Chapter 40: Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids

Diffusion of Oxygen from Alveoli to Capillaries diagram


Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 40

To read more about check out Chapter 23:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-23-heart-valves-and-heart.html

Hemoglobin within red blood cells serves a fundamental role in this transport system by dramatically increasing the oxygen-carrying capacity of blood far beyond the limits of simple physical dissolution. Simultaneously, specific biochemical pathways within erythrocytes convert carbon dioxide into highly soluble forms, ensuring rapid gas exchange and maintaining systemic acid-base balance during both resting and high-metabolic conditions.

Diffusion of Oxygen from Alveoli to Capillaries

  • Oxygen diffuses down partial pressure gradients.
  • Alveolar P_{\mathrm{O}_2} averages 104 mm Hg.
  • Incoming venous P_{\mathrm{O}_2} averages 40 mm Hg.
  • Initial diffusion gradient equals 64 mm Hg.
  • Blood P_{\mathrm{O}_2} rises rapidly in capillaries.
  • Full oxygenation occurs in one-third capillary length.
  • Remaining capillary length provides safety margin.
  • Exercise increases oxygen demand twentyfold.
  • Capillary transit time decreases during exercise.
  • Oxygen diffusing capacity increases threefold during exercise.
  • Capillary surface area expands significantly in exercise.
  • Ventilation-perfusion ratio improves during strenuous exercise.
  • Blood remains fully oxygenated even during exercise.

Transport of Oxygen in Arterial Blood

  • Ninety-eight percent blood originates from alveolar capillaries.
  • Alveolar capillary blood P_{\mathrm{O}_2} equals 104 mm Hg.
  • Two percent blood comes via bronchial circulation.
  • Bronchial circulation bypasses gas exchange areas completely.
  • Non-oxygenated bypass flow is termed shunt flow.
  • Shunt blood P_{\mathrm{O}_2} measures about 40 mm Hg.
  • Venous admixture reduces arterial P_{\mathrm{O}_2} to 95 mm Hg.

Oxygen Diffusion into Interstitial Fluid

  • Arterial blood enters tissue capillaries at 95 mm Hg.
  • Interstitial fluid P_{\mathrm{O}_2} averages 40 mm Hg.
  • Steep pressure gradient drives rapid oxygen diffusion.
  • Capillary P_{\mathrm{O}_2} falls to 40 mm Hg.
  • Venous blood leaves tissues at 40 mm Hg.
  • Increased blood flow raises interstitial P_{\mathrm{O}_2}.
  • Maximum possible interstitial P_{\mathrm{O}_2} equals 95 mm Hg.
  • Decreased blood flow lowers interstitial P_{\mathrm{O}_2}.
  • Increased cellular metabolism lowers interstitial P_{\mathrm{O}_2}.
  • Tissue P_{\mathrm{O}_2} reflects delivery and consumption balance.

Diffusion from Capillaries to Cells

  • Intracellular P_{\mathrm{O}_2} remains lower than capillary P_{\mathrm{O}_2}.
  • Intracellular P_{\mathrm{O}_2} ranges between 5 and 40 mm Hg.
  • Average intracellular P_{\mathrm{O}_2} equals 23 mm Hg.
  • Cellular metabolism requires only 1 to 3 mm Hg.
  • Intracellular P_{\mathrm{O}_2} provides large safety margin.

Diffusion of Carbon Dioxide

  • Carbon dioxide diffuses opposite to oxygen direction.
  • Carbon dioxide diffuses twenty times faster than oxygen.
  • Small pressure differences drive carbon dioxide diffusion.
  • Intracellular P_{\mathrm{CO}_2} averages 46 mm Hg.
  • Interstitial P_{\mathrm{CO}_2} averages 45 mm Hg.
  • Arterial blood P_{\mathrm{CO}_2} entering tissues equals 40 mm Hg.
  • Venous blood P_{\mathrm{CO}_2} leaving tissues equals 45 mm Hg.
  • Pulmonary capillary blood P_{\mathrm{CO}_2} equals 45 mm Hg.
  • Alveolar air P_{\mathrm{CO}_2} equals 40 mm Hg.
  • Pulmonary P_{\mathrm{CO}_2} equilibrates in one-third capillary length.
  • Decreased blood flow increases tissue P_{\mathrm{CO}_2} to 60 mm Hg.
  • Increased blood flow decreases tissue P_{\mathrm{CO}_2} to 41 mm Hg.
  • Increased metabolism greatly elevates tissue P_{\mathrm{CO}_2}.

Reversible Combination of Oxygen and Hemoglobin

  • Ninety-seven percent oxygen binds to hemoglobin.
  • Three percent oxygen dissolves in blood plasma.
  • Oxygen binds loosely and reversibly with heme.
  • High P_{\mathrm{O}_2} promotes hemoglobin oxygen binding.
  • Low P_{\mathrm{O}_2} releases oxygen from hemoglobin.
  • Systemic arterial blood saturation averages 97 percent.
  • Systemic venous blood saturation averages 75 percent.
  • Blood contains 15 grams hemoglobin per 100 ml.
  • One gram hemoglobin binds 1.34 ml oxygen.
  • Full saturation carries 20 ml oxygen per 100 ml.
  • Arterial blood carries 19.4 ml oxygen per 100 ml.
  • Venous blood carries 14.4 ml oxygen per 100 ml.
  • Normal oxygen delivery equals 5 ml per 100 ml.

Oxygen Transport During Exercise

  • Muscle interstitial P_{\mathrm{O}_2} falls to 15 mm Hg.
  • Venous oxygen content falls to 4.4 ml per 100 ml.
  • Exercise oxygen delivery increases to 15 ml per 100 ml.
  • Volume delivery increases threefold during exercise.
  • Cardiac output increases six to sevenfold in athletes.
  • Combined factors increase oxygen delivery twentyfold.
  • Normal utilization coefficient equals 25 percent.
  • Exercise utilization coefficient reaches 75 to 85 percent.
  • Local utilization coefficients can approach 100 percent.

Hemoglobin as Tissue Oxygen Buffer

  • Hemoglobin stabilizes tissue oxygen partial pressure.
  • Tissue P_{\mathrm{O}_2} upper limit is 40 mm Hg.
  • Small P_{\mathrm{O}_2} drops release large oxygen amounts.
  • Tissue P_{\mathrm{O}_2} stays tightly between 15 and 40 mm Hg.
  • Low alveolar P_{\mathrm{O}_2} barely changes tissue P_{\mathrm{O}_2}.
  • At 60 mm Hg alveolar P_{\mathrm{O}_2}, saturation is 89 percent.
  • Venous P_{\mathrm{O}_2} drops only to 35 mm Hg at altitude.
  • High alveolar P_{\mathrm{O}_2} cannot exceed 100 percent saturation.
  • Buffer mechanism maintains stable tissue P_{\mathrm{O}_2}.

Factors Shifting Oxygen-Hemoglobin Curve

  • Acidity shifts dissociation curve to the right.
  • Higher pH shifts curve to the left.
  • Increased carbon dioxide shifts curve to the right.
  • Increased blood temperature shifts curve rightward.
  • Increased 2,3-biphosphoglycerate shifts curve rightward.
  • Rightward shift enhances tissue oxygen unloading.
  • Leftward shift enhances pulmonary oxygen binding.
  • Bohr effect describes CO2 and H+ rightward shift.
  • Carbon dioxide forms carbonic acid in blood.
  • BPG increases during prolonged hypoxic conditions.
  • Muscle exercise factors shift curve strongly rightward.
  • Muscle temperature rises 2 to 3°C in exercise.

Cellular Oxygen Utilization

  • Intracellular P_{\mathrm{O}_2} above 1 mm Hg is non-limiting.
  • Cellular ADP concentration controls oxygen usage rate.
  • ATP breakdown forms ADP during activity.
  • ADP stimulates mitochondrial oxygen consumption rate.
  • Energy expenditure ultimately regulates oxygen usage.
  • Long diffusion distances limit cellular oxygen supply.
  • Zero blood flow limits cellular oxygen supply.
  • Blood flow limitation causes tissue hypoxia.

Oxygen Transport in Dissolved State

  • Arterial blood contains 0.29 ml dissolved oxygen per 100 ml.
  • Venous blood contains 0.12 ml dissolved oxygen per 100 ml.
  • Dissolved transport supplies 0.17 ml per 100 ml.
  • Dissolved state accounts for 3 percent total transport.
  • Exercise reduces dissolved transport percentage to 1.5 percent.
  • High alveolar P_{\mathrm{O}_2} increases dissolved oxygen significantly.
  • Excessive dissolved oxygen causes severe oxygen poisoning.
  • Oxygen poisoning causes brain convulsions and death.

Carbon Monoxide Poisoning

  • Carbon monoxide binds hemoglobin at oxygen sites.
  • Carbon monoxide binds 250 times more tenaciously.
  • Alveolar P_{\mathrm{CO}} of 0.4 mm Hg causes half saturation.
  • Alveolar P_{\mathrm{CO}} of 0.6 mm Hg is lethal.
  • Blood P_{\mathrm{O}_2} remains normal in carbon monoxide poisoning.
  • Cyanosis is absent in carbon monoxide poisoning.
  • Brain disorientation occurs without warning symptoms.
  • Pure oxygen therapy displaces carbon monoxide rapidly.
  • Five percent carbon dioxide stimulates alveolar ventilation.

Chemical Forms of Carbon Dioxide Transport

  • Normal carbon dioxide transport equals 4 ml per 100 ml.
  • Dissolved CO_2 transports 0.3 ml per 100 ml.
  • Dissolved form accounts for 7 percent transport.
  • Carbonic anhydrase accelerates CO_2 water reaction 5000-fold.
  • Carbonic anhydrase exists inside red blood cells.
  • Carbonic acid dissociates into hydrogen and bicarbonate ions.
  • Hydrogen ions bind hemoglobin buffer sites.
  • Bicarbonate ions diffuse out into blood plasma.
  • Chloride ions diffuse into red blood cells.
  • Bicarbonate-chloride carrier protein facilitates ion exchange.
  • Venous red cells contain higher chloride content.
  • Chloride shift maintains red cell ionic balance.
  • Bicarbonate mechanism transports 70 percent carbon dioxide.
  • Acetazolamide inhibits carbonic anhydrase enzyme activity.
  • Carbonic anhydrase inhibition raises tissue P_{\mathrm{CO}_2} to 80 mm Hg.
  • Amine radical binding forms carbaminohemoglobin.
  • Carbamino combination transports 20 to 30 percent CO_2.

Haldane Effect and Acid-Base Balance

  • Total blood carbon dioxide equals 50 volumes percent.
  • Exchange removes 4 volumes percent carbon dioxide.
  • Oxygen binding displaces carbon dioxide from hemoglobin.
  • Oxygenation turns hemoglobin into a stronger acid.
  • Acidic hemoglobin reduces carbaminohemoglobin formation capacity.
  • Acidic hemoglobin releases excess hydrogen ions.
  • Released hydrogen ions convert bicarbonate to CO_2.
  • Haldane effect doubles lung carbon dioxide release.
  • Haldane effect doubles tissue carbon dioxide pickup.
  • Normal arterial blood pH equals 7.41.
  • Normal venous blood pH equals 7.37.
  • Venous pH drops 0.04 units normally.
  • Heavy exercise causes 0.50 pH drop.

Respiratory Exchange Ratio

  • Normal oxygen uptake equals 5 ml per 100 ml.
  • Normal carbon dioxide output equals 4 ml per 100 ml.
  • Respiratory exchange ratio equals CO_2 output divided by O_2 uptake.
  • Pure carbohydrate metabolism yields 1.00 exchange ratio.
  • Pure fat metabolism yields 0.70 exchange ratio.
  • Fat metabolism oxygen forms water instead of CO_2.
  • Normal balanced diet exchange ratio equals 0.825.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.

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