USMLE Step 1 Lecture Notes 2016: Physiology (Usmle Prep)
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sequence of pressures, from highest to lowest. l Viscosity (poise) Blood Viscosity () Viscosity is a property of a fluid that is a measure of the fluid’s internal resistance to flow: l The greater the viscosity, the greater the resistance. l The prime determinant of blood viscosity is the hematocrit. Figure V-1-5 shows how viscosity varies with hematocrit. 8 7 6 5 4 3 2 1 Note What is the hematocrit? Answer: If a blood sample from an adult is centrifuged in a graduated test tube, the
(severe stenosis)—due to very high velocity of flow During inspiration and expiration, turbulent flow occurs in the large airways of the conducting zone. Series Versus Parallel Circuits If resistors are in series, then the total resistance is the sum of each individual resistor. RT = R1 + R2 + R3… l If resistors are in parallel, then the total resistance is added as reciprocals of each resistor. 1/RT = 1/R1 + 1/R2 + 1/R3… l Thus, total resistance is less in parallel circuits compared
discussed in Chapter 1 of Section IV. There is an inverse relation between afterload and ventricular output, thus there is generally an inverse relation between afterload and CO. Preload As discussed in Chapter 1 of Section IV, there is a direct relation between preload and ventricular output (Frank-Starling). Presuming there is no change in contractility or afterload, increasing preload increases CO and vice versa. Cardiac Output (CO)/Venous Return (VR) Curves CO/VR curves (Figure V-1-12)
series. l Systemic pressure decreases slightly through the arteries, decreases markedly through the arterioles, and then decreases only slightly more through the major veins. The loss of pressure is determined by regional resistance. l T he cross-sectional area increases from a minimum in the aorta to a maximum in the capillaries. Velocity of the blood is inversely related to a region’s cross-sectional area. l The main blood reservoir is the systemic veins. l Of the factors affecting
(OSA) since the lung and upper airways (nasopharynx) remain at a larger volume throughout the respiratory cycle. Inspiration Expiration PA +5 0 Figure VII-1-8b. CPAP PNEUMOTHORAX The following changes occur with the development of a simple pneumothorax. The pneumothorax may be (1) traumatic: perforation of the chest wall, or (2) spontaneous: rupture of an alveolus: l Intrapleural pressure increases from a mean at –5 cm H O to equal atmo2 spheric pressure. l Lung recoil decreases to zero as