Air Breathing Versus Water Breathing – Answer Sheet
1: Oxygen not very soluble in water. A single liter of air contains about 209 ml of oxygen whereas one liter of water contains only about 5 ml of dissolved oxygen.
2:Water is about 1800 times more dense than air. This means that oxygen diffuses much more slowly in water than in air. (Actually about 300 000 times more slowly at 20°C)
3: Viscosity is the term used to describe how strongly the molecules of a substance stick together. The more they stick, the higher the viscosity . Water is about 100X more viscous than air. This means it takes about 100X more energy to move 1L of water than it does to move 1L of air. Because of 1, 2 & 3, aquatic animals have to work much
harder to ventilate their respiratory surfaces. Fish can spend up to 20 % of their total metabolic energy moving water across their gills.
Fish get the molecular oxygen they breathe from the molecular oxygen (O2) that is dissolved in the water , not from the atomic oxygen that is part of the oxygen molecule. Gills are outfoldings of the body wall. There are four gill arches that loop from just under the eye to the bottom edge of the mouth on each side. From the downstream side of each gill arch a double series of gill filaments extend, resembling a series of V=s when laid on their sides and facing upstream. The ends of these filaments from adjacent gill arches touch at their tips. Sticking up at right angles from the upper and lower surfaces of each gill filament is a series of
lamellae . Gas exchange takes place between the water and the blood through the thin walls of the lamella. The lamella have all the characteristics of efficient gas exchange tissues:
- their surface is very large
- well-supplied with blood vessels
- very short diffusion distance
Air Breathing Versus Water Breathing– Answer Sheet(Contd.)
A countercurrent exchange system is one in which water flows over the gill lamellae in a direction
opposite to the flow of blood in the lamellae.
Water is pumped over the gills from front to back and the blood
flows in the opposite direction. This arrangement maintains the largest difference in concentration
between the oxygen in the water and the oxygen in the blood over the entire length of the gill. As a result of this exchange, oxygen-rich water is in contact with the beginning part of the gill where the blood has the highest oxygen content; oxygen-poor water at the back of the gill is in contact with the part of the gill where the blood has the lowest oxygen content.
This countercurrent flow is extremely efficient in extraction of oxygen. Water and blood move in opposite directions. Water leaving the gill can have given up 80 to 90 % of its initial oxygen content.
Before exhaling, mammals remove only 25 % of the oxygen present in the air in their lungs. Gills are not suited for air breathing because the thin lamellae stick together when deprived of the support of water. The effective surface area for gas exchange is thus reduced . Some O2 can be extracted from air by gills, but not enough for a fish to survive on.