Bernd Pelster

The Physiology of the Root Effect

In 1931 Root reported that the presence of CO2 markedly reduced the oxygen content of the blood of toadfish, sea robin, and mackerel, and caused the oxygen dissociation curve of the hemoglobin to become “nearly asymptotic with respect to the abscissa before saturation is complete”, which is still a valid description of the Root effect (cf. Brittain 1987). The same effect is obtained by acid addition to the blood (Root and Irving 1943).

5 Buoyancy At Depth

Publisher Summary This chapter describes the problem of buoyancy faced at deep-sea levels and discusses the swim bladder function of fishes. Fish produce hydrodynamic lift mainly by using their pectoral fins as hydrofoils. The metabolic power needed to propel the hydrofoils through the water can be calculated from drag on the hydrofoils and speed. Another strategy to achieve neutral buoyancy is to build up and maintain a buoyancy device—that is, compensate for the high density of most tissues by including special structures or organs characterized by a very low density. In many species, the swim bladder consists of two chambers, including a thick-walled section in which gas can be deposited and a thin-walled chamber in which gas can be resorbed. In other fishes, the resorbing part of the swim bladder is reduced to a special section of the secretory bladder, called “oval,” which can be closed off by muscular activity. The reduction of the effective gas-carrying capacity of swim bladder blood is brought about by the metabolic and secretory activity of epithelial gas gland cells. The specific gravity of gas increases with gas pressure; hence, the difference between swim bladder gas density and water density decreases with increasing water depth.

CO2 back-diffusion in the rete aids O2 secretion in the swimbladder of the eel

In order to estimate the relative importance of back-diffusion of O2 and CO2 for their partial pressure enhancement in the swimbladder, we have determined O2 and CO2 partial pressure and content and pH in microsamples collected non-obstructively from the afferent and efferent rete vessels in the European eel. 1. The PO2 increased significantly along the arterial vessels of the rete (from 33 to 303 Torr), with no change in O2 content, suggesting O2 not to be exchanged in the rete counter-current system. 2. A corresponding increase of PCO2 (from 4 to 35 Torr) was accompanied by a significant rise in CO2 content (from 8 to 15 mmol.L-1), suggesting significant CO2 back-diffusion in the rete. 3. Changes in PO2 during passage of blood through the swimbladder epithelium were variable and small, and the PO2 in rete venous blood was similar to that in rete arterial blood, explaining the lack of O2 back-diffusion. 4. Using blood CO2 dissociation data, about 70% of the rise in arterial CO2 content was estimated to derive from diffusion of CO2, the remaining 30% from diffusion of HCO3-, from venous to arterial rete capillaries, or from H+ transport in the opposite direction. The data indicate that CO2 back-diffusion in the rete does not only raise the rete arterial PCO2; it also reduces the O2 capacity (Root effect) and thus enhances the arterial PO2.

Water and lactate movement in the swimbladder of the eel, Anguilla anguilla

Hemoglobin (Hb) and lactate (La) concentrations were measured in small (150 microliters) blood samples collected with micropipettes from the inflow and outflow vessels of the rete mirabile of the eel swimbladder. Hemoglobin was used as a marker of the intravascular space. 1. Hemoglobin concentrations suggest that there was no significant water movement between the arterial and venous capillaries in the rete, but a significant 6% water efflux from the vascular space into the swimbladder epithelium. No significant differences in osmolality were observed between the sites of measurement. 2. Of the lactate present in the blood entering the venous capillaries of the rete, 30% derived from release by the swimbladder epithelium; 38% of the lactate entering the venous capillaries diffused back in the rete tissue into its arterial capillaries. 3. Theoretical models suggest that any water movement in the swimbladder, leading to blood flow mismatch in the rete counter-current system, reduces its efficiency to concentrate inert gases, whereas lactate back-diffusion enhances this efficiency.

Significance of the Bohr effect for tissue oxygenation in a model with counter-current blood flow

Counter-current arrangement of afferent and efferent blood flow in tissues is commonly considered to be detrimental to tissue oxygenation, since O2 diffusion would shunt O2 away from the tissue. We have investigated the combined effects of counter-current CO2 and O2 exchange in a simple model, paying particular attention to the Bohr effect. We have obtained the following main results. (1) Back-diffusion of CO2 leads to increasing CO2 partial pressure (PCO2) and CO2 content along the afferent vessel. This is enhanced when fixed acid is released by the tissue into the venous blood, e.g. during hypoxia, which leads to a further PCO2 increase therein. (2) The increasing PCO2, with concomitant decrease in pH, in the afferent blood leads to a decrease in blood O2 affinity (Bohr effect) and thus results in increased PO2. (3) The resulting O2 diffusion shunt diminishes the O2 content in afferent blood, but for most conditions its PO2 remains higher than without the Bohr effect. (4) During hypoxia, both the PO2 in blood reaching the tissue (Pta) as well as in that leaving it (Ptv) are significantly elevated above the level without the Bohr effect. Moreover, with fixed acid release both Pta and Ptv for O2 can be higher than the arterial PO2 value. (5) During hyperoxia, O2 diffusion shunt prevents the tissue PO2 levels from increasing to levels that might be regarded as toxic. It is concluded that a diffusion shunt in tissues stabilizes the O2 partial pressure at the tissue when it varies in arterial blood (hypoxia or hyperoxia).

Solute back-diffusion raises the gas concentrating efficiency in counter-current flow

We have extended the counter-current model of the rete mirabile of the fish swimbladder to include the effects of inert gas secretion into the swimbladder as well as solute back-diffusion in the rete capillaries. (1) Gas secretion attenuates the inert gas concentrating efficiency of the rete, i.e. its ability to produce high inert gas partial pressures in blood at the swimbladder pole. The maximum attainable gas secretion rate depends on the salting-out effect, i.e. on the ratio of solubility in venous and arterial blood of the rete. (2) Solute back-diffusion leads to a significant increase in the concentrating efficiency, and this is due to the salting-out effect produced by the solute when it diffuses back into the arterial capillary blood. This enhancement is particularly prominent when gas and solute permeabilities of the rete vessels are high. (3) Estimates for physiologic parameters in the European eel suggest that lactate back-diffusion may contribute significantly to the gas concentrating efficiency of the rete mirabile.

Gas Exchange in the Fish Swimbladder

The fish swimbladder acts as a device to adjust for neutral buoyancy at various depths. High gas pressures, corresponding to the ambient hydrostatic pressure, are encountered, most of which is made up by O2 and N2. To prevent gas loss, the swimbladder wall is made impermeable by guanine crystals in its wall. Gas deposition is made possible by lactic acid production in the swimbladder epithelium, which increases blood gas partial pressures of inert gases (salting-out effect), O2 (Bohr and Root effects) and CO2 (conversion from HCO3-). The hairpin counter-current blood flow in the rete mirabile enhances this partial pressure increase to the tremendous values, up to several 100 atm, encountered in deep sea dwellers. Flow balance in the rete capillaries is found to be crucial, and salt back-diffusion to be advantageous, for the concentrating efficiency in the rete mirabile.

Reduction of Gas Solubility in the Fish Swimbladder

The gas filled swimbladder serves many fish as a hydrostatic organ to achieve neutral buoyancy. The gas enters the bladder by diffusion from the swimbladder vessels. The high gas partial pressures necessary for establishing a diffusion gradient from the vessels to the swimbladder is achieved by reducing the solubility of gases in the swimbladder blood. This ‘single concentrating effect’ (Kuhn et al., 1963), the increase in gas partial pressure induced by a change in solubility, is then multiplied by countercurrent multiplication in the rete mirabile (Steen, 1970; Fange, 1983).

Chapter 5 Metabolism of the swimbladder tissue

Publisher Summary This chapter focuses on the metabolism of the swimbladder tissue in fish. Many fish posses a gas-filled swimbladder as a hydrostatic organ to achieve neutral buoyancy. As a compliant bladder, it obeys Boyles law and changes pressure and volume with changes in hydrostatic pressure—that is, with water depth. The hydrostatic pressure increases by about 1 atmosphere for every 10 m of water depth. A crucial step in the functioning of the swimbladder is the production and release of metabolites that modify the gas carrying properties of blood passing the tissue. The morphological arrangement of the various layers of swimbladder tissue provides close contact of the cells producing these metabolites—the gas gland cells—to the vascular system and also ensures the accumulation of gas molecules in the bladder and not in the surrounding tissues. The chapter presents a brief discourse on the morphology and histology of the bladder. It elaborates the metabolism of the swimbladder tissue, especially of the gas gland cells, and discusses the role of carbonic anhydrase in the swimbladder epithelium.

Diffusion and perfusion limitation in alveolar O2 exchange: shape of the blood O2 equilibrium curve.

The limitations imposed by diffusion (Ldiff) and perfusion (Lperf) on alveolar gas exchange can be estimated using a simple model of alveolar-capillary gas transfer (Piiper and Scheid (1981) Respir, Physiol. 46: 193-208). These limitations indicate the fractional increase of gas exchange that would occur by raising pulmonary conductances for diffusion or perfusion to functionally infinite values. The (simple) model assumes linear relations between concentration and partial pressure for the gases studied. We have investigated in this study the effects of this assumption for estimating Ldiff and Lperf for O2 whose blood equilibrium curve is particularly non-linear in normoxia. The calculations suggest that Lperf is only slightly overestimated by the assumption of linear blood O2 binding. For Ldiff, there is a significant overestimation in normoxia, but in hypoxia the linear equilibrium curve yields sufficiently accurate estimates. Calculations for data estimated for man on the summit of Mt. Everest suggest that alveolar O2 uptake in deep hypoxia at rest is mainly limited by perfusion and to a lesser degree by diffusion (Lperf greater than Ldiff). For the sustained exercise of climbing, on the other hand, diffusion limitation is more prominent than perfusion limitation (Ldiff greater than Lperf). Large values of Ldiff are estimated for normoxic O2 uptake across the skin of the gill-less and lung-less salamander, and here, the effects of the alinearity of the O2 equilibrium curve are pronounced. It is concluded that the simplified model of alveolar-capillary gas transfer, with linear O2 equilibrium curve, can be very useful to estimate diffusion and perfusion limitations from experimental data.