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Lecture 5

Lecture 5. Blood flow to the lungs (BF) Physiologic shunt (PS) Air flow (AF) Laminar flow (LF) Turbulent flow (TF) Ventilation-perfusion relations. Blood flow (BF). BF means simply the quantity of blood that passes a given point in the circulation in a given period of time.

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Lecture 5

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  1. Lecture 5 • Blood flow to the lungs (BF) • Physiologic shunt (PS) • Air flow (AF) • Laminar flow (LF) • Turbulent flow (TF) • Ventilation-perfusion relations

  2. 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 5 l/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.

  3. 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. • (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. • - The output of the right ventricle is equal to 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.

  4. 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 ↑, PVR ↓. There are 2 possible reason for this; • 1- Vascular distension: Because pulmonary vessels are distensible, as flow ↑, the diameter of the tubes also ↑ and thus PVR ↓. • 2- Vascular recruitment: As flow ↑, areas of the lung that received very little blood, now receive ↑ flow. This ↑ the cross-sectional area of the pulmonary vasculature and thus ↓ resistance.

  5. Physiologic shunt • A shunt is when perfused alveoli are not ventilated. Thus,the blood passing through those parts of the lung is not receiving oxygen at all. • This shunted blood (without O2) will eventually mix with the rest of the pulmonary blood. The result is a lower O2 saturation for the entire arterial blood. • There are 2 types of shunt; 1) Pulmonary Shunts:These shunts occur within the parenchyma of the lung (i.e. in the alveoli). 2) Extrapulmonary Shunts:This is a shunt that occurs outside the lung itself such as in a major bronchus.

  6. Air flow • Flow is defined as the quantity of molecules (or volume of air) moving per unit time (ml/sec). F = ∆P / R • Resistance is defined as a force opposing flow. It acts by decreasing the energy of molecules (created by a ∆P) and thus decreases flow. • Flow rate is defined as the amount of fluid moving per sec (ml/sec). • Velocity is defined as the distance fluid travels in 1 sec (cm/sec). • In the airways, flow will occur when a ∆P exists between one point along the airway and another, thus from a higher pressure to a lower one.

  7. Flow can be laminar , turbulent or marginally (disturbed) flow. • (1) Laminar flow (streamline flow): • It means blood flowing in all direction in the vessel and continually mixing within the vessel. • When blood flows at a steady rate through a long, smooth BV, it flows in streamlines, with each layer of blood remaining the same distance from the vessel wall. • Under laminar conditions, flow is determined by the ∆P and the resistance to flow. • Under LF conditions, the ∆P between two points a long a tube is directly proportional to the flow; • ∆P = K1 * V1 • Note; LF does not begin at the entrance of the tube. This fluid must travel a little before LF conditions are established. • LF in a tube of fixed dimension is defined by Poiseuilles Law; F = ∆Pr4 / 8nl

  8. (2) turbulent flow: • It means that the blood flows crosswise in the vessel as well as along the vessel, usually forming whorls in the blood called eddy currents. • In TF, fluid moves in all directions. It is chaotic fluid movement. In TF, the velocity of the fluid is, on the average, same everywhere in the tube. • In TF conditions, a higher ∆P is required to maintain a given flow rate; • ∆P = K2 * V2 • When eddy currents are present, the blood flows with much greater resistance than when the flow is streamline because eddies add tremendously to the overall friction of flow in the vessel. • The tendency for TF ↑ in direct proportion to the velocity of BF, the diameter of the BV, and the density of the blood, and is inversely proportional to the viscosity of the blood, in accordance with Reynold’s equation; Re = v.d.p / n

  9. For any fluids, flow tends to be turbulent when Re is greater than 2000 and laminar when Re is less than 2000. • During normal breathing, flow is very turbulent in the trachea and less turbulent in the narrower bronchi, and it becomes laminar in the small peripheral airways. Therefore, the total ∆P that occurs between the alveoli and the mouth is the sum of LF regions (small airways) and the TF regions (large airways).

  10. Fick’s law of diffusion • It state that the vol of gas per unit time moving across a tissue sheet is directly proportional to the area of the sheet and the diff in pp between the two sides but inversely proportional to the tissue thickness; Vgas = A/T X D X (p1-p2) • Fick’s law also stated that the rate of diffusion is directly proportional to the surface area X conc / distance. • Fick’s law of diffusion depends on the following factors 1- thickness 2- surface area 3- type of gas 4- pressure gradient • Factors that determine the rate of net flux by diffusion of a substance between two points in space includes; 1- the velocity of the individual particles 2- the magnitude of the conc gradient 3- the dimensions of the depth through which diffusion occurs.

  11. Ventilation-perfusion relations • VE is defined as the flow of air into and out of the alveoli.Perfusion is defined as BF to the alveoli. • VE and perfusion are normally matched in the lungs so that gas exchange (ventilation) nearly matches pulmonary arterial blood flow (perfusion). If mismatched, impairment of O2 and CO2 transfer results. • The concentration of oxygen in any lung unit is measured by the ratio of ventilation to blood flow: VENTILATION/PERFUSION (V/Q) • The Ventilation – Perfusion relationship can be measured by calculating the alveolar (A) – Arterial (a) PO2 difference.  PAO2 can be calculated using the equation: PAO2 = FIO2 (Patm – PH2O) – (PaCO2/R) at sea level, FIO2 = o.21, PH20 = 47, Patm = 760, PaCO2 measured by lab analysis, R = 0.8 PAO2 = 150 – (PaCO2/0.8)

  12. Effect of altering the VA / Q of the lung; • The inspired air has a PO2 of 150 mmHg and a PCO2 of 0. • The mixed venous blood entering the unit has a PO2 of 40 mmHg and a PCO2 of 45 mmHg. The PAO2 of 100 mmHg is determined by a balance between the addition of O2 by VE and its removal by BF. • Suppose that VA / Q reduced by obstructing its VE, leaving its BF unchanged (5.46B) → O2 will ↓ and CO2 will ↑, eventually VE is abolished (VA / Q of zero). • Suppose that VA / Q is ↑ by obstructing BF (5.46C) → O2 will ↑ and CO2 will ↓, eventually reaching the composition of inspired gas when BF is abolished (VA / Q of infinity). • Under normal conditions, the PAO2 averages 100 mmHg and the PCO2 averages 40 mmHg (5.46A).

  13. Abnormal V/Q in COLD: • Heavy smokers develops various degree of bronchial obstruction. A large share of those smokers develop serious alveolar air trapping→ emphysema. The emphysema in turn causes many of the alveolar walls to be destroyed. • There is 2 abnormalities occur in smokers to cause abnormal VA / Q; 1- because many of the small bronchioles obstructed, the alveoli beyond the obstruction are unventilated, causing VA / Q that approaches zero 2- in those areas of the lung where the alveolar walls have been mainly destroyed but there is still VA, most of the VE is wasted because of inadequate BF to transport the blood gases.

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