Blood Flow, Blood Pressure, Cardiac Output. Blood Vessels

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1 Blood Flow, Blood Pressure, Cardiac Output Blood Vessels

2 Blood Vessels Made of smooth muscle, elastic and fibrous connective tissue Cells are not electrically coupled Blood Vessels Arteries arterioles metarterioles capillaries venules veins Blood Vessels! Neurotransmitters, hormones, paracrines, etc. affect vessel diameter Endothelial muscle exhibits graded potentials Vascular smooth muscle is always partially contracted

3 Arteries and Arterioles

4 Arteries and Arterioles Arteries and arterioles Carry blood AWAY from the heart They are resistance vessels Blood here is under high pressure Veins and Venules Veins Carry blood toward the heart They are capacitance vessels; they do NOT provide resistance; they expand to allow blood flow through. How is blood pumped back to the heart under such low pressure?

5 Veins and Venules of note pulmonary veins and the umbilical vein carry blood rich in oxygen Capillaries Capillaries are one cell layer thick Capillaries are sites of exchange

6 Pressure, Flow, Resistance and Velocity

7 Basic Principles: Pressure = force exerted by a fluid (or a gas) on a container holding the fluid (or gas) Basic Principles: Measured in mm Hg the pressure exerted by a column of Hg that is 1mm high = 1 torr = 1/1760 atm = Pascals

8 Basic Principles Liquids (and gasses) flow from areas of high pressure to areas of low pressure until equilibrium is reached. Basic Principles The pressure of fluid (or a gas) in motion decreases over distance, due to friction of the fluid as it comes in contact with surfaces and other water molecules.

9 Basic Principles Volume and pressure are inversely related True for vessels too(!) Vasoconstriction = decreased volume, increased pressure Basic Principles Vasodilatation = increased volume, decreased pressure

10 Laws and Equations Based on Basic Principles Stoke s Law Flow Blood flows from an area of high pressure to of lower pressure until equilibrium is reached Flow = volume of fluid passing one point per unit time (gal / hour) or (ml / min) Blood flow P P = P - P 1 2 The greater the pressure differential, the faster the flow

11 What Reduces Flow? Resistance Resistance (R) = tendency of the system to oppose flow Flow 1/R Blood flowing through vessels encounters friction from: 1. Vessel walls 2. Blood cell collisions Flow P / R

12 Resistance to Flow is Determined by Flow and Resistance What factor most greatly affects blood flow in the body? Poiseuille's Law: This law describes the relative importance of the three factors affecting resistance: R = 8L v / π r 4 8 / π is a constant, so simplify: R L v / r4

13 Blood Velocity Flow = volume of blood passing one point per unit time (ml / min) Velocity = Flow / total cross-sectional area (distance / time) miles / hour m / sec Blood Velocity

14 Blood Velocity and Total Cross-Sectional Area Damage or weakening of a vessel puts strain on the others

15 Is Blood Velocity Highly Variable? During exercise, blood pressure and cardiac output increase significantly. What about blood velocity? Blood Pressure Ventricular contraction creates driving pressure Arteries are high-pressure vessels in which blood moves with a pulsatile flow. If a patient is bleeding, how might you tell if it was an artery or vein that was cut?

16 Blood Pressure Measurements Systolic Pressure: Avg. high of 120 mm Hg Diastolic Pressure: Avg. high of 80 mm Hg Sphygmomanometry is read as systolic / diastolic (120 / 80) Pulse = rapid pressure increase that occurs when ventricles push blood into the aorta MAP = Mean Arterial Pressure MAP measures the average arterial pressure during cardiac cycle MAP = Diastolic P + 1/3 (Systolic - Diastolic) Abnormally low MAP indicates impaired blood flow and oxygen delivery Abnormally high flow indicates risk of hemorrhage

17 MAP is determined by: 1. Balance between blood flow into and out of arteries 2. Cardiac output (CO) Cardiac Output = stroke volume x cardiac rate 3. Arteriole resistance Remember: R 1/r 4 4. Total blood volume Kidneys and ANS regulate

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19 Capillary Exchange Capillary density is proportional to the metabolic needs of the tissues served Velocity of blood flow is lowest at capillaries, allowing diffusion to go to equilibrium Total cross-sectional area of ALL capillaries determines velocity Capillary Exchange Two types of capillaries: Continuous capillaries Cells joined close together Fenestrated capillaries Cells have large pores Found in kidney and intestine

20 Defining Blood Flow Direction Filtration Absorption Capillary Exchange Question: What drives the flow of materials into or out of capillaries? Capillary Exchange Question: What drives the flow of materials into or out of capillaries?! Answer: GRADIENTS (what else?!)

21 Capillary Exchange Two opposing gradients: 1. Hydraulic pressure decreases along the length of the capillary as energy is lost to friction 2. (Colloid) osmotic pressure due to solute differences between the two sides of the capillary

22 Capillary Exchange Net filtration at arterial end, net absorption at venous end 85% of the capillary filtrate is directly returned via capillaries What happens to the rest?

23 Angiogenesis Angiogenesis = process by which new blood vessels develop Understanding angiogenesis can lead to treatment of 1. Cancer 2. Coronary artery disease Hypertension Hypertension is blood pressure that is in excess for one's age and gender Treatment: 1. modification of lifestyle is first. 2. diuretic drugs 3. Beta blockers are used to lower heart rate

24 Shock Shock occurs when low blood volume reaches an area of the body Hypovolemic Shock: circulatory shock due to low blood volume (due to bleeding, burns, dehydration) Septic Shock: circulatory shock due to sepsis (infection) Anaphylactic shock: due to an allergic reaction Cardiogenic shock: due to heart problem