RESPIRATION Dr Zainab H H Dept of Physiology

RESPIRATION Dr. Zainab H. H Dept. of Physiology Lec. 8

objectives n n Discriminate between O 2 & CO 2 transport. Describe O 2 - Hb dissociation curve

Transport of O 2 & CO 2

Hb and O 2 Transport q q q Each Hb has 4 polypeptide (2 α and 2 b) chains, each combined with heme group. In the center of each heme group is 1 atom of iron that can combine with 1 molecule O 2. One Hb molecule thus combine with four O 2 molecules. There about 280 million Hb molecules/RBC each RBC can carry over a billion molecules of O 2

Hemoglobin q Oxyhemoglobin: q q q Normal heme contains iron in the reduced ferrous form (Fe 2+). Fe 2+ shares electrons and bonds with O 2. Deoxyhemoglobin: q q When oxyhemoglobin dissociates to release O 2 to the tissues, heme iron is still in the reduced form. Hb does not lose an electron when it combines with O 2.

Hemoglobin (continued) q Methemoglobin: q Has iron in the oxidized ferric form (Fe 3+). lacks the electron it needs to form a bond with O 2 and cannot participate in O 2 transport. q Blood normally contains a small amount. q q Carboxyhemoglobin: The reduced heme is combined with CO. q The bond with CO is 210 times stronger than the bond with O 2. q Transport of O 2 to tissues is impaired. q

Hb Concentration q O 2 -carrying capacity of blood determined by its Hb concentration: 1) In anemia: q Hb concentration below normal abnormally low O 2 content of the blood. 2) In polycythemia: q Hb concentration above normal abnormally low O 2 content of the blood.

Hb Concentration q q q Erythropoietin: hormone produced by the kidneys controls production of RBCs in the bone marrow. If the amount of O 2 delivered to the kidneys is lower than normal stimulates secretion of erythropoietin production of RBCs. Androgens: promotes RBCs production q explains why the Hb concentration in men is from 1 to 2 g per 100 ml higher than in women.

The Loading and Unloading Reactions q q q Deoxyhemoglobin + O 2 Oxyhemoglobin (loading reaction) occurs in the lungs Oxyhemoglobin deoxyhemoglobin + O 2 (Unloading reaction) occurs in the tissues. Loading and unloading shown as a reversible reaction: (tissues) Deoxyhemoglobin + O 2 Oxyhemoglobin (lungs)

The Loading and Unloading Reactions q q The extent to which the reaction will go in each direction depends on: 1) PO 2 of the environment High PO 2 drives the equation to the Rt. (favors the loading reaction). Low PO 2 in the systemic capillaries drives the reaction in the opposite direction (promote unloading). The extent of this unloading depends on how low the PO 2 values are.

The Loading and Unloading Reactions affinity (bond strength) between Hb and O 2. Very strong bond would favor loading but inhibit unloading Weak bond would hinder loading but improve unloading. The bond strength between Hb and O 2 is normally strong enough so that 97% of the Hb leaving the lungs is in the form of oxy. Hb. BUT the bond is sufficiently weak so that adequate amounts of O 2 are unloaded to sustain aerobic respiration in the tissues. 2) q q

O 2 - Hb Dissociation Curve (OHDC) At PO 2 of 100 mm. Hg, blood in the systemic arteries, has a percent oxy. Hb saturation of 97% (97% of the Hb is in the form of oxy. Hb) delivered to the systemic capillaries O 2 diffuses into the cells and is consumed in aerobic respiration. q Blood leaving the systemic veins is reduced in O 2; it has a PO 2 of about 40 mm. Hg and a percent oxy. Hb saturation of about 75%. q

OHDC (continued) q q q Expressed another way: gram of Hb carry about 1. 34 ml of O 2, so if Hb were 100% saturated, it carry about 20 ml of O 2 normally it is only about 97 -98% saturated so it actually carries about 19. 4 ml of O 2 Blood entering the tissues contains 20 ml O 2/100 ml blood, and blood leaving the tissues contains 15. 5 ml O 2/100 ml blood (14. 4). Thus, 22%, or 4. 5 ml of O 2 out of the 20 ml of O 2 per 100 ml blood, is unloaded to the tissues.

OHDC (continued) q q q The relatively large amount of oxy. Hb remaining in the venous blood at rest serves as an O 2 reserve. If a person stops breathing, a sufficient reserve of O 2 in the blood will keep the brain and heart alive for about 4 to 5 minutes without using cardiopulmonary resuscitation (CPR) techniques. This reserve supply of O 2 can also be tapped when a tissue’s requirements for O 2 are raised.

OHDC (continued) q q q A graphic illustration of the percent oxy. H saturation at different values of PO 2 The values in this graph are obtained by subjecting samples of blood in vitro to different PO 2. The percent oxy. Hb saturations obtained can be used to predict what the unloading percentages would be in vivo with a given difference in arterial and venous PO 2 values.

OHDC (continued) q A. It is S-shaped, or sigmoidal due to Heme- Heme interaction. Plateau portion of the curve: q Range that exists at the pulmonary capillaries. q relatively flat at high PO 2 values indicates that changes in PO 2 within this range have little effect on the unloading reaction. q Minimal O 2 transported until the PO 2 falls below 60 mm Hg.

OHDC (continued) B. Steep portion of the curve: q Range that exists at the systemic capillaries q small changes in PO 2 produce large differences in % saturation (unload more O 2)

A 36 -year-old woman is found comatose at her home and is lifeflighted to the nearest regional medical center. Blood gases reveal a normal Pa. O 2 but a lower-than normal arterial O 2 saturation. Which of the following conditions is most consistent with the findings? n n n a. Anemia b. Carbon monoxide poisoning c. Hypoventilation d. Low V/Q ratio e. Right-to-left shunt