What is Gill? — Biology

Definition 21.4 University Biology — Year 1 · Chapter 21 — Respiratory Gas Exchange

A gill is an evagination of the body surface into the water, folded into filaments and lamellae a few micrometres thick through which blood flows in capillaries. A fish’s gills hang from four bony arches on each side; water drawn in at the mouth and pushed out under the gill covers by a two-pump system flows between the lamellae in one direction, and the blood in the lamellae flows the other way: a counter-current. A trout of a kilogram has some $2000\,\mathrm{cm}^{2}$ of lamellar surface, $2\,\text{µ}\mathrm{m}$ thick.

A gill arch with its filaments and their lamellae. Water flows between the lamellae; blood flows through them the opposite way.

Co-current and counter-current exchange with the same surface and flows. Opposing the flows keeps a gradient along the whole length and lets the blood approach the fresh water’s oxygen pressure.

Examples

Example 21.6 (The price of water)

A resting trout of $1\,\mathrm{kg}$ takes about $50\,\mathrm{mL}$ of oxygen an hour. From water at $7\,\mathrm{mL}/\mathrm{L}$ , with $80\,\%$ extraction, it must pump $9\,\mathrm{L}$ of water an hour across its gills — nine kilograms of a medium fifty times more viscous than air, through channels a fraction of a millimetre wide. A tenth of its metabolism goes to breathing; a human at rest spends a fiftieth. Warm the pond to $30\,{}^{\circ}\mathrm{C}$ and the same fish, needing more oxygen from water holding less, must pump three times as much.

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