Chapter 19: Blood Vessels 63 slides 1
Blood Vessels The Blood Vessels are essentially a series of tubes. Three types of blood vessel tubes: Arteries (carry blood away from heart) Arterioles are the smallest arteries before capillaries. Veins (carry blood to the heart) Venules are the smallest veins after capillaries. Capillaries 2
Structure of Blood Vessel Walls vessel walls made of layers or tunics tunica intima innermost tunic aka tunica interna. Next to the lumen. Contains the following layers: Endothelium: the simple squamous epithelium Subendothelial Layer: a basement membrane & loose C.T.» only in vessels larger than 1 mm in diameter. tunica media circularly arranged smooth muscle cells sheets of elastin surrounded by internal and external lamina. tunica externa connective tissue, nerves, lymphatics, elastin fibers contains vasa vasorum vessels of the vessels. 3
Artery & Vein (70x) 4
Artery, Vein & Capillary 5
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Relationship of Blood Vessels & Lymphatics 8
Relative Proportion of Blood Volume Throughout the Cardiovascular System 9
Arterial System Elastic (conducting) Arteries nearest the heart (Aorta parts, major branches). largest diameter at 1 to 2.5 cm. are the most elastic with elastin in all 3 tunics. act as a pressure reservoir, expanding & recoiling. allows blood to flow continuous vs spurting with hardening, this pressure-smoothing effect is lost. smooth muscle here is relatively inactive in vasoconstriction. walls can weaken and balloon over lifetime. 10
Arterial System Muscular (distributing) Arteries distal to the elastic arteries account for most of the named arteries in the body. internal diameter range from 0.3 mm to 1 cm. have the thickest tunica media (i.e. most sm. mus.). big role in vasoconstriction. has an elastic lamina on both sides of tunica media 11
Arterial System Arterioles smallest of the arteries lumen diameter range of 10 um to 300 um. larger arterioles have all 3 tunics tunica media is mostly smooth muscle with a few scattered elastic fibers. smaller arterioles may not have all 3 tunics are little more than endothelium with a single layer of smooth muscle spiraling around it. controls blood flow into capillaries. 12
Capillaries just consists of the tunica intima. surrounded in some spots by pericytes. a smooth muscle-like cell that stabilizes wall. average length is just 1 mm. diameter is 8 to 10 um. remember the average RBC was only 7.5 um. 3 basic types of capillaries: continuous fenestrated sinusoidal 13
Capillaries Continuous Capillaries: abundant in skin and muscles most common type endothelial cells provide an uninterrupted lining. adjacent cells attached by tight junctions. usually incomplete and have gaps of unjoined membrane called intercellular clefts. only small amount of fluid and small solutes can pass. NO intercellular clefts in Brain... blood-brain barrier. 14
Continuous Capillary 15
Capillaries Fenestrated Capillaries similar to the continuous except: the endothelial cells are riddled with oval pores or fenestrations. fenestrations covered by a delicate membrane called the diaphragm (basal lamina material). makes them more permeable to fluids, solutes. GI tract to absorb larger molecules endocrine organs to receive the larger hormones kidneys where high filtration rate is occurring. 16
Fenestrated Capillary 17
Capillaries Sinusoidal Capillaries highly modified, leaky capillaries found only in: liver bone marrow lymphoid tissue some endocrine organs have large irregular lumens & fenestrations. allow very large molecules and even cells to leave. in liver the endothelium is discontinuous and have large macrophages called kupffer cells which remove and destroy bacteria Blood flow in this capillary type is slowest. 18
Sinusoidal Capillary 19
Capillaries tend to form into interweaving networks called capillary beds. has a vascular shunt through it so circulation can be cut-off as needed to rest of capillaries. see figure 19.4 pre-capillary sphincters control entry into capillaries so blood gets shunted past an area. Note: this explains why if you eat just before swimming, the blood that was busy processing digesting is shunted to the now exercising muscles and abdominal cramps and indigestion set in. 20
The Capillary Bed 21
Venous System Venules the smallest veins entered after capillaries. WBCs adhere to the walls of the postcapillary venule in areas that are inflamed. walls are very porous and WBCs can easily pass. Veins have 3 distinct tunics walls much thinner than artery. lumens look collapsed compared to artery. tunica externa is most developed wall. 22
Venous System Venous system is able to hold up to 65% of all blood in body. they are still partially filled at this volume veins called capacitance vessels or blood reservoirs. low pressure here requires help in returning blood: large diameter lowers blood flow resistance. Venous Valves to prevent back flow in limbs no valves in veins of head, chest cavity, abdominal pelvic cavity. contracting skeletal muscle in limbs pushes blood back. 23
Venous System Varicose Veins dilated, tortuous veins, mostly found in lower extremities due to incompetent valves. precipitated by: genetics hinderance of venous return conditions prolonged standing in one position obesity... external iliac veins kinked off by abdomen. pregnancy... same. straining increase pressure causes hemorrhoids. 24
Venous System Venous Sinuses coronary sinus on heart dural sinuses in brain highly specialized, flattened veins with extremely thin walls of just endothelium. supported by the tissue around them. 25
Vascular Anastomoses arterial anastomoses merging arteries into one. venous anastomoses merging veins into one. collateral channels alternate pathways for blood to one area. can be tissue and even life saving if one is cut or blocked by disease. arteriovenous anastomoses pathway to bypass capillary bed. 26
Physiology of Circulation Blood Flow volume of blood flowing in an area over time. if referring to the whole body is same as CO. relatively constant under resting conditions. blood flow through individual body organs may vary widely and is related to their immediate needs. Blood Pressure blood s force per unit area exerted on vessel wall expressed in mmhg. unless specified as pulmonary blood pressure, BP refers to the systemic BP in the large arteries near the heart. the pressure gradient (from high pressures upstream to the low pressures down stream) keeps blood moving. 27
Physiology of Circulation Resistance opposition to flow... the friction blood encounters. most friction is encountered in systemic circulation. referred to as peripheral resistance. 3 main sources of resistance: blood viscosity the thicker the fluid (blood), the greater the resistance. constant in health people. can vary up or down in disease. vessel length the longer the vessel, the greater the resistance. constant in healthy people. usually increases with obesity. vessel diameter the smaller the diameter, the greater the resistance. can vary with tissue need in healthy and abnormal people. 28
Physiology of Circulation more on blood vessel diameter: fluid flowing next to a wall is slowed by friction. fluid flowing in the center flows faster. therefore, in smaller tubes, a greater percentage of the blood volume is next to the vessel wall and will move slower due to the friction it encounters. this phenomenon is called laminar flow or streamlining. resistance varies inversely with the fourth power of the vessel radius. example: if we double the radius of a vessel, the resistance will drop to 1/16 of its original value. R =1/r 4 = 1/ (2x2x2x2) = 1/16. this means the small diameter arterioles have the most impact. turbulent flow (encountered with rough or protruding areas) is an irregular fluid motion... dramatically increases resistance. 29
Physiology of Circulation Relationship between Flow, Pressure & Resistance: Blood flow is directly proportional to the difference in blood pressure between two points in circulation, that is, the blood pressure gradient. Formula: F = P/R when there is a bigger drop in blood pressure between point A and point B (the P), flow (F) will increase. when resistance (R) increases the flow (F) will decrease. of the ways to affect flow... resistance will affect flow the most. 30
Systemic Blood Pressure the pumping action of the heart creates flow. pressure results when flow meets resistance. systemic blood pressure is highest in the aorta and declines all the way to zero in the right atrium! the steepest pressure drop occurs in the arterioles. 31
Blood Pressure in Various Blood Vessels 32
Arterial Blood Pressure Arterial BP reflects two factors: how much elastic arteries can be stretched this is their compliance or distensibility. the volume of blood forced into them. systolic pressure due to the forces of left ventricular contraction. 120 mmhg in healthy adult. diastolic pressure due to the recoil forces of the large arteries 70 to 80 mmhg in healthy adult. 33
Arterial Blood Pressure pulse pressure is the difference between the systolic and diastolic pressures. it is felt as a throbbing pulsation in an artery during systole as artery walls are stretched. is increased the more stretch resistant a blood vessel becomes: arteriosclerosis in arteries decreases their stretchiness. 34
Arterial Blood Pressure mean arterial pressure (MAP) is the pressure that propels blood to the tissues. because diastole usually lasts longer than systole, the MAP is not simply the average. formula: MAP = diastolic pressure + pulse pressure 3 example: person with a BP of 120/80 mmhg: MAP = 80 mmhg + = 93 mmhg NOTE: as you get to the capillaries, the blood flow is steady and pulse pressure is gone. 35
Capillary Blood Pressure BP has dropped to about 33 mmhg at the beginning of the capillaries and is only about 15 mmhg as it enters the venules. too high and you push filtrate fluid into the tissues and cause edema. 36
Venous Blood Pressure flow is steady and non-pulsatile. goes from about 15 mmhg to 0 mmhg as it enters the right atrium. Assistance to help the venous return: large lumens of veins to hold more blood. valves to prevent back flow of blood. respiratory pump inhalation will increase venous return. muscular pump contracting muscles increase venous return. smooth muscle in veins constricts to increase venous return. 37
The Muscular Pump 38
Maintaining Blood Pressure Cardiac Output, Blood Pressure and Peripheral Resistance have interrelating variables. changes in one variable that affects blood pressure will be corrected not by one variable, but many variables. major factors enhancing cardiac output in figure 19.7 remember that the heart is primarily controlled by the cardioinhibitory center in the medulla. stroke volume is controlled mainly by venous return (EDV). during stress, the cardioacceleratory center in the medulla, activates the sympathetic n.s. increases heart rate by acting on SA node increases stroke volume by enhancing cardiac contractility (ESV) the enhanced cardiac output results in an increased MAP, which represents an increased blood flow to the tissues. 39
Factors Increasing Cardiac Output 40
Maintaining Blood Pressure Short-term neural control mechanisms: operate via a reflex arc baroreceptor to vasomotor center of medulla then to vascular smooth muscle. (see figure 19.8) two main goals of short-term neural control: maintain adequate MAP by altering vessel diameter. if BP is low, blood flow to less necessary tissues is diverted. altering blood distribution for demands and situations. if exercising blood goes to muscles more than digestive organs. Warning: vigorous rubbing of the carotid bifurcation where the baroreceptors are can quickly drop a person s heart rate (and therefore CO). Clinically used if person is in supraventricular tachycardia and no drugs ready. 41
Baroreceptors Maintain Blood Pressure Baroreceptor Locations: the bifurcation of the common carotids in the aortic arch in the walls of the large arteries of the thorax and neck. activate when stretched (or rubbed vigorously) sends signal to stimulate cardioinhibitory center in medulla. end result is vasodilation of arteries & veins thus dropping BP. also sends signal to inhibit cardioaccelatory center in medulla. end result is a drop in heart rate and heart contractile forces. Main function of Baroreceptors is to protect you from sudden drops in BP with changes of position: laying to standing (BP would drop quickly if not for them). the carotid sinus reflex preserves blood flow to the brain. 42
Baroreceptor & BP Homeostasis 43
Baroreceptor & BP Homeostasis 44
Hormonal Control of BP 45
Maintaining Blood Pressure Long-term renal regulation of BP: done by altering blood volume by controlling how much you urinate. direct renal mechanism alters blood volume independently of hormones. increase blood volume and BP will increase the rate of fluid filtration through the kidney and this leads to more fluid ending up in the urine. indirect renal mechanism the renin-angiotensin mechanism. activated by dropping BP, ends with vasoconstriction. Aldosterone secretion to increase Na reabsorption 46
Renal Control of Blood Pressure 47
Factors Causing an Increased MAP 48
Pulses & Blood Pressure Pulse sites can also be used as pressure points to stop blood flow distal to them in case of a severe bleeding injury. Blood Pressure Measurement done by sphygmomanometer and auscultation. pressure is increased above systolic the slowly released listening for the sound of blood to begin flowing again in artery. This is the systolic BP. flow at this point has gone from laminar flow to turbulent flow... this makes Korotkoff sounds! the pressure at which turbulent flow goes back to laminar the sounds vanish (usually) is diastolic BP. 49
Easily Palpated Pulse Sites 50
Blood Pressure Abnormalities Hypertension systolic BP over 140 mmhg. diastolic BP over 90 mmhg. transient HTN is normal if due to pain, fever, exercise or stress!!! persistent HTN is what needs medical treatment. Hypotension values vary, need a symptom like dizziness. orthostatic HoTN due to slow to respond SNS as we age. i.e. you get dizzy getting up to quick. you will stop urine formation if it gets too low! 51
Tissue Perfusion: Rest vs Exercise 52
Blood Flow Velocity vs Total Cross-Sectional Vessel Area 53
Control of Arteriole Smooth Muscle Diameter 54
Blood Flow in Special Location Skeletal Muscle: when muscles become active blood flow increases. is in direct proportion to their metabolic activity is called active or exercise hyperemia. Brain: blood flow is ~750 ml/min and is relatively constant. most sensitive to Carbon Dioxide levels increasing. less sensitive to Oxygen levels dropping. Fainting occurs when MAP is < 60 mmhg. Cerebral edema occurs if MAP is > 160 mmhg. 55
Blood Flow in Special Locations Skin: capillaries are in the papillary layer of dermis. epidermis has no blood vessels (diffusion). shunting blood to skin can release heat more heat loss if skin wet (sweating). Lungs: the arteries here look like veins microscopically systolic pressure is 24 mmhg. diastolic pressure is 8 mmhg. low oxygen in lung will constrict blood vessels. consistent with gas exchange done here. areas with disease have blood shunted past them! 56
4 Capillary Transport Mechanisms 57
4 Capillary Transport Mechanisms 58
Capillary Flow Dynamics Bulk Flow: about 20 Liter of fluid is filtered of the capillaries daily before returning to the blood stream. direction of this flow and the volume is affected by: Capillary Hydrostatic Pressure (HPc): this force filters blood through the capillary walls leaving behind the cells and most proteins. Arterial End HPc is 35 mmhg Venous End HPc is 17 mmhg the Interstitial Fluid Hydrostatic Pressure (HPif) pushes fluid back into the capillary also, the capillary colloid osmotic pressure (OPc) or oncotic pressure at about 26 mmhg pulls fluid into it. 59
Fluid Flows at Capillaries Remember that unlike HP, the OP will not vary much as the proteins predominately stay inside the capillary and help contribute to the ability to pull water back in. 60
Fluid Flows at Capillaries 61
Events & Signs of Compensated Hypovolemic Shock 62
Pulmonary Circulation 63