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Lecture 3

Lecture 3. O 2 transport CO 2 transport Surface tension Role of surfactant Gas exchange Blood flow. O 2 transport. O 2 is carried in the blood in 2 forms; 1) bound to Hb (approx 98.5 %) 2) dissolved in the plasma (approx 1.5 %).

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Lecture 3

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  1. Lecture 3 • O2 transport • CO2 transport • Surface tension • Role of surfactant • Gas exchange • Blood flow

  2. O2 transport • O2 is carried in the blood in 2 forms; • 1) bound to Hb (approx 98.5 %) • 2) dissolved in the plasma (approx 1.5 %). • The amount of any gas that dissolves in blood is directly proportional to the PP of the gas and the solubility of the gas. Therefore, CO2 = SO2 * PO2 (Henry's law). • At 37C, the SO2 is 0.003 ml O2 / 100 ml blood / mmHg. Therefore, The content of O2 dissolved in blood = 0.003 * PO2. If plasma PO2 is 100 mmHg (approx normal arterial blood), the amount of O2 dissolved is 0.3 ml. This small amount of O2 will not sustain normal human metabolism. Another method is needed to transport O2 to tissues in sufficient quantity to meet metabolic demands. The Hb mol meets this requirement.

  3. Each mol of Hb can carry 4 mol of O2. Fully sat Hb can carry approx 1.36 ml O2 / g Hb, and normal human blood contains about 15 g Hb / 100 ml blood. Multiplying these two constants yields 20.4 ml O2 / 100 ml blood. • Hb is a protein in which a haem group is attached to each of 4 subunit polypeptide chain (2 alpha & 2 beta). Hb contains 4 iron atoms (4 haem group). Each one contain a Fe2+ within a haem group.

  4. If 100 ml of plasma is exposed to an atmos with a PO2 of 100 mmHg, only 0.3 ml of O2 would be absorbed. However, if 100 ml of blood is exposed to the same atmos, about 19 ml of O2 would be absorbed. WHY? • The total quantity of O2 bound with Hb in normal systemic arterial blood is about 19.4 ml /100 ml of blood. On passing through the tissue capillaries, this amount is reduced to approx 14.4 ml. Therefore, 5 ml is the quantity of O2 that are transported from the lungs to the tissues by each 100 ml of BF. • During heavy exercise, there might be upto 20 times ↑ in O2 transport to the tissues compared to normal.

  5. Factors which affect the O2-Hb dissociation curve: • These factors may shift the curve to the right, indicating lower affinity of Hb to O2, or shift the curve to the left, indicating an ↑ affinity of Hb to O2. These factors includes; • 1) PCO2:↑ PCO2 → ↓ affinity of Hb to O2 → shift the curve to the right (this is called Bohr effect). • 2) PH: ↓ PH (or ↑ [H+]) → ↓ affinity of Hb to O2 → shift the curve to the right. • 3) Temp : ↑ Temp → shift the curve to the right. • 4) 2,3- diphosphoglycerate (2,3-DPG): :↑ 2,3-DPG → ↓ affinity of Hb to O2 → shift the curve to the right. • P50 is the pp of O2 required to achieve 50% Hb sat.

  6. CO2 transport • CO2 transported from the body cells back to the lungs in 3 forms; • (1) Dissolved in the plasma (approx 7-10%). • (2) Reacts with the amino group of plasma proteins to form carbamino proteins (carbaminohemoglobin) (approx 23-30%). • (3) Reacts with H2O to form H2CO3 (approx 60-70%) CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+

  7. CO2 dissociation curve • The relationship of CO2 content of blood to PCO2 is known as CO2 dissociation curve. • The vol of CO2 carried in the blood is determined by PCO2. The dissolved form is directly proportio to PCO2 (0.06 ml dissolved in 100 ml of blood/1 mmHg PCO2). • The curve is affected by Hb sat with O2 (Haldane effect). • Oxyhemoglobin shifts the curve to the right, i.e. in the lungs CO2 is released from the blood. • Reduced Hb shifts the curve to the left, i.e. more CO2 is taken up by the blood in the tissues.

  8. Directional movement of CO2 • All movement across membrane is by diffusion. • Note that most of CO2 entering the blood in the tissues ultimately is converted to HCO3-. This occurs almost entirely in the erythrocytes because CA enzyme is located there, but most of the HCO3- then moves out of the erythrocyte into the plasma in exchange for chloride ions “ the chloride shift”.

  9. Chloride shift • The rise in the HCO3- content of red cell is much greater than that in plasma as the blood passes through the capillaries. The excess of HCO3- leaves the red cell in exchange for cl-. This change is called the chloride shift. • The chloride shift occurs rapidly and essentially complete in 1 second. • The cl- content of the red cells in venous blood is therefore significantly greater than in arterial blood. • In pulmonary capillaries; cl- leaves the red cell and move into the plasma in exchange for HCO3-; in systemic capillaries, the reverse occurs.

  10. The Haldane effect • It results from the simple fact that the combination of O2 with Hb in the lung causes the Hb to become a stronger acid. This displaces CO2 from the blood and into the alveoli in 2 ways; • (1) The more highly acidic Hb has less tendency to combine with CO2 to form carbaminohemoglobin, thus displacing much of the CO2 that is present in the carbamino form from the blood. • (2) The ↑ acidity of Hb also causes it to release an excess of H+, and these bind with HCO3- to form H2CO3; this then dissociate into H2O and CO2, and the CO2 is released from the blood into the alveoli and, finally, into the air.

  11. Surface tension and role of surfactant • ST is defined as the collapsing pres exerted upon the alv. • ST in the alveolus is created by interacting H2O mol which direct a force inward and could caused the alv to collaps. • An important factor affecting the compliance of the lungs is the ST of the film of the fluid that lines the alv. • One way to think of a ST is to imagine that the surface consists of a thin rubber membrane under stretch. If an incision 1 cm long were made in this membrane it would gape, and if sutures were put in it to bring the two cut sides together the total force of the sutures would be equal to the ST.

  12. According to Laplace's Law: • P = 2T / r. where P is the pres within an alv, T the tension in the alveolar wall and r the radius of the alv. • This formula shows that the pres inside a small alveolus is larger than that inside a large alveolus. Thus, if 2 alv of diff sizes were connected, we would expect the smaller one to collapse and empty its gas contents into the larger one. • This formula also shows that the tension in the alveolar wall is directly proportional to the pres within the alv. The tension in the alveolar wall has 2 components: • 1-The tension generated in the wall of the spherical alv. • 2- the ST created by the watery liquid that lines the walls of the alv.

  13. As seen above, surfactant lowers the ST of the lining fluid and thus decreases the overall tension in the alveolar walls. By decreasing the tension, the pres decreases proportionally. Thus the pres diff between 2 alv that are connected is decreased , and the smaller alv will not collapse. • Surfactant has two very important functions: • 1) It lowers ST of the lining fluid so we can breath without too much effort. • 2) It prevents the alv from collapsing.

  14. Gas exchange • It takes place at a respiratory surface. For unicellular organisms the RS is simply the cell membrane, but for a large organisms it is the respiratory system. • In humans, respiratory GE or VE is carried out by mechanisms of the lungs. The actual GE occurs in the alv. • The matching of VE and perfusion is the critical determinant of GE, and a deficiency of excess of VE relative to the amount of BF → either inadequate or wasteful respiration.

  15. Gas exchange at the alveoli: • Alv are designed for rapid GE. • Alv are found at the end of the branching bronchioles and so they have a good air supply. • The alv walls are very thin and have a moist surface. They are covered by a network of capillaries which transport the gases. • Blood takes about 1 sec to pass through the lung capillaries. In this time the blood becomes nearly 100% saturated with O2 and loses its excess of CO2. • When you breath in the first 150 ml fills the tubes which are outside the alv (anatomic VD). There is also a functional VD (air in these alv doesn’t exchange with the blood and is part of the VD). • The amount of air reaching the alv with each breath is equal to VT – VD.

  16. B) Gas exchange in the lungs: • Inspired air contains about 21% O2 and 78% N2, almost no CO2. • Blood returning to lungs is high in CO2 and is low in O2. • Blood leaving lungs is enriched with O2, low in CO2. • In pulmonary capillaries, O2 diffuses into capillary blood, while CO2 diffuses into alveolar air. • No exchange of gases occurs in heart, arteries, or arterioles. • The ratio of CO2 produced / O2 consumed is known as RQ.

  17. Blood flow (BF) • BF means simply the quantity of blood that passes a given point in the circulation in a given period of time. • The overall BF in the circulation of an adult person at rest is about 5000 ml/min. This is called cardiac output because it is the amount of blood pumped by the heart in a unit period of time. • BF through a blood vessels is determined by 2 factors; (1) Pressure difference of the blood between the two ends of the vessel, also sometimes called pressure gradient along the vessel, which is the force that pushes the blood through the vessel, and (2) the impediment to BF through the blood vessel, which is called vascular resistance.

  18. Circulation of blood through the lungs is as follow; (1) Pulmonary circulation: • - Unoxygenated BF from the right atrium to the right ventricle and leaves through pulmonary arteries to the lungs. The blood then oxygenated in the lungs and returns to the left atrium through the pulmonary veins. • - The output of the right ventricle is equal to that of the left ventricle and, like that of the left ventricle, average 5.5 l/min at rest. Thus, the pulmonary vasculature is unique in that it accommodates a BF equal to that of all other organs in the body. • (2) Systemic circulation: - The oxygenated blood in the left atrium flows into the left ventricle and is then pumped out to the rest of the body through the arteries. The tissues of the body use the oxygen and then return unoxygenated blood back to the right ventricle through the veins.

  19. Pulmonary vascular resistance (PVR) plays an important role in determining BF as indicated from the following equation; Flow = Pa – Pla / PVR Where Pa = pulmonary artery pressure Pla = left atrial pressure • PVR depends on alveolar diameter and the flow rate. So it appears that as flow increases, PVR decreases. There are 2 possible reason for this; • 1- Vascular distension: Because pulmonary vessels are distensible, as flow increases, the diameter of the tubes also increases and thus PVR is decreased. • 2- Vascular recruitment: As flow increases, areas of the lung that received very little blood, now receive increased flow. This increase the cross-sectional area of the pulmonary vasculature and thus reduces resistance.

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