The Vascular Physiology

Vascular Physiology Functional Morphology of arteries: Circulation divided into pulmonary circulation and systemic (or peripheral) circulation. A. 84% of the blood found in systemic circulation including:  64% of body blood found in veins.  13 % is in the systemic arterial system  7 % is in the systemic capillaries B. 7 % is in the heart C. 9% is in the pulmonary vessels Blood found in arterial system and Left ventricles (High pressure system) Blood found in systemic veins, pulmonary circulation, heart(Low pressure system) chambers other than Left ventricle 1. Arteries (distribution vessels): includes a. large elastic arteries (windkessel vessels) b. medium muscular arteries (distribution vessels) 2. Arterioles (resistance vessels) Arterioles are the smallest branches of the arteries. Arterioles are under high pressure. Arterioles blood volume called the stressed volume. Arteriolar act as control conduits through which blood released into the capillaries. Arterioles have a high smooth muscle wall that extensively innervated by autonomic nerve fibers (the grates effect on autonomic is when there is highest concentration of smooth muscle). Alpha 1 Adrenergic receptors are found on the arterioles of the skin,  splanchnic (Splanchnic organs - including the stomach, small intestine, large intestine, pancreas, spleen, liver, and may also include the kidney.) renal circulations. Beta 2 Adrenergic receptors found on arterioles of skeletal muscle. Sheet Note 1 >> stressed volume : pressure against the walls of the vessels : >> arterioles are considered as resistance vessels because they have the largest amount of smooth muscles . Arterioles precise diameter of the lumen at any given moment determined by neural (sympathetic) and chemical (norepinephrine) The vasoconstriction and vasodilation in the arterioles are the primary mechanisms for of both resistance and regulation of blood pressure. Therefore, Arterioles are the site of highest resistance in the cardiovascular system (resistance vessels). Arterioles are critical in slowing down (or resisting) blood flow to the capillary Arterioles muscle fibers are normally slightly contracted, causing arterioles to maintain a consistent (muscle tone) in this case referred to as vascular tone 3. Capillaries (exchange vessels) Capillaries have the largest total cross-sectional and surface area. Capillaries wall is only 1 cell thick and is simple squamous epithelium surrounded by basal lamina. Capillaries are thin walled. Capillaries are the site of exchange of nutrients, water, and gases. Capillaries have no muscle, or elastic valve. Capillaries total thickness wall is only about 0.5 micrometer Capillaries internal diameter is 4 to 9 micrometers, barely large enough for red blood cells and other blood cells to squeeze through. Capillaries have a typical length of only 0.3 to 1 millimeter, so the blood remains in the capillaries for only 1 to 3 seconds, which is surprising because all diffusion of nutrient food substances and electrolytes that occurs through the capillary walls must be performed in this short time Sheet Note 2 >> we classified arterioles as resistance vessels, because given their small lumen, they dramatically slow the flow of blood from arteries. In fact, arterioles are the site of greatest resistance in the entire vascular network. >> The surface area for the arteriole itself is very small compered to other vessels , but we look at the surface area for all the arterioles in the body and we find it the largest surface area compered to other vessels in the body , and that’s explains why the exchange process occurs mostly in the arterioles (cause it has the largest surface area so makes the diffusion faster and easier). Functional morphology of capillaries: The arterioles divided into smaller muscle-walled vessels (meta-arterioles), and these in turn feed into capillaries. Each metarteriole arises from a terminal arteriole and branches to supply blood to a capillary bed that may consist of 10– 100 capillaries. A meta-arteriole  slightly larger than the typical capillary (10-20 µm)  has structural characteristics of both an arteriole and a capillary.  the smooth muscle of the tunica media of the metarteriole is not continuous have individual smooth muscle cells placed a short distance apart when constricted reduces or shuts off blood flow through their respective capillary beds. Sheet Note 3 meta-arteriole : >> >> smooth muscles in meta-arteriole are not continuous , instead they are separated to conroll the blood flow between the arterioles & capillaries so when they are constricted they reduces or shut off blood flow through their respective capillary The precapillary sphincters, circular smooth muscle cells that surround the capillary at its origin with the metarteriole, tightly regulate the flow of blood from a metarteriole to the capillaries it supplies. Their function is critical: If all of the capillary beds in the body were to open simultaneously, they would collectively hold every drop of blood in the body and there would be none in the arteries, arterioles, venules, veins, or the heart itself. Normally, the precapillary sphincters are closed. When the surrounding tissues need oxygen and have excess waste products, the precapillary sphincters open, allowing blood to flow through and exchange to occur before closing once more. If all of the precapillary sphincters in a capillary bed are closed, blood will flow from the metarteriole directly into a thoroughfare channel and then into the venous circulation, bypassing the capillary bed entirely. This creates what is known as a vascular shunt. In addition, an arteriovenous anastomosis may bypass the capillary bed and lead directly to the venous system. Blood dose not flow continuously through the capillaries. Instead, it flows intermittently, turning on and off every few seconds, this is called (vaso-motion). Vasomotion: intermittent contraction and relaxation of precapillary and met-arteriole smooth muscle (5-10 time/min) which is regulated both locally (metabolically) and by sympathetic control. . Vasomotion regulated by chemical signals that are triggered in response to changes in internal conditions, such as oxygen, carbon dioxide, hydrogen ion, and lactic acid levels. Sheet Note 4 >> if the blood flow from the artery to the vein directly through capillary we call it (arteriovenous shunt) which is an abnormal motion of blood. Capillaries and post-capillaries venules have (Pericytes) outside the endothelial cells. These cells have long processes that warp around the vessels. They have the following functions: They are contractile  Release a wide variety of vasoactive substances (vasoconstriction & vaso-dilation)  Synthesize and release constituents of the basement membrane and extra-cellular matrix  Regulate the flow through the junction between endothelial cells. Sheet Note 5 Pericytes : are cells that surround the capillaries , they have the ability of contraction Types of capillaries: There are three main types of capillaries: A. Continuous capillary: By “continuous,” this definition describes: • endothelial cells that have a lining that is uninterrupted with • tight junctions between endothelial cells. Continuous capillary is the most common type of capillary and found in almost all vascularized tissues. Continuous capillaries only allow smaller molecules to pass (like gases, water, glucose, ions, and some hormones) through their intercellular clefts (small gaps in between their endothelial cells). Lipid-soluble molecules can passively diffuse through the endothelial cell membranes along concentration gradients. Tight junction capillaries can be further subdivided into: 1. Those with numerous transport vesicles, which found primarily in skeletal muscle, fingers, gonads, and skin 2. Those with few vesicles, which primarily found in the CNS (are a constituent of the blood Brain Barrier). Sheet Note 6 >> continuous type of capillaries have continuous endothelial structure and continuous basement membrane . B. Fenestrated Fenestrated capillary (derived from fenestra Latin for "window") have 1. Endothelium: a. pores in the endothelial cells (60-80 nm in diameter) allow small molecules and limited amounts of protein to diffuse b. tight junctions in the endothelial lining 2. continuous basal laminae. The number of fenestrations and their degree of permeability vary, however, according to their location. Fenestrated capillaries are common in the small intestine, glomeruli of the kidney, choroid plexus of the brain and many endocrine structures, including the hypothalamus, pituitary, pineal, and thyroid glands. The types of fenestrations are: With diaphragm which is radially oriented fibers With no diaphragm (as in the renal glomerulus), instead have podocytes foot processes or pedicles, which have slit pores with a function analogous to the diaphragm of the capillaries. C. Sinusoidal (also known as a discontinuous capillary): Sinusoidal capillary is the least common type.  Endothelium are flattened,  incomplete basement membranes  extensive intercellular gaps.  intercellular clefts and fenestrations (a special type of openpore capillary); that have larger openings (30-40 µm in diameter) increasing the permeability of the capillary Sinusoid blood vessels are primarily located in the  bone marrow allow RBC and WBC (7.5 µm - 25 µm diameter)  liver allows various serum proteins to pass, aided by a discontinuous basal lamina. lymph nodes adrenal glands spleen 4. Venules Venule is an extremely small vein, generally 8–100 micrometers in diameter. Post-capillary venules join multiple capillaries exiting from a capillary bed Capillaries and post-capillaries venules have (Peri-cytes) outside the endothelial cells. 5. Veins (capacitance vessels) • Progressively merge to form larger veins. The largest vein, the vena cava, returns blood to the heart. • are thin-walled and easily distended. Because they contain little elastic tissue and smooth muscles, but considerable vasoconstriction produced by activity in the nor-adrenergic nerve • are under low pressure. • contain the highest proportion of the blood in the cardiovascular system (the unstressed volume). • have ά1-adrenergic receptors. • equipped with valves Sheet Note 8 >> veins have less amount of smooth muscles than arteries so its affected by sympathetic stimulation less than parasympathetic stimulation . Biophysical consideration: Hemodynamics branch of physiology dealing with forces involved in the circulation of the blood. Blood rheology study of flow properties of blood and its elements of plasma and cells Blood flow & velocity Blood flow refers to the movement of blood through the vessels from heart ►arteries ►arterioles ►capillaries ► Venules ►veins ►heart Blood flow means the quantity of blood that passes a given point in the circulation in a given period (mm3/sec or litter/min or litters/sec). Blood flow (mm3/sec) = velocity (mm/sec) X cross-section area (mm2) The overall blood flow in the total circulation of an adult person at rest is about 5000 ml/min. This called the cardiac output because it is the amount of blood pumped into the aorta by the heart each minute. Blood flow velocity is the speed, which blood moves along the circulation in any particular segment and expressed in unit of distance per time. V= Q / A Where V: Velocity of blood flow (cm/sec), Q: Flow (mL/sec), A: Cross-sectional area (cm2) The velocity of blood is inversely relation to cross sectional area The highest velocity is seen in aorta with a small cross-sectional area= 2.5 cm average velocity of 4050 cm/s) The lowest velocity is seen in capillaries (8 to 10 microns in diameter, just large enough for red blood cells to pass through them in single file and a very slow flow (0.03 cm/s) (taken in consideration that the sum of all of the capillaries (large cross-sectional area = 4500 cm2). The lower velocity of blood in the capillaries optimizes conditions for exchange of substances across the capillary wall. The cross-sectional areas of the veins are much larger than the arteries, averaging about four times those of the corresponding arteries. This difference explains the large blood storage capacity of the venous system in comparison with the arterial system. Types of flow: Laminar (Streamline) flow Laminar flow (or viscous flow) describes the movement of fluid through a tube in concentric layers that slip past each other. Laminar flow characterized by concentric layers of blood moving in parallel down the length of a blood vessel. Laminar flow is the most efficient pattern of flow velocities, in that the fluid exerts the least resistance to flow in this configuration. When laminar flow occurs, the velocity of flow in the center of the vessel is far greater than that toward the outer edges. The highest parabolic laminar flow velocity (Vmax) found in the center of the vessel. The lowest parabolic laminar flow velocity (V=0) found along the vessel wall. The cause of the parabolic profile is the following: The fluid molecules touching the wall move slowly because of adherence to the vessel wall. The next layer of molecules slips over these, the third layer over the second, the fourth layer over the third, and so forth. Therefore, the fluid in the middle of the vessel can move rapidly because many layers of slipping molecules exist between the middle of the vessel and the vessel wall; thus, each layer toward the center flows progressively more rapidly than the outer layers. Turbulent flow has crosscurrents , and the fastest velocities are not necessarily in the middle of the stream. Several factors contribute to the tendency for turbulence: high flow velocity, large tube diameter, high fluid density, low viscosity 1. All of these factors can be combine to calculate Reynolds number (NR), which quantifies the tendency for turbulence: Where v is the mean velocity, d is the tube diameter, ρ (rho), is the fluid density, and η (eta) is the fluid viscosity. Turbulent flow occurs when NR exceeds a critical value. When Reynolds’ number rises above approximately 2000, turbulence will usually occur even in a straight, smooth vessel. Conditions are appropriate for turbulence: (1) High velocity of blood flow, (2) Pulsatile nature of the flow, (3) Sudden change in vessel diameter (rapid narrowing of blood vessel) (4) Large vessel diameter. (5) Sharp turns in the circulation (6) Rough surface in the circulation Turbulent flow means that blood flow in a laminar and crosswise forming whorls in the blood , called eddy currents. When eddy currents are present, the blood flows with much greater resistance than when the flow is streamlined, because eddies add tremendously to the overall friction of flow in the vessel. Eddy current occurs in A. Normal condition: As in, the beginning of aorta and pulmonary artery Eddy currents generated because blood is forced through an orifice into a broad vessel, surrounding the valve and cusps of the valve and keep the cusps in the stream and not against the chamber wall. the Reynold number can rise to several thousands due to high velocity during ejection phase bifurcation of artery (as common carotid artery bifurcation) the Reynolds’ number rises above 200 to 400, turbulent flow will occur at some branches of vessels but will die out along the smooth portions of the vessels Reynolds number less than or equal to 2000 indicates laminar flow. Reynolds number more than or equal to 2000 turbulence will usually occur even in straight smooth vessel. Reynolds’ number is almost never high enough, in small vessels, to cause turbulence Reynolds number is without units B. Abnormal conditions: Murmurs Murmur in the heart is an abnormal, extra sound during the heartbeat cycle Murmur generated by turbulent flow of blood in the heart where Eddy currents generated because blood is forced through an orifice into a broad chamber, surrounding the valve and cusps of the valve and keep the cusps in the stream and not against the chamber wall Murmur generated due to a. pathological changes in the cardiac valves either dilation (regurgitation) narrowing (stenosis) b. septal defect that raise flow velocity often induce turbulent flow. Murmur important in diagnosing cardiac valvular lesions.  Bruits (or vascular murmur) Bruits is the abnormal sound generated by turbulent flow of blood in an artery Bruits occurs in vessels stenosis, occurs in atherosclerosis due to narrowing of arterial diameter or rough surface of arterial wall. arterio-venous shunts. Bruits occurs due high blood flow velocity. (7) High fluid density (8) Low viscosity. Viscosity Isaac Newton described viscosity in 1713 as internal friction to flow in a fluid or lack of slipperiness. Viscosity reflects to the thickness of fluid. Viscosity is a property of fluids that indicates resistance to flow. Viscosity can be measure in vitro by viscometer Unit of viscosity Poise (after Poiseuille). A fluid of 1 Poise viscosity has a force of 1 dyne/cm2 of contact between layers when flowing with a velocity gradient of 1 cm/sec. The poise (P) is the unit of dynamic viscosity in the gram –centimetersecond The analogous unit in the International System of Units is the pascalsecond (Pa·s): Relative viscosity: Relative viscosity is a more often used term. Relative viscosity refers to the viscosity of fluid relative to viscosity of water at body temperature (37°C) Viscosity of water at 21 °C is 0.01 poise or 1centipoise. Viscosity of water at body temperature (37 °C) is 0.695 centipoise Plasma has a viscosity of 1.2 centipoise at 37°C Plasma has a relative viscosity of 1.7. Blood has a viscosity of 2.8-3.8 centipoise at 37°C Blood (plasma plus cells) has a relative viscosity of about 4-5. Several factors affecting viscosity including: (1) Blood composition changes: RBC mass: increase the number of RBC, Hematocrit (or Packed cell volume), and hemoglobin all will increase viscosity. Examples: a. anemia decrease viscosity b. polycythemia increases viscosity Those factors considered as most important factors that increase viscosity The most important factor of these is the RBC because each RBC exerts frictional drag against adjacent cells and against the wall of the blood vessel. When the hematocrit rises to 60 or 70%, which it often does in polycythemia, the blood viscosity can become as great as 8-10 times that of water, and its flow through blood vessels is greatly retarded Plasma protein: increase plasma protein will increase viscosity. Changes in Plasma protein (such as hyper-gamma-globulin-emia) has less effect on viscosity than RBC changes. Resistance of cell to deformation: viscosity increase in hereditary spherocytosis and sickle cell If clotting mechanisms are stimulated in the blood, platelet aggregation and interactions with plasma proteins occur. This leads to entrapment of red cells and clot formation, which dramatically increase blood viscosity. All these factors will explain the higher viscosity of blood than water. (2) Temperature:  body temperature ► blood viscosity Blood viscosity increase 2% for each oneº C decrease in temperature. When the hand kept in ice water regional blood viscosity show a threefold increase. (3)Shear rate or blood flow velocity gradient. Viscosity of the blood decrease as the shear rate or velocity gradient increase and vice versa. Shear rate is the rate of change of velocity at which one layer of fluid passes over an adjacent layer. As an example, consider that a fluid placed between two parallel plates that are 1.0 cm apart, the upper plate moving at a velocity of 1.0 cm/sec and the lower plate fixed. Newtonian fluids such as Water, air, alcohol, glycerol, and thin motor oil and plasma their viscosity remains the same whether they are flowing fast or slowly. Non- Newtonian fluids such as Whole blood its viscosity changes with its velocity. A. At high blood flow velocity (or high shear rate) such as (exercise, during systole) blood viscosity decrease. This is because red cells tend to collect in the center of the lumen of a vessel and move with their long axis parallel to the direction of flow where flow rate is fastest leaving cell free zone of plasma at periphery, an arrangement known as axial streaming. Axial streaming ►reduces the viscosity ► reduces resistance to flow. The RBC tend to accumulate along the axis of the blood vessel has consequence that result in decrease of hematocrit as blood approaching the micro-vessel. The phase separation due to axial migration affects the cellular content of blood flowing into the side branches of blood vessels. A branch originating from a vessel of higher order fed mainly by the marginal stream (which contains plasma) of the higher order vessel so receives blood with a lower hematocrit. This is called plasma skimming which has the following effects: Larger branches with higher flow rates receive relatively more RBC and therefore have higher hematocrit blood Smaller breaches with low flow rates receive relatively less RBC and therefore have lower hematocrit blood . This explains why the hematocrit of capillary blood is about 25% less than the whole– body hematocrit.