Monash University NUR1112 Circulatory System Lectures 2024 PDF

Steve Gschmeissner / Photo Researchers Inc. The Circulatory System NUR1112 Fundamental Skills and Knowledge for Nursing and Midwifery Practice 1  Lectures prepared and delivered by: Dr Natalie Bennett, Peninsula Campus Unless otherwise stated, all imag...

Steve Gschmeissner / Photo Researchers Inc. The Circulatory System NUR1112 Fundamental Skills and Knowledge for Nursing and Midwifery Practice 1  Lectures prepared and delivered by: Dr Natalie Bennett, Peninsula Campus Unless otherwise stated, all images are the property of Pearson Education Limited and are sourced from: Marieb & Hoehn, Human Anatomy & Physiology, 10th ed., 2016 and Martini, Ober & Nath, Visual Anatomy & Physiology, 2011 Warning: This material has been reproduced and communicated to you by or on behalf of Monash University under Part VB of the Copyright Act 1968 (the Act). The material in this communication may be subject to copyright under the Act. Any further reproduction or communication of this material by you may be the subject of copyright protection under the Act. Do not remove this notice. Learning Objectives 1. Compare the structure of arteries, arterioles, capillaries, venules and veins, and relate the structure of each vessel type to its function. 2. Define blood flow, blood pressure, and resistance, and explain the relationship between these factors. 3. Explain systolic pressure, diastolic pressure, mean arterial pressure, and pulse pressure. 4. List and explain the factors that influence blood pressure and describe how blood pressure is regulated. 5. Explain the movement of fluid, respiratory gases and nutrients between capillaries, tissues and back again. 6. Locate clinically relevant blood vessels. Why is this important? ¡ This module is foundational for informed clinical practice ¡ Blood pressure measurement enables an assessment of the effectiveness of the cardiovascular system ¡ Trends in vital signs, such as blood pressure, over time enable accurate decision making in the planning of treatment The Circulatory System What is the overall purpose of the cardiovascular system? To provide adequate blood flow to all tissues/organs according to their immediate needs Blood Vessels Objective 1: Compare the structure of arteries, arterioles, capillaries, venules and veins, and relate the structure of each vessel type to its function. Venous system Arterial system Large veins Heart (capacitance vessels) Elastic arteries (conducting vessels) Lymphatic Muscular arteries system (distributing vessels) Small veins (capacitance vessels) Lymphatic capillary Arterioles (resistance vessels) Postcapillary venule Terminal arteriole Vascular shunt Capillaries Precapillary sphincter (exchange vessels) M&H Figure 19.1 Blood vessel structure ¡ Blood vessel walls are typically formed from three layers or tunics (coverings) 1. Tunica intima – inner layer 2. Tunica media – middle layer 3. Tunica externa – outer layer ¡ Tunics surround central space = vessel lumen ¡ Tunics provide specific physical properties that facilitate vessel function: STRUCTURE FUNCTION Blood vessel structure A photomicrograph of an artery and a vein Artery Artery Tunica intima Inner endothelium Outer basement Elastin Vein membrane Tunica media Endothelium Smooth muscle Elastin à Vasoconstriction à Vasodilation Tunica externa Vein Tough connective tissue Collagen fibres Tunica intima Tunica media Tunica externa  Heart Elastic arteries ¡ Thick-walls ¡ Located near the heart ¡ Diametre: large (1 – 2.5 cm) ¡ Elastin preset in all tunics ¡ Conducting vessels – conduct blood away form the heart Heart Elastic arteries Muscular Arteries ¡ Distal to elastic arteries Heart ¡ Thick tunica media ¡ Distributing vessels - Change diametre to control blood flow to body regions and organs  http://slideplayer.com/9502577/30/images/40/Dynamic+adjustments+in+the+blood+distribution+to+the.jpg Arterioles ¡ Smallest arteries ¡ Diametre 10 µm – 0.3 mm Pressure reserve (“arteries”) ¡ Predominantly tunica media ¡ Resistance vessels: change their diametre to Fluid exits via variable control resistance to blood flow à controls resistance outflow tubes (“arterioles”) flow into capillary beds within specific tissues/organs ¡ Vasoconstriction à decreases blood flow ¡ Vasodilation à increases blood flow Flow to “tissues” Summary Elastic arteries Muscular arteries Arterioles Medium Diametre Large (1 - 2.4 cm) Small (10 µm – 0.3 mm) (0.3 mm - 1 cm) Structure Thin walls, mainly High elastic content Thick tunica media tunica media Resistance vessels – Distributing vessels – Conducting vessels – change diameter to change diameter to use elastic recoil to control resistance to Function(s) control blood flow to conduct blood away blood flow and flow body regions and from the heart into tissues/capillary organs beds  Heart Capillaries ¡ Microscopic vessels ¡ Average length 1 mm, diametre 8-10 µm ¡ Thin walls of tunica intima and a supportive basement membrane ¡ There are ~ 40 billion capillaries in an adult body ¡ Exchange vessels à exchange of nutrients, wastes, gases, hormones etc. with interstitial fluid and thus with cells ¡ Rich supply in most tissues – but there are exceptions … ¡ Three structurally and functionally distinct types of capillaries 1. Continuous capillaries 2. Fenestrated capillaries 3. Sinusoidal capillaries Continuous Capillaries ¡ Endothelial cells joined by tight junctions to Basal lamina form a smooth, lining Endothelial cell Nucleus ¡ Intercellular clefts - some gaps between endothelial cells allow limited passage of fluids and small solutes A continuous capillary ¡ Pinocytotic vesicles Vesicles containing materials transported ferry fluids and larger across the endothelial solutes across the cell Intercellular cleft capillary wall Basement membrane (basal lamina) Fenestrated Capillaries ¡ Endothelial cells contain pores (fenestrations) ¡ Pores increase permeability to allow rapid exchange of fluids and small solutes ¡ Found in areas of active filtrations (kidneys), absorption (small intestine) or in endocrine glands Sinusoidal Capillaries ¡ Most leaky capillaries ¡ Large spaces between endothelial cells (sinusoids) and large fenestrations, incomplete basement membrane ¡ Slow blood flow allows large molecules and cells to pass between the blood and tissues ¡ Found in the liver, lymphoid organs, adrenal medulla Summary Continuous capillaries Fenestrated capillaries Sinusoidal capillaries Structure Endothelial cells joined As for continuous Endothelial cells with by tight junctions capillaries but large fenestrations (smooth, low friction endothelial cells have (pores) and large lining), gaps = fenestrations (pores) spaces between cells intercellular clefts (sinusoids) Liver, lymphoid organs Skin, muscles, lungs, Kidneys, small intestine, (i.e. spleen), bone Location central nervous system endocrine glands marrow, adrenal medulla Filtration, absorption or Capillary exchange - secretion – allow larger Very leaky capillaries – allow fluid and small volumes of fluid and allow cells and large Function solutes to enter/exit via slightly larger solutes to proteins to enter/exit via intercellular clefts enter/exit via clefts and sinusoids fenestrations Venules & Veins ¡ Capillaries unite to form venules ¡ Venules unite to form veins ¡ Large lumen à easy blood flow ¡ Tunica intima folds to form valves ¡ Little smooth muscle or elastin ¡ Thick tunica externa of collagen fibres ¡ Capacitance vessels: thick tunica externa provides support for accommodating a large blood volume Capillaries 5% M&H Figure 19.5 Summary Venules Veins Diametre 8 – 100 mm Medium (100 mm – 1.5 cm) Structure Thin walls Thin walls, tunic intima forms valves, some tunic media, relatively thick tunic externa Return blood to the heart, large capacity to hold blood and act Function(s) Drain capillary beds as a blood reserve (can hold 60% of blood volume at one time) à capacitance vessels Flow, Pressure & Resistance Objective 2: Define blood flow, blood pressure and resistance, and explain the relationships between these factors. Blood flow, blood pressure and resistance determine the dynamics of circulation. Blood Flow Blood flow is the volume of blood flowing through a vessel, organ or the entire circulation (= cardiac output) in a given time period. ¡ Measured in ml/min ¡ Relatively constant when at rest ¡ Varies in individual tissues/organs, depending on need ¡ Blood flow is determined by ¡ Blood pressure ¡ Resistance The goal of the cardiovascular system is to maintain adequate blood flow  Blood Pressure Blood Pressure Blood pressure is the force exerted on a vessel wall by the blood in that vessel. ¡ Expressed in mmHg ¡ Measured as systemic arterial blood pressure in large arteries near the heart ¡ Force, generated by the pumping action of the heart, that keeps blood moving (i.e. maintains blood flow) à blood moves from an area of high pressure to lower pressure, i.e. down a pressure gradient https://commons.wikimedia.org/wiki/File:Sphygmomanometer.jpg  Peripheral resistance Resistance Resistance is the opposition to blood flow. Resistance is a measure of the amount of friction blood encounters as it flows through a vessel. ¡ Three primary sources of total peripheral resistance (TPR) ¡ Blood viscosity Blood Blood vessel Vessel Obstructions viscosity length diameter in vessels ¡ Total blood vessel length ¡ Blood vessel diametre  Blood viscosity TPR: Viscosity ¡ Thickness or stickiness of a fluid by JohnnyMrNinja, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=18145986 Q. Which requires more effort to drink through a straw – a milkshake or a By CamEvans from Sacramento,California, California – MilkshakeUploaded thickshake? ¡ Due to the concentration of blood cells and plasma proteins ¡ Normally fairly constant but alters with changes in ¡ Blood cell numbers (i.e. changes in RBC) ¡ increased haematocrit (e.g. polycythaemia) à ­ viscosity ¡ decreased haematocrit (e.g. anaemia) à ¯ viscosity ¡ Plasma volume, e.g. dehydration à ­ viscosity  Blood vessel length TPR: Vessel length By krzyboy2o - milkshake anyone?Uploaded by JohnnyMrNinja, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=18146645 ¡ Resistance to flow increases as the vessel length increases Q. Is it easier to drink a milkshake through a short straw or a long straw? ¡ Relatively constant in adults ¡ Changes over time in children with growth  Vessel diameter TPR: Vessel Diametre ¡ The amount of contact between two surfaces determines the amount of friction – friction determines ease of movement ¡ In a blood vessel, the more contact the blood has with Greatest resistance, the walls of the vessel slowest flow near surfaces means more friction Least resistance, between the blood and greatest the vessel wall à more flow at center resistance to blood flow TPR: Vessel Diametre ¡ As vessel diametre changes, the degree of resistance to blood flow changes, depending on how much of the blood in the vessel is contacting the vessel walls: i.e. ­ vessel diametre à ¯ contact between blood and vessel walls à ¯ friction à ¯ resistance to blood flow and vice versa A SMALL CHANGE IN VESSEL DIAMETRE à A BIG CHANGE IN RESISTANCE AND THUS BLOOD FLOW  Vessel diameter TPR: Vessel Diametre ¡ Changes in blood vessel diametre are ¡ Frequent ¡ Significantly alter resistance and blood flow ¡ Small diametre arterioles act as resistance vessels ¡ Vasoconstriction à decreases diametre à increases resistance à decreases blood flow ¡ Vasodilation à increases diametre à decreases resistance à increases blood flow ¡ Changes in resistance (via changes in arteriole diametre) are the primary means of altering local blood flow TPR: Vessel Diametre ¡ Obstructions/protrusions of the vessel wall à decrease vessel diametre and induce turbulent blood flow à increase resistance and interferes with blood flow i.e. damaged endothelium or atherschlerotic plaques Turbulence Plaques Plaque deposit Flow, Pressure & Resistance ¡ Blood flow (F) is determined by: ¡ The difference in blood pressure between two points in the circulation (i.e. the pressure gradient) ¡ Resistance (R) P F= R à Direct relationship between F and P, i.e. if the pressure gradient increases, flow increases à Indirect relationship between F and R, i.e., if the resistance increases the flow decreases  P Flow, Pressure & Resistance F= R ¡ Our cardiovascular system tries to maintain adequate blood flow by altering TPR (R) ¡ During exercise, blood flow to skeletal muscles must increase … HOW? 1. Vasodilation will reduce resistance and increase blood flow, BUT if this is not sufficient … 2. Pressure must increase … HOW? … By increasing the action of the heart, i.e. increasing cardiac output (remember – the heart generates pressure that drives blood flow) ¡ Resistance is altered/adjusted continually to provide adequate blood flow  Blood Pressure Systemic Blood Pressure Objective 3: Explain systolic pressure, diastolic pressure, mean arterial pressure and pulse pressure. ¡ The pumping action of the heart generates pressure, which in turn, drives blood flow ¡ Blood flows down a pressure gradient – from an area of high pressure to an area of low pressure ¡ Blood flow is opposed by resistance ¡ Blood pressure decreases with distance from the heart as it overcomes resistance to drive blood flow … Systemic Blood Pressure BP highest Steepest drop in in aorta resistance vessels Pressure drops to almost 0 mmHg Pressure declines throughout system (“energy” that is reduced as it overcomes resistance to keep blood flowing) Direction of blood flow (down pressure gradient) Arterial Blood Pressure ¡ Arterial pressure reflects two factors: 1. How much elastic arteries can be stretched, i.e. COMPLIANCE 2. The volume of blood forced into the elastic arteries by ventricular contraction, i.e. STROKE VOLUME (e.g. increased SV à increased pressure as more blood moves into the arteries and pushes on the artery walls) ¡ Blood pressure in elastic arteries near the heart is pulsatile (i.e. not constant) – due to the pumping action of the heart à gives rise to two extremes of pressure … Arterial blood pressure ¡ Systolic pressure = peak pressure generated in the large arteries when the ventricles contract ¡ Average 120 mmHg (in a healthy, young adult) ¡ Normal range 90-120 mmHg ¡ Systolic pressure increases as compliance decreases (afterload) ¡ Diastolic pressure = pressure in the large arteries during ventricular relaxation ¡ Average 80 mmHg ¡ Normal range 60-80 mmHg Pulse Pressure ¡ Pulse pressure = systolic pressure Superficial temporal minus diastolic pressure artery i.e. 120 – 80 = 40 mmHg Facial artery Common carotid ¡ Felt as a throbbing pulsation in artery an artery (a pulse) Brachial artery ¡ Pulse points - superficial Radial artery arteries, often overlying bone Femoral artery ¡ Declines with increasing distance from the heart Popliteal artery Posterior tibial artery Dorsalis pedis artery  Blood Pressure Mean Arterial Pressure ¡ Mean arterial pressure (MAP) = the pressure that propels blood through the vessels ¡ Average between systolic and diastolic pressure (as diastole lasts longer than systole, MAP is not halfway between these two pressures – see next slide) ¡ Declines with increasing distance from the heart ¡ Defined as: MAP = diastolic pressure + (1/3 x pulse pressure) = 80 + (1/3 x 40) = 93 mmHg  Blood Pressure Systolic Beca use spend the hear s twic t Pulse pressure, 120 long e a s in the difference Mean arterial pressure it do diastole es in a between systolic (MAP), the sum of the systol s e and diastolic 100 diastolic pressure and pressures one-third of the pulse pressure 80 MAP = DP + 1/3PP Diastolic i.e. 90 + (120 – 90 )/3 60 or mm Hg 90 + 10 = 100 mmHg 40 MAP is the driving force for blood 20 flow 0 Aorta Elastic Muscular Arterioles Capillaries Venules Medium- Large Venae arteries arteries sized veins veins cavae Mean Arterial Pressure ¡ MAP can also be calculated as follows: 1. If blood Flow F = P/R or F = MAP/R 2. and total blood flow = cardiac output à CO = MAP/R 3. Then rearrange the equation à MAP = CO x R ¡ Anything that ­ CO or R will ­ BP … why? ¡ ­ R via vasoconstriction à there is less room in the vessel for the blood (¯ lumen volume), thus blood pushes harder against the vessel walls à ­ BP and … ¡ Vasoconstriction will ­ venous return à ­ CO à ­ MAP ¡ ­ CO (i.e. via ­ HR or SV) à ­ MAP Alterations in Blood Pressure ¡ Hypotension ¡ Systolic pressure is below 90 mmHg ¡ May result in dizziness and fainting ¡ Hypertension ¡ Transient elevation due to exercise, illness, emotions ¡ Chronic hypertension ¡ Sustained systolic pressure > 140 mmHg ¡ Major cause of heart failure, vascular disease, stroke ¡ Risk factors: smoking, stress, diet, obesity, age, health https://cdn.pixabay.com/photo/2017/05/13/22/28/blood-pressure-2310824__340.jpg Capillary Blood Pressure ¡ Ranges from ¡ 35 mmHg at the arterial end of the capillary bed to ¡ 15 mmHg at the venous end of the capillary bed ¡ Low capillary pressure is required because 1. High pressure would damage thin-walled, fragile capillaries 2. Most capillaries are very permeable so low pressure is adequate for fluid exchange with tissues Arteriole Vein Capillaries Venous Blood Pressure ¡ Fairly constant at ~ 15 mmHg ¡ Does not change significantly with cardiac cycle ¡ Very small pressure gradient, too low to provide adequate venous return to the heart à So how does blood return to the heart ??? Venous Return 1. Valves compartmentalise blood to shift it in small volumes and prevent blood backflow 2. Muscular pump – skeletal muscle contraction squeezed veins and helps push blood toward the heart 3. Respiratory pump – pressure changes during breathing help blood move toward the heart by squeezing abdominal veins as thoracic veins expand 4. Pulsation of nearby arteries 5. Venoconstriction of tunica media under sympathetic control  Blood Pressure Regulation of Blood Pressure Objective 4: List and explain the factors that influence blood pressure and describe how blood pressure is regulated. ¡ Regulation of blood pressure is essential to ensure adequate blood flow to vital organs à the brain must co-ordinate the activities of the heart, blood vessels and kidneys ¡ Factors that determine blood pressure (and thus blood flow) include: Peripheral resistance 1. Cardiac output (rapid, short-term regulation) Cardiac 2. Peripheral resistance (rapid, short-term regulation) output 3. Blood volume (slower, long-term regulation) Blood volume Q. What happens to BP when these factors change? Factors that Determine Blood Cardiac output Pressure – Cardiac Output ¡ Cardiac output = rapid, short-term regulation of BP and blood flow ¡ CO = stroke volume x heart rate ¡ Determined by EDV (preload), blood volume, venous return, ESV, contractility, ANS, hormones, plasma electrolytes … ¡ ­ SV or HR à ­ CO (= an ­ in the volume of blood moving from the heart into the arteries) à ­ BP ¡ ¯ SV or HR à ¯ CO (= a ¯ in the volume of blood moving the heart into the arteries) à ¯ BP ¡ Cardiac output directly determines blood pressure  Peripheral resistance Factors that Determine Blood Pressure - Resistance ¡ Peripheral resistance (TPR) = rapid, short-term regulation of BP and blood flow ¡ Primarily altered by changing arteriole diametre ¡ Vasoconstriction à ­ R à ¯ flow (Note: ­ BP due to ¯ lumen volume) To maintain flow more pressure must be applied – generated by the heart à ­ R + ­ CO à ­ MAP à maintains/increases flow ¡ Vasodilation à ¯ R à ­ flow (Note: ¯ BP due to ­ lumen volume)  Factors that Determine Blood Blood volume Pressure – Blood Volume ¡ Blood volume (BV) = slower, long-term regulation of BP ¡ Controlled by renal and endocrine mechanisms ¡ ­ BV à more blood pushing on vessel walls à ­ BP ¡ ¯ BV à less blood pushing on vessel walls à ¯ BP ¡ Changes in blood volume alter venous return, EDV and preload, and thus SV, CO and BP ¡ Small increases in blood volume offset by vessel compliance (stretch) but compliance decreases with age/atherosclerosis (i.e. afterload increases), thus small ­ BV à ­ BP https://cdn.pixabay.com/photo/2012/04/13/20/39/test-tube-33570__340.png Summary: Factors that Determine Blood Pressure Blood vessel Vessel Blood Obstructions Stroke Heart rate Water loss Water gain length diameter viscosity in vessels volume Blood Aorta Urine Time determine determine determine Peripheral Cardiac Blood resistance output volume determine Blood Pressure  Blood Pressure Regulation of Blood Pressure Blood pressure, and thus blood flow, are regulated at multiple levels: 1. Autoregulation – occurs within tissues and is dependent on local conditions 2. Neural regulation – involves the cardiovascular centres, the autonomic nervous system and the baroreceptor reflex 3. Renal mechanisms 4. Endocrine regulation The above mechanisms alter cardiac output, peripheral resistance and/or blood volume in order to regulate blood pressure and thus maintain adequate blood flow Autoregulation: Local Regulation of BP and Flow ¡ Tissues regulate their own blood pressure and flow in response to local conditions by altering arteriole diameter to regulate blood flow into capillary beds ¡ Intrinsic regulation: (control from within) ¡ Metabolic control ¡ ­ CO2, ¯ O2, ¯ pH à arteriole dilation à ­ blood flow into capillary bed à restores homeostasis Arteriole Capillaries Venule ¡ opposite if conditions reversed (a) Arterioles dilated—blood flows through capillaries. ¡ Myogenic (muscle) control ¡ High systemic BP à arteriole stretch à reflex constriction à ¯ blood flow into capillary bed à prevent damage ¡ reduced stretch à dilation … (b) Arterioles constricted—no blood flows through capillaries. Neural Regulation Medulla oblongata Vagus nerves (CN X) Sympathetic ¡ The cardiovascular centres in (parasympathetic) ganglia the medulla oblongata of the Sympathetic brainstem contain 3 centres: nerves 1. Cardioinhibitory centre Spinal cord ¡ Parasympathetic input (CN X) into SA and AV nodes ¡ Slows heart rate à ¯ CO 2. Cardioacceleratory (cardiostimulatory) centre ¡ Sympathetic input into SA and AV nodes à ­ heart rate à ­ CO ¡ Sympathetic input into ventricular myocardium à ­ force of contraction à ­ stroke volume à ­ CO  3. Vasomotor centre ¡ Sympathetic vasomotor fibres to the smooth muscle of arterioles à changes in vasomotor tone à changes in vessel diameter – for peripheral (most) blood vessels: Increased sympathetic activity à ­ vasomotor tone (i.e. muscle contraction) à vasoconstriction à ­ BP Vasomotor centre Decreased sympathetic activity à ¯ vasomotor tone (i.e. muscle relaxation) à vasodilation à ¯ BP Note: the opposite response occurs in arteries/arterioles of the heart, skeletal muscles and liver due to inhibitory β2 receptors for NA à vasodilation ¡ Input from the hypothalamus (ANS control centre) - regulates ¡ blood flow to maintain body temperature ¡ cardiovascular function in a fight or flight response Neural regulation relies on the baroreceptor reflex Baroreceptors: ¡ Stretch receptors ¡ Detect changes in pressure ¡ Inform the medullary cardiovascular centres ¡ Locations: ¡ Carotid artery sinuses ¡ Aortic arch ¡ Walls of most large arteries in the neck and thorax ¡ Initiate the baroreceptor reflex Medical Dictionary, © 2009 Farlex and Partners  3. ­ frequency of impulses stimulates cardioinhibitory centres, inhibits the 4a. ­ parasympathetic cardiostimulatory centre and and ¯ sympathetic inhibits vasomotor centre impulses to heart cause ¯ HR, ¯ contractility (SV) and thus ¯ CO The baroreceptor reflex … 2. Baroreceptors in carotid sinuses and aortic arch are stimulated 4b. ¯ sympathetic impulses to tunica media results in 1. Stimulus: ­ BP vasodilation, causing ¯ R (arterial pressure (peripheral vessels) 5. ¯ CO and ¯ R rises above return BP to normal range) homeostatic range 5. ­ CO and ­ R return 1. Stimulus: ¯ BP BP to homeostatic (arterial pressure range falls below normal range) 4b. ­ sympathetic impulses to tunica media results in vasoconstriction, causing ­ R (peripheral vessels) 2. Baroreceptors in 4a. ¯ parasympathetic carotid sinuses and ­ sympathetic and aortic arch impulses to heart are inhibited cause ­ HR, ­ contractility (SV) and thus ­ CO 3. ¯ frequency of impulses inhibits cardioinhibitory centres, stimulates the cardiostimulatory centre and Summary notes stimulates vasomotor centre on next slide The baroreceptor reflex responses … Cardiovascular centre in Response to high BP Response to low BP medulla oblongata (goal = decrease BP) (goal = increase BP) Cardioinhibitory centre Stimulated à ­ Inhibited à ¯ (parasympathetic à SA & parasympathetic parasympathetic AV nodes) stimulation of heart à stimulation of heart à ¯ HR ­ HR Cardiostimulatory centre Inhibited à ¯ Stimulated à ­ (sympathetic to SA & AV sympathetic sympathetic nodes and ventricular stimulation of heart à stimulation of heart à myocardium) ¯ HR and SV ­ HR and SV Vasomotor centre Inhibited à ¯ Stimulated à ­ (sympathetic to tunica sympathetic sympathetic media of blood vessels) stimulation of tunica stimulation of of media à vasodilation tunica media à of peripheral vessels vasoconstriction of peripheral vessels Baroreceptor Reflexes ¡ Function rapidly to protect against (short-term) changes in blood pressure (and thus maintain blood flow) e.g. posture changes – standing from a reclining position à temporary pooling of blood due to gravity à ¯ BP à blood flow to brain reduced à inadequate oxygen supply à dizziness and fainting ¡ Carotid sinus baroreceptor reflex à monitors BP to ensures adequate blood flow to the brain ¡ Aortic baroreceptor reflex à monitors BP to maintains blood flow in the systemic circuit ¡ Note: baroreceptor reflexes are ineffective against sustained blood pressure changes, e.g. chronic hypertension Renal Mechanisms 1. Direct mechanism ¡ Blood pressure directly determines the rate of urine formation à alters blood volume à alters blood pressure i.e. ­ BP à activates baroreceptor reflex à peripheral vasodilation à ­ renal blood flow à ­ filtration rate in kidneys à ­ urine production à ¯ BV and thus ¯ BP or ¯ BP à activates baroreceptor reflex à peripheral vasoconstriction à ¯ renal blood flow à ¯ filtration rate in kidneys à ¯ urine production à preserves/maintains BV à prevent further decrease in BP Renal Mechanisms 2. Indirect mechanism – involves hormones: Activated by a DECREASE Renin-angiotensin-aldosterone system (RAAS) in MAP ¡ ¯ BP à à à angiotensin II production, which: i. Stimulates peripheral vasoconstriction à ­ R à ­ BP (also ­ venous return and thus CO) ii. Stimulates aldosterone secretion à ­ renal reabsorption of Na+ ions and thus water from the filtrate à ¯ urine production à maintains BV and BP iii. Stimulates ADH release à ­ renal water reabsorption from the filtrate à maintains BV and BP (also promotes peripheral vasoconstriction) iv. Stimulates thirst à ­ BV and thus BP  Arterial pressure Initial stimulus Physiological response Result Inhibits baroreceptors Renin-angiotensin-aldosterone system (RAAS) Sympathetic NS activity (via b. reflex) Angiotensinogen (an inactive precursor) Target for anti- Renin release hypertensive drugs, from kidneys e.g. captopril Angiotensin I Angiotensin converting enzyme (ACE) Angiotensin II ADH release by Thirst via Vasoconstriction; Adrenal cortex posterior pituitary hypothalamus peripheral resistance Secretes Aldosterone Renal Na+ Renal water Water intake reabsorption reabsorption (increases blood volume) ­ Venous return (maintains blood volume) and cardiac output Notes on Blood volume the previous MAP slide… Endocrine Regulation Adrenal gland Kidney Hormones that increase BP: Hormones that decrease BP: ¡ Adrenalin and noradrenalin à rapid ­ in CO (­ both SV & HR) ¡ Atrial natriuretic peptide and peripheral vasoconstriction (ANP) ¡ Produced by the ¡ Angiotensin II à heart in response to vasoconstriction, thirst, and high BP promotes secretion of: ¡ Opposes the actions o Aldosterone à ­ renal sodium of angiotensin II à ¯ ion and water reabsorption BV and BP o ADH à peripheral vasoconstriction and ­ renal water reabsorption  Central Regulation Central regulation involves neural and renal/endocrine Short-term Stimulation of mechanisms. baroreceptors Activation of elevation of BP by sympathetic Neural mechanisms elevate and cardiovascular simulation of the cardiac output and reduce Neural chemoreceptors centers heart & peripheral blood flow to nonessential or mechanisms vasoconstriction inactive tissues via peripheral vasoconstriction. Stimulation Long-term Renal/endocrine mechanisms Of RAAS increase in BV and thus BP involve long-term increases in Renal-Endocrine mechanisms blood volume and thus blood pressure. If autoregulation is ineffective Autoregulation Autoregulation HOMEOSTASIS HOMEOSTASIS involves changes in RESTORED RESTORED patterns of blood Local decrease flow within capillary in resistance beds as arterioles and increase HOMEOSTASIS in flow dilate or constrict Normal in response to Local arterioles dilate blood pressure chemical changes Inadequate and volume in the interstitial fluid. local blood flow HOMEOSTASIS DISTURBED Start Increased tissue activity Chemical changes (decreased O2 or pH, increased CO2 ) Physical stress (trauma, high temperature) Martini et al., 2012, Figure 18.18 Capillary Dynamics Objective 5: Explain the movement of fluid, respiratory gases and nutrients between capillaries, the tissues and back again. ¡ Blood flow through capillaries is slow and intermittent – controlled by arteriole diamtere in response to local conditions (e.g. low O2 or high CO2 levels) ¡ When blood flows through capillary beds exchange occurs ¡ Capillary exchange involves: 1. Exchange of solutes - respiratory gases, nutrients and wastes 2. Bulk flow of fluid (which carries solutes)  Capillary lumen Pinocytotic vesicles Endothelial fenestration Intercellular (pore) 4. Active transport cleft via vesicles: endocytosis and exocytosis (large substances, e.g. proteins) 3. Movement through Basement fenestrations (small, water- membrane soluble substances, e.g. electrolytes, glucose, amino acids) 2. Movement through intercellular clefts 1. Diffusion through (water-soluble endothelial substances, e.g. membranes electrolytes, glucose, (lipid-soluble amino acids) substances, e.g. O2, CO2) Capillary Exchange Bulk Flow of Fluid ¡ Fluid moves across capillary walls by bulk flow ¡ out of the capillary at the arterial end ¡ into the capillary at venous end ¡ Fluid moves through: ¡ Intercellular clefts (between endothelial cells in all capillaries) ¡ Fenestrations (pores within endothelial cells in some capillaries) ¡ Sinusoids (big gaps between endothelial cells in some capillaries) ¡ Bulk fluid flow determines the relative fluid volumes of the blood and ISF ¡ Direction and volume of fluid movement is determined by two opposing forces: hydrostatic and colloid osmotic pressures Hydrostatic Pressure ¡ Force exerted by fluid pushing against a tissue wall ¡ Capillary hydrostatic pressure (HPc) ¡ Force of the blood plasma on the capillary walls = blood pressure ¡ Pushes fluid and solutes out of capillaries through intercellular clefts/fenestrations/sinusoids at the arterial end of bed ¡ Some fluid, cells and most proteins remain in the capillary https://cdn.pixabay.com/photo/2016/02/16/19/28/burpee-1203906_960_720.jpg Colloid Osmotic Pressure ¡ Force related to the tonicity (solute concentration) of a solution à pulling force ¡ Capillary colloid osmotic pressure (OPc) ¡ Due to the presence of solutes within the plasma that are unable to diffuse out of the capillary, e.g. proteins ¡ These solutes pull fluid back into the capillaries at the venous end of the bed https://cdn.pixabay.com/photo/2015/04/16/16/41/fitness-725881__340.jpg Capillary bulk flow Tissue cells PUSH > PULL à fluid moves out ¡ The difference Capillary between the H2O HP (PUSH) = 15 mm Hg push and pull Arteriole HP (PUSH) = 35 mm Hg OP (PULL) = 25 mm Hg Venule forces OP (PULL) = 25 mm Hg determines net H2O fluid loss from, Interstitial fluid PUSH < PULL à fluid moves in or gain to, capillaries © 2016 Pearson Education, Ltd. PUSH – PULL = ¡ ~ 20 L/day pushed out net filtration pressure ¡ ~ 17 L/day pulled in ¡ ~ 3 L/day drained from ISF by lymphatic system Oedema ¡ Oedema is an abnormal increase in the volume of interstitial fluid ¡ Due to an increased push, or decreased pull, force: ¡ Increased capillary hydrostatic pressure (i.e. BP) ¡ High blood volume, local vessel blockage, incompetent venous valves ¡ Inflammation à causes increased capillary permeability ¡ Decreased colloid osmotic pressure ¡ Low levels of plasma proteins due to liver disease or malnutrition ¡ Blockage of lymphatic vessels à preventing fluid drainage from tissues ¡ Parasites, surgical removal Clinically Relevant Blood Vessels Objective 6: Locate clinically relevant blood vessels. Helpful information: ¡ the name of a vessel may reflect the body region where it is found (e.g. brachial, femoral), the organ it serves (e.g. renal or hepatic), or the bone it follows (e.g. radial or tibial) ¡ arteries are usually deep, while veins are both deep and superficial ¡ deep veins tend to run parallel with arteries (and nerves) and share the same names, while superficial veins tend to have distinct names Pulse Superficial temporal artery points Facial artery Common carotid Compression of artery the superficial region of an Brachial artery artery against firm underlying Radial artery tissues enables pulse pressure, Femoral artery and thus heart Popliteal artery rate, to be easily Posterior tibial monitored. artery Dorsalis pedis artery Blood pressure measurement Systemic arterial blood pressure is most commonly measured indirectly in the brachial artery. Veins for venepuncture The median cubital vein, located in the cubital fossa (fold of the elbow) connects Cephalic)vein) the basilic and cephalic veins. All three of these veins are Basilic)vein) common locations for venepuncture, particularly the median cubital vein. Median)cubital)vein)