October 13, 2008.
How do fish get O2 into their swimbladders?
Why doesn’t it leak out?
1. First observation – fish have evolved mechanisms to maintain neutral buoyancy – swimbladders.
A. Why bother maintaining neutral buoyancy?
1. minimizes the cost of staying at a particular depth (don’t have to swim to keep at a certain spot
2. Allows for a high proportion of skeletal muscles to be used for locomotion
3. Allows a high degree of maneuverability in 3D
B. Two types of swimbladders – physostomous & physoclistus.
1. Physostomous bladders have a connection to the gut. Fish go to the surface to gulp air and put into swimbladder
2. Physclistus bladders have no connection to the gut. Get air out of the bloodstream. This allows them to go quite deep.
2. Problem. Fish that use a physoclistus swimbladder go to great depths. To maintain neutral buoyancy, they put air into their swim bladder.
- At 500 m depth → 50 Atm of pressure on fish (50 Atm is 735 pounds)
Fish must have equivalent force upwards to be neutrally buoyant
draw this out –
40 Atm O2 and 10 Atm N2 in SB
in surrounding water, 0.2 Atm O2 and 0.8 Atm N2
Fish must pump O2 and N2 into bladder against a diffusial gradient and maintain it there.
3. Question: How do fish keep the gas in the swimbladder?
Draw out a small swim bladder, but do not label with gases yet. Label gas gland and rete mirabile
Solution #1: fat layer and guanine crystals around swim bladder maintain gases inside.
Solution #2: bump up [O2] in blood so it is higher than that in swimbladder – O2 diffuses into swimbladder
- gas gland
-very few mitochondria
- anaerobic metabolism à make & release lactic acid
-- also produce lots of CO2 -- more acid
recall CO2 +H2O ß> H2CO3 ß-à H+ + CO3-
Swimbladder #2 – draw big – show what happens in gas gland tissue, H+, CO2+, & O2
The counter current gradient acts as a trap to keep gases in the swim bladder.
Step 1 – Gas gland secretes lactic acid and CO2
- this happens because gas gland tissue has very few mitochondria – anaerobic respiration produces lactate; - also has another pathway to produce CO2
Step 2 – H+ ions in blood; CO2 also functions to increase H+ in blood
Step 3- Root & Bohr Shifts turn on – O2 is released from Hb
Step 3 – [O2] higher than that in swimbladder
Step 4 – As blood leaves gas gland, passes along counter current multiplier
-- H+ diffuses into arterial blood
-- O2 diffuses into arterial blood
Important thing here – if the “Root On” and “Root Off” processes happened at the same time, then O2 would be liberated in arterial blood and the trap wouldn’t work.
However, “Root On” is slower than “Root Off”. The effects of H+ don’t kick in until the gas gland (or possibly later). It’s o.k. if it’s on the venous side, because it will still diffuse across and bump up concentration in gas gland.
As CO2 and H+ diffuse to arterial side, pH ↑ and O2 binds to Hb.
Also, solutes in blood cause “salting out effect” by lowering O2 solubility in blood.
Draw in conditions along the blood vessels. Point out that the book has your figure flipped upside down.
Important to note
1. difference between total O2 in blood versus O2 tension in blood
Blood leaving rete mirable has a higher tension of O2 (due to root effect) but lower total O2 content.
-- O2 was “given up” to swim bladder
2. Small difference in O2 tension between venous & arterial blood is multiplied along the length of the capillaries.
a. longer capillaries produce higher multiplication effect
b. fish species in deep water have longer capillaries than those in shallow water
1. Capillaries in muscles (which are thought to be long) are often 0.5 mm
2. Capillaries in some deep sea fish are 25 mm long (50X that of muscle)
2. Other gasses besides O2
N2 and Argon are also high in swimbladders (higher than in water)
- may be due to “salting out” effect due to lactic acid – some fish may add NCl
- any solute added to blood will decrease the solubility of gas in blood
Review – 3 important things to get O2 into gas bladder
1. anaerobic respiration in gas bladder + release of lactic acid and CO2
2. counter current exchange along rete mirable
3. slow “Root on” (and to some extent, fast “Root off”
5. Unresolved issues
A. Some fish such as whitefish (Coregonus – in salmonid family) have nearly 100% N2 in swimbladder.
?How can this be? What type of mechanism only effect N2?
b. How is O2 (and N2) rapidly diffused from liquid into air form?
recall that diffusion in gills (and also in rete mirable) involves O2 and other gases dissolved in liquid. How do they rapidly get back into the gaseous stage?
6. Getting air out of the swim bladder.
draw swim bladder with both rete mirable and dorsal connection
dorsal connection involves small capillaries that sometimes make contact with wall of swim bladder. – surrounded by an oval patch which is a sphincter muscle
When keeping gas in, the sphincter is closed & capillaries don’t come into contact with swim bladder
When deflating, sphincter opens and diffusion into capillaries is possible.
6. Choroid Body in eye also has a rete mirable. Needs to maintain high levels of O2 for vision.