energy) and blood capillaries surrounding lung alveoli (where new oxygen is collected) than the smaller mass mammals.
Slide 9:
The implications of these observations are profound - all mammalian hearts have similar mechanical capability but they produce less metabolic energy per beat in larger mammals. The metabolic efficiency per heart beat decrease from 44% in a 2g shrew to 19% in a 6-ton elephant. If one extrapolates this trend to higher mass mammals then this reduction in energy production with body mass might put a limit as to how big a mammal might possibly be.
A second interesting observation is the heart beat rate - 1000 beats per minute in a shrew. This only allows 60 msec to complete a heart beat. Considering the muscle relaxation times and other physical constraints, there must be a limit to high fast a heart can pump blood round the body. This observation points to a lower limit to the mammal body size - possibly similar to the shrew at about 2 g.
Final Word: This feature has discussed how the gas transport system in mammals is organised. Most of the available data are for animals at rest (except for humans) - these are the easiest to measure. Animals do not always sit at one place and exert themselves that requires additional energy. They achieve this by increasing the heart rate - inspiring more oxygen per minute. During activity, animal's bones, muscles etc are put under additional pressure- we see ace athletes, tennis players suffering from broken bones, pulled muscles etc. - what is the safety factor in the design of individual body parts is an interesting question.
Another aspect that we have not looked at is adaptation. The discussion here has largely been for the animals living at sea level in temperate climates. Animals do adopt their physiology faced with adverse situations over lengthy periods of time. This is a subject I would like to return at some stage in the future.
Appendix 1: Lung structure and Respiratory Variables
A detailed description of the anatomy and function of the respiratory system is available in Chapter 6 of lumenlearning.com. I highly recommend this Online Resource.
Appendix 2: All about partial pressures of gases and the Hb-oxygen dissociation curve in blood
There are two main features that we need to understand here - (a) the properties of Hb and (b) partial pressures of oxygen and CO2 at the alveoli and the tissue cells. These are discussed below:
(a) The Haemoglobin molecule (Hb) : Hb is a globular protein composed of four subunits that contain a non-protein heme group with a central iron atom Fe++. Each iron atom can reversibly bind to one molecule of oxygen, allowing a single Hb molecule to carry up to 4 oxygen molecules.
Oxygen partial pressure pO2 is 104 mmHg in lungs sufficient for Hb to reach saturation. This is because air in the lungs is continuously replenished by the oxygen-rich air we breathe. Through the thin walls of alveolar blood capillaries, oxygen diffuses to arterial blood with a partial pressure of about 95-98 mmHG. In tissue cell, oxygen is consumed to produce metabolic energy resulting in a low pO2. The pressure gradient between oxygen across the thin cell wall causes diffusion to set-in forcing a release of some of the oxygen attached to the Hb (see slide above). Arterial oxygen diffuses into the cells, the outflowing blood in the veins has a lower pO2 of around 40 mmHg.
The oxygen dissociation curves vary with mammal size as shown in the next slide.
The curves shift to the right in smaller mammals that means that in the tissue at pO2 of 40 mmHg, Hb dumps a lot more oxygen in the tissue cell than it will for higher mass mammals. The extra oxygen helps to maintain a higher BMR* in smaller mass mammals.
Appendix 3: On Gas Partial Pressures
The partial pressure of a gas is the pressure that a single gas in a mixture would exert if it alone occupied the entire volume of the mixture at the same temperature.
The total pressure, P, of an ideal gas mixture is the sum of the partial pressures of all it constituent gases P = P1 + P2 + P3+ ...
Partial pressures of gases in the atmosphere
Gas % abundance Partial Pressure
(mmHg)
Nitrogen (N2) 78.6 597.4
Oxygen (O2) 20.9 158.8
Water (H2O) 0.04 3.0
CO2 0.004 0.3
Other Gases 0.0006 0.5
TOTAL 100 760 mmHg
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In the lungs & alveoli, the partial pressures are modified as the air we breath picks up moisture from the nose and trachea. Also CO2 diffuses in to the alveoli sacs from veinous blood.
Partial pressures of gases in the lungs, alveoli, arterial & veinous blood (in mmHg)
Gas Alveolar Arterial Veinous Tissue Cell
air blood blood blood
N2 560 570 570 570
O2 104 98 40 low
H2O 47 47 47 47
CO2 40 40 45 46
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Total pressure of gases adds to 760 mmHg at sea level to balance the outside pressure on the body. Blood is saturated with water vapour. pO2 in tissue cells, particularly in mitochondria, is ~10 mmHg as oxygen is consumed to produce aerobic energy.
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