In veterinary medicine, refillable CO₂ absorber canisters are still widely used; these contain granules that absorb the CO₂ exhaled by the patient. How full the canister should be filled depends, amongst other things, on the direction of the gas flow: If the respiratory gas flows into the canister from below (you can tell this by where it first turns purple: at the top or the bottom), a free distribution zone is automatically created there – the gas disperses before it even comes into contact with the lime, which makes the absorption more uniform. If the gas flows into the canister from above, the following must be taken into account: As a general rule, the canister must be filled to about two-thirds – never to the brim. This is because there must be sufficient air space between the lime granules to allow the gas to flow evenly through the entire lime bed. If the canister is too full, breathing resistance increases. If it is too empty, the gas seeks the path of least resistance and bypasses large sections of the lime – this is known as ‘channeling’. The result: poor CO₂ absorption, even though plenty of fresh lime is still present.