Bird’s eggs contain everything that the chick needs, except oxygen. So how exactly does the egg ‘breathe’?
First, the shell needs to be permeable to gases. The shell consists of a calcium carbonate outer layer, with two shell membranes beneath. A typical outer shell of a hen’s egg holds about 10,000 pores, each less than 0.02mm (approximately 0.017mm) in diameter, about 2.3mm2 pore area in total. Through these pores oxygen is taken in and carbon dioxide (CO2) released. However, the pores also allow water vapour to escape. Over the 21-day incubation period, a hen’s egg typically takes in 6 litres of oxygen, releases 4.5 litres of CO2 and loses 11 litres of water vapour. The rate of water loss depends on the porosity of the eggshell and the water vapour pressure gradient between the nest and egg.
Water vapour pressure is the pressure at which water evaporates at the same rate that it condenses. We can change this equilibrium point by changing temperature. At lower temperatures, the vapour pressure is less (fewer molecules have the kinetic energy to evaporate), and at higher temperatures the vapour pressure is higher (more molecules have the kinetic energy to evaporate). A liquid with a higher vapour pressure will evaporate more readily, and it will boil when its vapour pressure reaches the external, atmospheric pressure. For example, water has, at 100°C, a saturated vapour pressure equal to the atmospheric pressure at sea level (760 Torrs). If the atmospheric pressure decreases below 760 Torrs, the saturated vapour pressure of water and the atmospheric pressure would match at lower temperatures. For example, on Mount Everest, the low pressure associated with altitude means water will boil at about 70°C.
To avoid the egg losing too much water, whilst not jeopardising oxygen uptake, birds employ a number of strategies. The water vapour pressure gradient between nest and eggs is regulated by nest construction (insulation, humidity) and incubation patterns. The pore area of the egg is kept within certain biological limits: large enough for the egg to get enough oxygen, and small enough for the egg not to dry out. Species that lay their eggs at lower atmospheric pressures (higher altitude) tend to have lower pore area as the diffusion coefficient for gases is increased at altitude. This means that while more oxygen is delivered (good), more water vapour escapes (bad), and the lowered pore area at altitude is therefore necessary to prevent the egg from drying out. Some pollutants (such as aluminium) can increase shell porosity, causing water loss that is too great for the embryo to survive.
A pocket of air can be found between the inner and outer shell membrane on the blunt side of the egg, and this increases in size during the incubation as water is lost to the environment. This is believed to be the principal site of gas exchange, and there tends to be more and larger pores in this area. In the final day before hatching, the chick breaks through the barrier to this air space, and begins breathing with its lungs (pulmonary breathing).
Before pulmonary breathing is possible, gas exchange occurs via a membrane called the corioallantoic membrane. This is inside the inner membrane of the shell. Air diffuses through the shell and outer membrane into the area between membranes, where the capillary-rich corioallantoic membrane allows diffusion of oxygen into the blood and CO2 out of the blood. The blood is transported from the embryo to this membrane through corioallantoic arteries to the corioallantoic membrane, and oxygenated blood returns from the membrane to the embryo through allantoic veins.
The resistance to diffusion is mostly in the hard shell. The membranes have low permeability at first, but they increase in permeability quickly within the first few days after the egg is laid. After this, the egg doesn’t change its diffusion characteristics, and so the concentration of gases depends on the air composition outside of the egg and the metabolism (oxygen uptake, carbon dioxide production) of the embryo inside.
During the incubation period, the embryo grows and its metabolic demands increase. That means it uses more oxygen and produces more CO2 as it grows. As the partial pressures of gases outside of the egg don’t change, this means the gradients of gases between the inside and outside of the shell begins to increase. In short, oxygen concentration goes down and CO2 concentration goes up inside the egg over time. This change in pressure gradients of the gases across the shell and enables more oxygen to flow in (and more CO2 to flow out) as the embryo’s metabolism increases. Despite this, oxygen is reduced and CO2 increased inside the egg in a gradual change that continues almost until hatching. In this way, the ability of gas to diffuse through the shell places an upper limit on the metabolism of the growing embryo.
(Reptile eggs are a different kettle of …err fish, and tends to be more variable than bird’s eggs. Typically, reptiles use underground nests which have reduced oxygen availability, increased CO2 availability and increased humidity compared to the open nests seen in most birds. As discussed above, gas exchange depends in part on atmospheric conditions, and so reptile eggs’ respiration often depends greatly on the properties of the nest (and its interaction with the environment) as well as the properties of the egg itself. )