Transport Of Gases

Dr. Niranjan Murthy H.L Assistant Professor Dept of Physiology SSMC

Atmosphere Alveoli Blood Tissues

Transport of oxygen • Oxygen is carried by blood in two forms: (i) dissolved in plasma (ii) combined with hemoglobin

PO2 mm Hg

PCO2 mm Hg

Alveolus

104

45

Arterial blood

95

40

Venous Blood

40

45

Tissue

40

46

Uptake of oxygen by pulmonary blood

O2 uptake during exercise • Resting O2 requirement: 250ml/min • O2 requirement increases by 20 times during strenuous exercise • Increased cardiac output reduces blood exposure time to alveolar O2 • Blood is still almost saturated because: (i) increased diffusing capacity for O2 (ii) safety period for O2 diffusion

O2 transport in arterial blood • Pulmonary venous blood has PO2 of 104 mm Hg • Aortic blood has PO2 of 95 mm Hg • Admixture of deoxygenated blood from bronchial circulation

Diffusion of O2 into tissue fluids • Concentration gradient of 55 mm Hg • Tissue PO2 depends on: (i) rate of oxygen transport to tissues by blood (ii) rate of tissue metabolism • Intracellular PO2 ranges from 5 to 40 mm Hg • 1 to 3mm Hg of tissue PO2 is adequate to support chemical reactions of the cells

Transport of oxygen in blood • Chemical combination with hemoglobinoxygenation- reversible- 97% • Dissolved in plasma- 3%

Hemoglobin • • • •

Pigment present in RBCs Iron-protoporphyrin-globin Iron is in ferrous form Fe2+ binds 4 pyrrole rings, a polypeptide chain and a molecule of oxygen • Adult hemoglobin- HbA • Fetal hemoglobin- HbF

Oxygen-hemoglobin dissociation curve

• Loose and reversible combination of oxygen molecule with heme • 4 molecules of oxygen can be carried by each hemoglobin molecule • Each gram of Hb binds 1.34 ml of O2 • Each 100ml of blood carries 20.1ml of O2 • Arterial blood is having PO2 of 95mm Hg, saturation of 97 percent and O2 carrying capacity of 19.4ml/100ml of blood

• In venous blood, PO2 is 40 mm Hg, percentage saturation is 75% and O2 carrying capacity is 14.4ml/100ml • 5ml of O2 is transported from lungs to tissues under normal resting conditions • Utilization coefficient- percentage of blood that gives up O2 during tissue capillary passage- 25% at rest

• During strenuous exercise tissue PO2 may fall to 15 mm Hg thus increasing O2 delivery upto 15ml/100ml • Cardiac output may increase 7 fold thus giving an overall 20 times increase in O2 delivery • Utilization coefficient increases to 75-85%

Tissue oxygen buffer system • Hb is responsible for stabilizing oxygen pressure in tissues • Tissue pressure is held tightly between 15 and 40 mm Hg. • Buffer effect maintains tissue PO2 even when there is marked changes in atmospheric [O2]

Physiological significance of O2-Hb dissociation curve • Significance of flat top part- amount of O2 carried will not change significantly even if PO2 drops to 60mm Hg. This of advantage in high altitudes • Significance of steep part- small reduction in tissue PO2 will increase more release of O2 thus preventing tissue hypoxia

Significance of P50 • It is PO2 at which Hb is half saturated with O2 . • Normal P50 is 26mm Hg at PCO2 40mm Hg, pH 7.4 and body temperature 37°C • Hb affinity for O2 is inverse function of P50

Factors affecting O2-Hb dissociation curve

• Effect of pH- the bohr effect: CO2 entry to tissue capillaries Reduced pH Increased delivery of O2 to tissues (shift to right) Vice-versa in pulmonary capillaries (shift to left)

• Effect of 2,3diphosphoglycerate (DPG): Shifts curve to right Doesn’t bind to γ chain of HbF Increased levels in chronic hypoxia decreased levels in stored blood

• Shift to left Carbon monoxide Myoglobin HbF

Transport of O2 in dissolved form • At normal PO2 of 95 mm Hg, 0.29ml of O2 is dissolved in 100ml of plasma • At venous PO2 of 40mm Hg, 0.12ml of O2 is dissolved

Uptake of CO2 from tissues

Excretion of CO2 from the lung

Transport of CO2 • Normally, 4ml of CO2/100ml of blood is transported each minute to lungs • Forms of CO2 transport: (i) dissolved form- 0.3ml/100ml- 7% of CO2 transport (ii) bicarbonate ion form- 70% of total CO2 transport (iii) in combination with Hbcarbaminohemoglobin- 20 to 30% of total CO2 transport

Transport of CO2 in HCO3 form ¯

• RBCs are rich in carbonic anhydrase • CO2 from plasma diffuses into RBC • CO2+H2O=H2CO3=H++HCO3¯ • HCO3¯ is exchanged for Cl¯ • Chloride shift- Hamburger phenomenon • Acetazolamide- carbonic anhydrase inhibitor- diuretic

Transport of CO2 in carbaminohemoglobin form • Combination with Hb and other plasma proteins • Combination with plasma proteins is less significant

Carbon dioxide dissociation curve • Total quantity of CO2 in blood in all forms depends on PCO2 • PaCO2 is 40mm Hg and PvCO2 is 45mm Hg • [CO2] in veins is 52 volumes percent and in arterial blood is 48 volumes percent

Haldane effect • Binding of O2 to Hb will tend to displace CO2 • OxyHb is a stronger acid • Highly acidic Hb has less tendency to combine with CO2 to form carbaminohemoglobin • Increased acidity of Hb will displace H+ from Hb • It doubles the pickup of CO2 at tissues and release in the lungs

Respiratory quotient • R= rate of CO2 output rate of O2 uptake • R for carbohydrates is 1, fats is 0.7 • R for a person on normal diet is 0.825