Blood Vessels PDF

Blood Flow and Pressure ❖ Blood flow = volume of blood that flows through any tissue in a given time period (mL/min). Total blood flow is cardiac output (CO) = heart rate (HR) × stroke volume (SV). ❖ 2 factors affect blood flow: (1) the pressure difference that drives the blood flow...

Blood Flow and Pressure ❖ Blood flow = volume of blood that flows through any tissue in a given time period (mL/min). Total blood flow is cardiac output (CO) = heart rate (HR) × stroke volume (SV). ❖ 2 factors affect blood flow: (1) the pressure difference that drives the blood flow through a tissue (2) the resistance to blood flow in specific blood vessels. Blood flows from regions of higher pressure to regions of lower pressure ❖ As blood leaves the aorta and flows through the systemic circulation, its pressure falls progressively as the distance Copyright © John Wiley & Sons, Inc. All rights reserved. from the left ventricle increases.  Blood Pressure ❖ Although the venous circulation flows under much lower pressures than the arterial side, usually the small pressure differences (venule 16 mmHg to right atrium 0 mmHg), plus the aid of muscle and respiratory pumps is sufficient. Copyright © John Wiley & Sons, Inc. All rights reserved.  Mean Arterial Pressure (MAP) ❖ the average blood pressure in arteries, is roughly one‐third of the way between the diastolic and systolic pressures ❖ MAP = diastolic BP + 1/3 (systolic BP – diastolic BP) ❖ EX: MAP = 80 + 1/3 (120 – 80) = 93.3 ❖ ALSO: MAP = CO X R (resistance), which means if cardiac output rises due to an increase in stroke volume or heart rate, then the mean arterial pressure rises as long as resistance remains the Copyright © John Wiley & Sons, Inc. All rights reserved.  Pressure, Flow, And Resistance ❖ As we have already seen, vascular resistance is itself dependent on other factors like (1) the size of the lumen (2) blood viscosity, (3) total blood vessel length in the body (body size). Two of these factors (viscosity and the length of blood vessels) are unchangeable from moment to moment. The diameter, however, is readily adjusted if the body needs to change blood flow to a Copyright © John Wiley & Sons, Inc. All rights reserved.  Pressure, Flow, And Resistance ❖ Copyright © John Wiley & Sons, Inc. All rights reserved. Pressure, Flow, And Resistance “Hardening of the arteries” (loss of elasticity) seriously hampers the body’s ability to increase blood flow to meet metabolic demands. Copyright © John Wiley & Sons, Inc. All rights reserved.  Pressure, Flow, and Resistance ❖ Systemic vascular resistance (SVR), refers to all the vascular resistances offered by systemic blood vessels. ❖ The diameters of arteries and veins are large, so their resistance is very small because most of the blood does not come into physical contact with the walls of the blood vessel. ❖ The smallest vessels—arterioles, capillaries, and venules—contribute the most resistance. A major function of arterioles is to control SVR by changing their diameters. Arterioles need to vasodilate or vasoconstrict only slightly to have a large effect on SVR. Copyright © John Wiley & Sons, Inc. All rights reserved.  Venous Return ❖ The volume of blood returning through the veins to the right atrium must be the same amount of blood pumped into the arteries from the left ventricle – this is called the venous return. Besides pressure, venous return is aided by the presence of venous valves, a skeletal muscle pump, and the action of breathing. Copyright © John Wiley & Sons, Inc. All rights reserved.  Venous Return ❖ The skeletal muscle pump uses the action of muscles to milk blood in 1 direction (due to valves). ❖ The respiratory pump uses the negative pressures in the thoracic and abdominal cavities generated during inspiration to pull venous blood towards the heart. Copyright © John Wiley & Sons, Inc. All rights reserved. Proximal valve Distal valve 1 2 3 Copyright © John Wiley & Sons, Inc. All rights reserved.  Pressure, Flow, And Resistance (Interactions Animation) ❖ Vascular Regulation You must be connected to the internet to run this animation Copyright © John Wiley & Sons, Inc. All rights reserved. Factors that increase BP Copyright © John Wiley & Sons, Inc. All rights reserved.  Velocity of Blood Flow ❖ The speed or velocity of blood flow (in cm/sec) is inversely related to the cross‐sectional area. Velocity is slowest where the total cross‐sectional area is greatest. ❖ Each time an artery branches, the total cross‐sectional area of all its branches is greater than the cross‐sectional area of the original vessel, so blood flow becomes slower and slower as blood moves further away from the heart, and is slowest in the capillaries. ❖ Velocity of blood flow decreases as blood flows from the aorta to arteries to arterioles to capillaries, and increases as it leaves capillaries and returns to the heart. The slow rate through capillaries aids the exchange of materials between blood and interstitial fluid. Copyright © John Wiley & Sons, Inc. All rights reserved.  Autoregulation ❖ Autoregulation = the ability of tissues to regulate their own blood flow, independent of input from hormones or the nervous system Warming causes vessels to dilate, cooling causes them to constrict Myogenic response – when the stretch on arteriole walls increases (due to higher pressures), they constrict reflexively – reduces blood flow (negative feedback) Locally released vasodilators – acids (increased with metabolic activity), potassium (increases with intense cellular activity), adenosine (used-up ATP), nitric oxide (released by endothelial cells) Locally released vasoconstrictors – a variety of chemicals including endothelins andCopyright thromboxanes © John Wiley & Sons, Inc. All rights reserved.  Blood pressure and homeostasis ❖ The vascular system senses alterations of BP and blood flow and signals the cardiovascular centers in the brain. The heart then appropriately modifies its rate and force of contraction. Arterioles and the precapillary sphincters of the metarterioles adjust resistance at specific tissue beds. Copyright © John Wiley & Sons, Inc. All rights reserved.  Blood pressure and homeostasis ❖ For example, during emergencies, the autonomic nervous system will vasodilate the precapillary sphincters of metarterioles in the skeletal muscles, lungs, and brain, while constricting the precapillary sphincters found in tissues such as the skin, GI tract, and kidneys. This sends the majority of the cardiac output (blood flow) to those organs important in a fight or flight response, while temporarily depriving (through vasoconstriction) the nonessential organs. This response is mediated by the sympathetic Copyright © John Wiley & Sons, Inc. All rights reserved.  The Cardiovascular Center ❖ In the medulla oblongata, receives input both from higher brain regions and from sensory receptors ❖ Nerve impulses descend from the cerebral cortex, limbic system, and hypothalamus to affect the cardiovascular center. ❖ The 3 main types of sensory receptors that provide input to the CV center are proprioceptors, baroreceptors, and chemoreceptors. Proprioceptors monitor movements of joints and muscles and provide input to the cardiovascular center during physical activity. Their activity accounts for the rapid increase in heart rate at the beginning of exercise. Baroreceptors monitor changes in pressure and stretch in the walls of blood vessels, and Copyright © John Wiley & Sons, Inc. All rights reserved. Blood pressure and homeostasis Copyright © John Wiley & Sons, Inc. All rights reserved. Neural Regulation of Blood Pressure ❖ Two of the most important control points are the pressure receptors (called baroreceptors) located in the arch of the aorta and the carotid sinus. ❖ Two important baroreceptor reflexes are the carotid sinus reflex and the aortic reflex Copyright © John Wiley & Sons, Inc. All rights reserved.  Blood pressure and homeostasis ❖ Stimulation of the baroreceptors in the carotid sinus is called the carotid sinus reflex (CN IX & X), and it helps normalize blood pressure in the brain. Stimulation of the aortic baroreceptors (aortic reflex) helps normalize the systemic BP. Copyright © John Wiley & Sons, Inc. All rights reserved.  Baroreceptors When the blood pressure falls, baroreceptors are stretched less, and the input is sensed in the cardiovascular centers of the brain which respond with decreased parasympathetic and increased sympathetic stimulation. Blood pressure increases do the opposite. Copyright © John Wiley & Sons, Inc. All rights reserved.  Blood pressure and homeostasis ❖ Another type of sensory receptor important to the process of autoregulation of BP are the chemoreceptors. Copyright © John Wiley & Sons, Inc. All rights reserved.  Chemoreceptor Reflexes ❖ Chemoreceptors are found in the carotid bodies (located close to baroreceptors of carotid sinus) and aortic bodies (located in the aortic arch). ❖ When they detect hypoxia (low O2), hypercapnia (high CO2), or acidosis (high H+), they signal the cardiovascular centers. They increase sympathetic stimulation increasing heart rate and respiratory rate, and vasoconstricting the vessels (arterioles and veins) to increase BP. Copyright © John Wiley & Sons, Inc. All rights reserved. Hormonal Regulation of Blood Pressure ❖ The Renin-angiotensin-aldosterone (RAA) system is an important endocrine component of autoregulation. Renin is released by kidneys when blood volume falls or blood flow decreases. It is subsequently converted into the active hormone angiotensin II which raises BP by (1) vasoconstriction and by (2) stimulating secretion of aldosterone from the adrenal glands (inc. reabsorption by kidneys = inc. blood volume) Copyright © John Wiley & Sons, Inc. All rights reserved. Hormonal Regulation of Blood Pressure ❖ Epinephrine and norepinephrine are also released from the adrenal medulla as an endocrine autoregulatory response to sympathetic stimulation. They increase cardiac output by increasing rate and force of heart contractions. ❖ Antidiuretic hormone (ADH) is released from the posterior pituitary gland in response to dehydration or decreased blood volume. Causes vasoconstriction. Copyright © John Wiley & Sons, Inc. All rights reserved. Hormonal Regulation of Blood Pressure ❖ Atrial Natriuretic Peptide (ANP) is a natural diuretic polypeptide hormone released by cells of the cardiac atria. ANP participates in autoregulation by: ❑ Lowering blood pressure (it causes a direct vasodilation) ❑ Reducing blood volume (by promoting loss of salt and water as urine) Copyright © John Wiley & Sons, Inc. All rights reserved.  Hormones summary: ❖ Epinephrine/norepinephrine: increases vasoconstriction in some capillary beds, vasodilation in others – net result is increase in blood pressure. As well, increases heart rate and contractility ❖ Angiotensin II: increases thirst, increases water and solute reabsorption into blood, potent vasoconstrictor, causes release of aldosterone. Net effect = vasoconstriction, increased water retention, and therefore increased blood pressure ❖ Aldosterone: increases water and solute reabsorption into the blood ❖ ADH: increases water reabsorption Copyright © John Wiley & Sons, Inc. All rights reserved.  Pulse ❖ A measure of peripheral circulation can be done by checking the pulse. The pulse is a result of the alternate expansion and recoil of elastic arteries after each systole. It is strongest in arteries closest to the heart and becomes weaker further out. Normally the pulse is the same as the heart rate. Copyright © John Wiley & Sons, Inc. All rights reserved.  Measuring Blood Pressure ❖ Blood pressure is the pressure in arteries generated by the left ventricle during systole and the pressure remaining in the arteries when the ventricle is in diastole. Copyright © John Wiley & Sons, Inc. All rights reserved.  Measuring Blood Pressure ❖ Blood pressure is usually measured in the brachial artery in the left arm with a sphygmomanometer ❖ A cuff is inflated until the brachial artery is compressed and blood flow stops, about 30 mmHg higher than the person's usual systolic pressure. The technician slowly deflates the cuff. When the cuff is deflated enough to allow the artery to open, a spurt of blood passes through = systolic blood pressure (SBP) ❖ As the cuff is deflated further = diastolic blood pressure (DBP) represents the force exerted by the blood remaining in arteries during ventricular relaxation. ❖ The various sounds that are heard while taking blood pressure are called Korotkoff sounds. Copyright © John Wiley & Sons, Inc. All rights reserved.  Alterations Of Blood Pressure ❖ About 50 million Americans have hypertension (HTN). It is the most common disorder affecting the CV system and is a major cause of atherosclerotic vascular disease (ASVD), heart failure, kidney disease and stroke. Copyright © John Wiley & Sons, Inc. All rights reserved.  Alterations Of Blood Pressure ❖ Hypertension is defined as an elevated systolic blood pressure (SBP), an elevated diastolic blood pressure (DBP), or both. Depending on severity, it is classified as pre-hypertension, Stage 1 HTN, or stage 2 HTN. Copyright © John Wiley & Sons, Inc. All rights reserved.  Alterations Of Blood Pressure ❖ Hypotension is defined as any blood pressure too low to allow sufficient blood flow to meet the body's metabolic demands (to maintain homeostasis). ❖ consentespecially Many persons, and pain scale some thin, young women, have very low BP, yet experience no dizziness, fatigue, or other symptoms – they are not hypotensive, and in fact are probably very healthy (cardiovascular wise). ❖ Hypotension leading to hypo-perfusion of Copyright © John Wiley & Sons, Inc. All rights reserved.  Shock And Homeostasis ❖ The 4 basic types of shock are: Hypovolemic shock, due to decreased blood volume Cardiogenic shock, due to poor heart function Obstructive shock, due to obstruction of blood flow Vascular shock, due to excess vasodilation - as seen in cases of a massive allergy (anaphylaxis) or sepsis. In the U.S., septic shock causes >100,000 deaths/yr. and is the Copyright © John Wiley & Sons, Inc. All rights reserved.  Shock And Homeostasis ❖ The same negative feedback mechanism discussed in autoregulation of blood pressure/flow is activated to restore blood and nutrient flow in cases of shock. Heart will respond with ⭡ rate and force of contraction. Selective tissue beds will vasoconstrict to shunt blood flow to those tissue most necessary to life (brain). The other neural, hormonal, and chemical Copyright © John Wiley & Sons, Inc. All rights reserved.  Homeostatic Responses to Shock ❖ Most cases of shock call for the administration of extra fluids and emergency medications like epinephrine to help restore perfusion to the tissues. ❖ If the body is not able to do this quickly, with or without outside help, organs will fail (kidney failure, liver failure, coma) and damage may become permanent. Copyright © John Wiley & Sons, Inc. All rights reserved.  Homeostatic Responses to Shock 1. Activation of the renin–angiotensin–aldosterone system. 2. Secretion of antidiuretic hormone 3. Activation of the sympathetic division of the ANS 4. Release of local vasodilators Copyright © John Wiley & Sons, Inc. All rights reserved.  Signs and Symptoms of Shock ❖ Systolic blood pressure is lower than 90 mmHg. ❖ Resting heart rate is rapid due to sympathetic stimulation and increased blood levels of epinephrine and norepinephrine. ❖ Pulse is weak and rapid due to reduced cardiac output and fast heart rate. ❖ Skin is cool, pale, and clammy due to sympathetic constriction of skin blood vessels and sympathetic stimulation of sweating. ❖ Mental state is altered due to reduced oxygen supply to the brain. ❖ Urine formation is reduced due to increased levels of aldosterone and antidiuretic hormone (ADH). ❖ The person is thirsty due to loss of extracellular fluid. ❖ The pH of blood is low (acidosis) due to buildup of lactic acid. Copyright © John Wiley & Sons, Inc. All rights reserved. ❖ The person may have nausea because of impaired blood flow to