• Circulatory System functions to deliver oxygen from respiratory surface to tissues. Efficiency of circulatory system determined by:

1. Blood volume flowing by tissues (heart rate, stroke volume, capillarity, diameter and elasticity of vessels)
2. Oxygen concentration in blood
3. Ability of tissues to obtain oxygen (tissues from blood, blood from air in lung)

1. How is oxygen carried in the blood?

1. In many inverts. Carried as dissolved oxygen in plasma.
2. In more active inverts. and in vertebrates dissolved oxygen insufficient, so blood contains respiratory pigments.

• Mammals 14-30 vol%
• Birds 10-20 vol%
• Herps. 6-12 vol%
• Fish 0.8-20 vol%

*The lowest value for fish is from the Antarctic Icefish which has no respiratory pigments and no red blood cells. The loss of resp. pigment is an adaptation in this fish that lives in cold, well-oxygenated waters. This fish also has a low metabolic rate (and activity levels, MR = _ to _ of allometric predictions) and enormous gill surface area, so oxygen dissolved in plasma is sufficient to meet its metabolic demands.

1. Types of Respiratory Pigments

1. Hemoglobin (Hb) = iron porphyrin ("heme") + globin (protein). Vertebrate hemoglobins have 4 subunits (each with heme + globin) except for cyclostomes which have only 1 subunit. Invertebrate hemoglobins with 1 to >250 subunits.

• Occurs in at least 10 animal phyla (see Table).

1. Hb; occurs in polychaete worms

1. Hemerythrins = another iron-containing protein, nonporphyrin structure; occurs in

Sipunculid worms, Priapulids, some polychaetes, at least one brachiopod (clam-like

bivalves).

2. Hemocyanins = copper-containing protein; occurs widely in inverts. including

1. OXYGEN DISSOCIATION CURVES (Hb)

1. Number of oxygen binding sites is determined by number of subunits in resp. pigment molecule. Each subunit with 1 binding site. Hemoglobin with 4 subunits, myoglobin = a form of Hb found in muscle cells containing only one subunit.
2. Mb (and cyclostome Hb) are monomers and show hyperbolic dissociation curves (a.k.a. oxygen equilibrium curves); In Hb polymer, the binding of oxygen at one site facilitates the binding of oxygen at other sites (= cooperativity); results in a sigmoid curve (see Handout). The sigmoid response results from cooperativity and allows Hb to efficiently load or unload oxygen as conditions dictate (see Handout).
3. Oxygen Affinity = degree to which Hb and oxygen are attracted. A high affinity Hb has a dissociation curve further to the left than a low affinity Hb. Species differences in Hb-oxygen affinity result from differences in amino acid sequence of globin molecules. A convenient measure of oxygen affinity is P₅₀ = partial pressure of oxygen at which one-half of the heme groups have bound oxygen (see Handout).
4. Effectors of Oxygen Affinity

1. Temperature — binding of oxygen to Hb (by weak reversible chemical bonds) is an exothermic reaction; thus an increase in temperature weakens these bonds and favors dissociation of oxygen; an increase in temp. shifts the curve to the right.
2. pH — decreased pH shifts curve to the right
3. CO₂ — increased CO₂ shifts curve to the right; effects oxygen affinity directly by binding to Hb and altering comformation and indirectly by decreasing pH (CO₂ + H₂O à H⁺ + HCO₃^-). (See Handout).

4) Organic Phosphates and Other Modulators — Organophosphate compounds and inorganic

• Modulators vary among vertebrates: (see Handout)

• Fish = ATP, GTP, inositol pentaphosphate (IPP)

• Amphibs. = ATP, 2,3-diphosphoglycerate (DPG — from glycolysis), small amounts of GTP

• Reptiles = ATP, small amts. of GTP; Crocodiles — only HCO₃^-
• Birds = DPG (embryo), IPP (adults)
• Mammals = DPG

• For evolutionary scenario of organic phosphate modulation see Handout.

1. ADAPTATION OF OXYGEN TRANSPORT

1. Altitude — Increased elevation results in decreased partial pressure of oxygen.

1. Long-term Adaptation

1. High altitude species often have higher oxygen affinity (decreased P₅₀) relative to low altitude species. Increased oxygen loading at lungs, but decreased unloading at tissues. Results in a net increase in oxygen delivery.

1. Deer Mouse — decreased P50 at high altitude due to decreased DPG in RBC. (See Handout)
2. Bar-headed Goose — lower P50 than close relatives from moderate altitudes
3. Lake Titicaca Frog — low P50 and high hematocrit relative to other anurans

1. High altitude species often show increased hematocrit and hemoglobin conc. which

1. Humans — native to altitude show 5-15% increase in hematocrit, 10-20% increase in hemoglobin conc.
2. Some lizards also show increased hematocrit and hemoglobin conc. at moderately high altitudes.

1. Short-term Adaptation

1. increased blood oxygen carrying capacity (increased erythropoiesis) — takes weeks for full expression
2. Change in blood oxygen affinity — immediately see slight increase in affinity due to respiratory alkalosis, followed within hours by increased DPG which gives net rise in P50.

2. Body Size

1. Oxygen affinity of whole blood is negatively correlated with body mass in mammals and birds, at least on an interspecific basis.
2. Lower affinity Hb in small mammals and birds facilitates oxygen unloading at tissues and helps meet high metabolic demands of small endotherms.
3. Many deviations to this general trend exist.

1. High altitude animals tend to have relatively high oxygen affinity
2. Burrowing animals also tend to have relatively high oxygen affinity as an adaptation to potential low oxygen levels in burrows.

1. Reptiles generally show similar scaling relationships to endotherms; intraspecific comparisons in many verts. show varying results — positive and/or negative relationships have been documented.
2. Interspecific scaling effects apparently are due to variations in intraerythrocytic organic phosphate concentrations, as stripped Hb solutions do not show a similar correlation.

C. Diving

1. No adjustment of oxygen affinity

2. No clear relationship between diving and P₅₀ or diving and magnitude of Bohr effect.

3. Metabolic support of diving:

a) Short Dives - primarily aerobically supported

b) Long Dives - primarily anaerobically supported and often accompanied by a

reduction in total metabolism (See Overhead).

4. Primary Oxygen Stores during Dive:

1. In lungs - high affinity Hb advantageous, enhanced Bohr effect disadvantageous

1. If carry large amount of air in lungs, then diving to depths (increased pressure) increases dissolved gases (principally N₂) in blood, which upon resurfacing will come out of solution causing bubbles in the blood = bends.
2. Diving mammals and birds carry little air in lungs and may even exhale prior to diving. Reliance on oxygen in lungs during diving is practiced by some herps., but these don't dive to great depths.

b) In blood and tissues - high affinity Hb disadvantageous, enhanced Bohr effect advantageous.

(i) Most work has been done on whales and seals.

(ii) Generally higher oxygen carrying capacity in divers than in terrestrial

1. species, but there is a limit to increasing Hct and [Hb] as blood becomes too viscous. So, also get an increase in blood volume in divers.

1. Divers also have higher myoglobin levels in muscles than non-divers so oxygen stored in tissues is higher.
2. End Result = higher total blood and tissue oxygen stores in divers than in non-divers (e.g., seal with about double the O₂ stores of a human of equal mass).

D. Development - proceeds in a specialized microenvironment that is potentially hypoxic (e.g., outer layers of egg present significant diffusion barrier, uterine environment in mammals less well oxygenated than maternal tissues).

1. Fish - Many show decrease affinity and decreased temperature sensitivity as development proceeds. SEE OVERHEAD.

2. Amphibians - Decreased oxygen affinity and increased sensitivity to acid on metamorphosis from tadpole to adult. Changes due to replacement of larval Hb with adult

3. Reptiles - Decreased O₂ affinity throughout development, apparently involves

4. Birds - Complex pattern of changing oxygen affinity throughout development. Initial increase

• Adaptive value of this pattern difficult to determine. Increased O₂ affinity to a certain

5. Mammals - High oxygen affinity fetal blood, low oxygen affinity adult blood. Allows loading

of oxygen from maternal to fetal blood. SEE HANDOUT.

- Differences in affinity associated with:

a) Different types of Hb

(i) Ruminants show fetal Hb with intrinsically lower P₅₀.

(ii) Primates have fetal Hb that is less responsive to DPG, and therefore has a higher affinity.

b) No structural difference, but increased levels of DPG in adults (dog, horse, mouse, opossum, etc.).

I. Carried largely in chemical combination rather than as free dissolved CO₂ (although CO₂ much more soluble in aqueous solutions than oxygen).

A. Bicarbonate Ions - 80 to 90% of CO₂ in venous blood.

CO₂ + H₂O ® H₂CO₃ ® H⁺ + HCO₃^-

1. The vast majority of CO₂ from tissues diffuses through plasma into RBC.

2. Carbonic Anhydrase in RBC catalyzes conversions of:

CO₂ + H₂O ® H⁺ + HCO₃^-

- So that reaction proceeds rapidly to the right (rapid conversion impt. so that Bohr effect can enhance tissue unloading).

3. Build up of bicarbonate ions within RBC causes diffusion of bicarbonate ions into plasma; Chloride ions diffuse in from plasma to balance charges = Chloride Shift.

4. Most CO₂ in the plasma is also carried as bicarbonate ions, but conversion is much slower.

B. Combined with Hemoglobin - CO₂ binds to terminal amino groups on globin molecules, not to oxygen binding site. SEE HANDOUT FOR REVIEW.

• End Result is that the amount of CO₂ carried in the blood is greatly increased over the amount

of CO₂ dissolved in water at normal partial pressures (40-45 torr for mammals).

• CO₂ Dissociation Curves = total CO₂ concentration in blood relative to pCO₂; oxy- and deoxy hemoglogin differ slightly since oxyHb is a slightly stronger acid and therefore binds less CO₂ (Haldane Effect = decreased CO₂ affinity as oxygen saturation increases).
• Release of CO₂ into air or water is essentially a reversal of the process at the tissues.

II. CO₂ RELEASE (from blood to respiratory medium)

1. Assume RQ = 1.0 (O₂ taken in = CO₂ released).

2. Air

150 torr O₂ ® 100 torr O₂

Inspire Expire

0.2 torr CO₂ ® 50.2 torr CO₂

3. Water

150 torr O₂ ® 100 torr O₂

Inspire Expire

0.2 torr CO₂ ® 3 torr CO₂

- Difference in CO₂ concentration on expiration is due to high solubility of CO₂ in water.

4. Therfore, it is much easier to dump CO₂ to environment for water-breathers than it is for air-breathers because the gradient is much steeper. SEE HANDOUT.

5. Aquatic organisms also capable of actively eliminating HCO₃^- through gill epithelium. SEE HANDOUT.

6. Since aquatic organisms can more easily eliminate CO₂, they tend to have much lower blood pCO₂ than terrestrial vertebrates (therefore blood buffering capacity lower and they are more sensitive to changes in pCO₂).

III. BLOOD BUFFERS

1. Protons produced during the formation of bicarbonate ions as well as lactate produced during activity must be buffered to prevent large changes in blood pH.
2. The major buffering substances in the blood (require negatively charged groups to accept protons) are:

a) Carbonic Acid - Bicarbonate System - (H⁺ + HCO₃^-)

b) Blood Proteins - Hb, plasma proteins

c) Phosphates - modifiers of oxygen affinity

3. Active animals have increased blood buffering capacities relative to sluggish animals. This relationship is true both within and among vertebrate Classes.

CO₂ has less effect on elevating lethal pO₂ in fishes which inhabit warm,

somtimes stagnant water.