The swimbladder has an interesting way of retaining gases. The inner lining of the swimbladder consists of numerous gland cells collectively called the gas gland. When the gas volume within the swimbladder is low this gland reacts by releasing lactic acid into the blood, acidifying it. With acidification, the blood releases oxygen where it accumulates in the rete mirabile (Pough 1999). The rete mirabile ("wonderful net") is a bundle of close-lying arterial and venous capillaries that diffuse gases between one another as blood is carried through them, in and out of the gas gland. The rete mirabile, much like the oval, requires a large surface area for optimal contact between the arterial and venous capillaries (Marshall 1966). The diffusion of oxygen out through the bladder wall is blocked by a thin layer of crystalline guanine, thus also maintaining the volume of the swimbladder.
In order to restore gases to a deflated, closed swimbladder, the rete mirabile and the gas glands must create a combined pressure greater than that pressure within the swimbladder. Oxygen, having been forced off hemoglobin by lactic acid, is retained in the venous capillaries of the rete. Because of the rete's parallel arrangement of venous and arterial capillaries, a countercurrent multiplier effect occurs (Figure 4). This is when gases are able to diffuse from a higher oxygen volume in the venous to the lower oxygen volume in the arterial capillaries, therefore keeping more oxygenated blood within the loop of the rete (Schmidt-Nielsen 1997). When the pressure in the rete exceeds that of the swimbladder, oxygen diffuses out of the rete and into the bladder. The idea of multiplication comes into effect when we look at the varying lengths of capillaries that make up the rete. The longer the capillaries are, there is a greater surface area for gas exchange. So, with longer capillaries, a greater oxygen pressure can be created (Pough 1999). This can be most important to those fish who spend the majority of their time at great depths. With an increase in swimbladder pressure as a result of the greater water depth, the fish would have a harder time keeping gases from being lost from the system. Therefore, those fish found in deep waters, where the pressure is greatest, generally have longer capillaries that make up their rete so that there is a greater surface area for gas exchange (Marshall 1966).
Figure 4: Swimbladder vascular organization.
*Adapted from Fig 8-5,
Pough, 1999 and Fig 10.38 Schmidt-Nielsen
1997.