Fish can breathe underwater. We all know this, right! But have you ever stopped to think how incredible that is? We can only breathe air and need a wealth of SCUBA equipment to survive underwater! Fish, like sharks, have a completely different respiratory system to our's - they extract oxygen from water. So how do they do this? How do gills actually work?
O My!
Many (but not all!) creatures require oxygen to live, as oxygen is vital for "respiration". Sift through those deep memories of your high school education and you might remember that respiration is a chemical reaction which occurs inside every single cell of the body to produce energy from sugars (Glucose + Oxygen -----> Carbon Dioxide + Water + Energy). This energy is critical for movement, thinking, metabolism ... everything! Without oxygen, we die.
Human beings breathe oxygen into the lungs from the air, but sharks and other fish are able to extract oxygen from water via their gills. The gills are also important for ion exchange, pH balance and excretion of waste. Being able to extract oxygen from water is quite a remarkable feat when you consider that oxygen levels are very low in water; only about 1% where air is about 20% oxygen saturation (Ebert et al, 2020, Abel & Grubbs, 2021).
The vast majority of sharks have five "gill slits" (a rare few species have six or seven gills), located on each side of their body, between the head and the pectoral fins. Sharks' gills are different to those of boney fishes, as they can be covered up by a "gill flap", where other fishes' gills are naked (Ebert et al, 2020, Abel & Grubbs, 2021).
The gills themselves are made of stiffened "gill arches", which support millions of reticuated "gill filaments". These structures are the site of gas exchange. Similarly to how a towel has fluffy fibres to maximise water absorption, these finger-like projections increase the surface area of the gills to maximise how much oxygen can be absorbed. In mako sharks (Isurus oxyrinchus), for instance, this surface area of the gill filaments would be 50,000 square cm if we flattened them out! (Ebert et al, 2020).
It's in my Blood
The circulatory system of sharks and other fishes is very different to that of humans. We have a double pump system whereby one half of the heart pumps deoxygenated blood from the heart to the lungs (where it picks up oxygen) and the other half of the heart pumps the oxygenated blood from the heart to the body (where it drops off it's oxygen).
Sharks have one closed circulatory system, with the heart pumping blood directly from the gills (where it picks up oxygen) to the body (where it deposits oxygen). The oxygenated blood does not need to go back to the heart first (Ebert et al, 2020, Abel & Grubbs, 2021).
The gills contain an enormous amount of capillaries to maximise the absorption of oxygen out of the water. These blood vessels carry the oxygenated blood away from the gills to fuel respiring tissues around the body (Ebert et al, 2020, Abel & Grubbs, 2021).
Going with the Flow
A shark's gills go all the way through the wall and open up into the shark's mouth. Therefore, water can flow into the shark's open mouth and back out through the gill slits (Ebert et al, 2020, Abel & Grubbs, 2021).
A few sharks move forwards to keep this water flowing. This is known as "ram ventilation". More commonly, sharks are able to draw water in by flexing the muscles of their mouth, so they don't need to keep moving. This is called "buccal pumping". Some sharks also have specialised openings called "spiracles" on the side of their head, behind the eyes, that aid in water flow (Ebert et al, 2020, Abel & Grubbs, 2021). To learn more about all these processes check out Myth Busted: Sharks DO NOT Have to Keep Swimming to Breathe.
Countercurrents
As water runs over the gill filaments on its way back out of the gill slits, dissolved oxygen moves from the water, though the surface membrane and into the shark's blood stream via "diffusion". At the same time, carbon dioxide goes the other direction (Abel & Grubbs, 2021).
But how does this happen? It's possible thanks to a remarkable physiological adaptation known as a "countercurrent exchange system"; where blood flow inside the gill filaments is in the opposite direction to the water flow on the outside of the membrane (Ebert et al, 2020, Abel & Grubbs, 2021).
If both of these fluids were flowing the same direction, the large difference in oxygen levels in the water versus the deoxygenated blood would mean that gas exchange via diffusion would happen very quickly at first. However, as oxygen concentration got higher in the blood, the oxygen level in the gill filaments would start to match that of the water; the concentration gradient would reach equilibrium and diffusion across the membrane would slow (or stop) (Abel & Grubbs, 2021).
However, the counter current system means that the oxygen gradient remains high all along the length of the gill filament. As the gills absorb oxygen, the concentration in the water goes down, but as the blood is flowing in the opposite direction, even when the oxygen concentration in the water is relatively low, it is still higher than in the blood. Similarly, even when blood oxygen concentration is relatively high, it is still lower than that of the water. This makes for very efficient gas exchange, as the whole length of the gill filament can absorb oxygen (Ebert et al, 2020, Abel & Grubbs, 2021). What an incredible feat of evolution!
References
Abel DC & Grubbs RD (2020). Shark Biology and Conservation: Essentials for Educators, Students, and Enthusiasts. Johns Hopkins University Press, Canada. IBAN: 9781421438368.
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
By Sophie A. Maycock for SharkSpeak
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