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

2. objectives Discriminate between O2 & CO2 transport. Describe O2- Hb dissociation curve

3. Transport of O 2 & CO 2

4. Hb and O 2 Transport  Each Hb has 4 polypeptide (2 α and 2  ) 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 are about 280 million Hb molecules/RBC  each RBC can carry over a billion molecules of O 2

6. Hemoglobin  Oxyhemoglobin:  Normal heme contains iron in the reduced ferrous form (Fe 2+ ).  Fe 2+ shares electrons and bonds with O 2.  Deoxyhemoglobin:  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.

7. Hemoglobin (continued)  Methemoglobin:  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.  Blood normally contains a small amount.  Carboxyhemoglobin:  The reduced heme is combined with CO.  The bond with CO is 210 times stronger than the bond with O 2.  Transport of O 2 to tissues is impaired.

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

9. Hb Concentration  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  explains why the Hb concentration in men is from 1 to 2 g per 100 ml higher than in women.

10. The Loading and Unloading Reactions  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 + O2  Oxyhemoglobin (lungs)

11. The Loading and Unloading Reactions  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.

12. The Loading and Unloading Reactions 2) 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 oxyHb.  BUT the bond is sufficiently weak so that adequate amounts of O 2 are unloaded to sustain aerobic respiration in the tissues.

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

15. OHDC (continued)  Expressed another way:  gram of Hb carry about 1.34ml of O 2, so if Hb were 100% saturated, it carry about 20ml of O 2  normally it is only about 97-98% saturated so it actually carries about 19.4ml 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.

18. OHDC (continued)  The relatively large amount of oxyHb 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.

19. OHDC (continued)  A graphic illustration of the percent oxyH 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 oxyHb 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.

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

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

23. A 36-year-old woman is found comatose at her home and is life- flighted to the nearest regional medical center. Blood gases reveal a normal PaO2 but a lower-than normal arterial O2 saturation. Which of the following conditions is most consistent with the findings? a. Anemia b. Carbon monoxide poisoning c. Hypoventilation d. Low V/Q ratio e. Right-to-left shunt