Chapter 21: Blood Vessels and Circulation

(Blood Vessels and Circulation) Chapter 21 Interstitial Fluid · fluid between the blood vessels and cells ↳ Similar composition to plasma · consists of water, nutrients (amino acids, sugars, fatty acids, Intracellular fluid Plasma salts etc.), hormones, enzymes, waste products... · functions to deliver nutrients, remove wastes, intercellular communication Blood Flow Between the Heart and Lungs Interstitial fluid Blood Vessels · need to be resilient enough to withstand changes in pressure, and flexible enough accommodate underlying tissues · anastomosis: the joining of two blood vessels (creates capillary beds) · collateral: more than one artery supplying an area or capillary bed · angiogenesis: the formation of new blood vessels from existing ones ↳ cancer cells can use angiogenesis to feed themselves (bad) Types of Blood Vessels · Artery: carries blood AWAY from heart to peripheral capillary ↳ big → small ↳ big arteries are highest pressure vessels (contain elastic fibers) · Arteriole: a small arterial branch that delivers blood to a capillary network ↳ less pressure than artery b/c its farther from the heart · Capillary: a small blood vessel, located between an arteriole and a venule, with thin walls allowing for the diffusion of gases, nutrients, and wastes between plasma and interstitial fluid (exchange vessels) ↳ not present in avascular areas (epithelia, cartilage, eye etc. ) ↳ lower pressure vessels · Venule: thin walled vein that receives blood from capillaries, unites to form veins · Veins: a blood vessel carrying blood from a capillary bed TOWARDS the heart ↳ small → big Arteries vs. Veins · most arteries and veins run parallel to each other · walls of arteries are thicker than veins (thicker tunica media) ↳ therefore arteries can maintain shape and veins collapse when cut · endothelial lining of arteries have pleated folds, veins do not have this · Veins have valves, and a larger lumenal diameter Vessel Walls · vessel walls have 3 layers and they gradually change in each location ① Tunica Intima (inner layer) ↳ endothelium of simple squamous epithelium and thin layer of connective tissue with elastic fibers - if arteries contract, the endothelium does not move → creates folds ② Tunica Media (middle layer) ↳ composed of smooth muscle tissue and elastic fibers which cause vasoconstriction and vasodilation ↳ thickest layer in arteries ③ Tunica Externa (aka adventicia) (outermost layer) ↳ composed of connective tissue with elastic fibers ↳ stabilize and anchor blood vessel to surrounding tissues Vasa vasorum: some blood vessels are too thick for diffusion so they have their own blood supply ↳ "vessels of vessels" Arteries 1. Elastic arteries: the largest arteries; transport blood away from the heart. Rich in elastic fibers and few smooth muscle fibers ↳ resiliency to change in pressure 2. Muscular arteries: medium arteries; has thicker tunica media (more smooth muscle), allowing for vasoconstriction and vasodilation ↳ distributes blood to skeletal muscles and internal organs 3. Arterioles: small arteries; carries blood from muscular arteries to the capillaries. Has much less smooth muscle and less substantial tunica externa ↳ branch into metarterioles that connect to capillaries ↳ "resistance vessels" Vasodilation and Vasoconstriction · vasodilation: an increase in the diameter of arterioles due to relaxation of smooth muscle in tunica media (decreased pressure + resistance) · vasoconstriction: a decrease in the diameter of arterioles due to the contraction of smooth muscles in tunica media (increased pressure + resistance) · occurs in response to local factors, hormones, stimulation of vasomotor center · will impact afterload, peripheral blood pressure, and capillary blood flow Capillaries · exchange vessels, smallest in diameter · single layer of simple squamous epithelium (endothelial tube) surrounded by a basement membrane ↳ no tunica media or tunica externa · found throughout body, especially vascular tissues · 3 types of capillaries: ① Continuous: have complete endothelial lining (no gaps), allows for simple diffusion and bulk transport (endocytosis and exocytosis) ② Fenestrated: have pores for rapid exchange between blood and interstitial fluid ③ Sinusoid: have a discontinuous endothelial lining, permitting free movement of water and solutes (including large plasma proteins) ↳ have phagocytic cells that are fixed or mobile to grab pathogens Capillary Beds · network of capillaries connecting metarterioles to venules · precapillary sphincter: surrounds opening of each capillary and controls the blood flow ↳ contracts (constricts) and relaxes (dilates) several times causing blood to flow in a pulsing fashion → autoregulation controlled at tissue level Capillary Exchange · chemical and gaseous exchange done to maintain homeostasis · exchange between blood and interstitial fluid → cells (and vice versa) ↳ deliver nutrients and wastes, O2 and CO2 gas exchange · close proximity to cells allows for rapid delivery · Diffusion: the net movement from higher to lower concentration down a concentration gradient ↳ via channels, fenestrated capillaries, sinusoid capillaries · Filtration: removal of solutes as solution moves across a porous membrane due to hydrostatic pressure ↳ hydrostatic pressure: the pressure exerted by the fluid in the capillaries, pushing solutes and solvent into interstitial fluids → cells · Reabsorption: a result of osmosis (diffusion of water) Venules · the smallest veins · carry blood from capillaries to larger veins · tend to lack media (no smooth muscle) except in larger venules ↳ cannot contract without smooth muscle Veins · veins return blood to the heart through a low pressure system that requires valves ↳ blood pressure is too low to overcome gravity, so tunica intima have folds that form venous valves to prevent backflow (skeletal muscle movements push values open) · Medium veins: tunica media is thin with relatively few smooth muscle fibers; thickest layer is adventitia (tunica externa) · Large veins: all layers are thick, especially tunica externa eg. Superior and inferior vena cava Venous valves: used in veins to prevent backflow since veins are a low pressure system ↳ for example, muscles in your legs contract as you walk and they help push the blood up. Valves are open and then they close to prevent back flow. This continues all the way up. The Distribution of Blood · throughout vessels: ↳ 30-35% in arteries and capillaries ↳ 65-70% in veins (reservoirs) · veins are more distensible (stretchy) than arteries and show high capacitance ↳ they can stretch and hold lots of blood without an extreme increase in BP ↳ this is how they are reservoirs Tissue perfusion · tissue perfusion refers to adequate blood flow to the tissues, ensuring that the needs for nutrients and oxygen are being met · impacted by cardiac output, peripheral resistance, and blood pressure · When tissues are active, circulation must increase to meet these demands · adequate blood flow is needed for adequate perfusion of tissues. Under normal conditions, blood flow mirrors cardiac output: ↳ CO = HR x SV ↳ ↓CO = ↓Capillary blood flow · capillary blood flow depends on pressure and resistance ↳ adequate pressure → overcome resistance → ensures blood flow ↳ the rate of blood flow through a vessel is proportional to the pressure gradient, and inversely proportional to vascular resistance therefore: pressure↑blood flow↑, and resistance↓blood flow↑, BUT↑resistance causes↑pressure Think of it like this: if pressure were to increase between two points of a blood vessel, like when the heart contracts more or if cardiac output increases, that would cause a pressure gradient. There would be more pressure by the section of the vessel nearest to the heart and lower pressure farther away. Blood flows from high to low pressure, so blood flow would increase. Resistance is easier: if a blood vessel dilated (decreasing resistance) blood flow would increase because its bigger and easier to move through Pressure and Resistance · pressure (P): generated by the heart to overcome resistance ↳ leads to pressure gradients · resistance (R): anything the heart has to pump against · flow (F): refers to the flow of blood · the cardiovascular center in your brain can alter pressure gradients by adjusting cardiac output or peripheral resistance (as mentioned in previous example above) Blood pressure (BP): usually refers to arterial blood pressure and it is the pressure exerted by blood against the walls of arteries as the heart pumps it around the body Capillary hydrostatic pressure (CHP): the pressure exerted by blood within the capillaries and capillary walls Venous pressure: the pressure within the venous system (veins and venules) Blood Pressure · blood pressure generated by the heart must be enough to overcome resistance present in the blood vessels (vascular resistance, peripheral resistance) · BP is the force exerted by blood against a vessel wall ↳ depends on volume of blood and compliance of vessel walls · BP can be separated into systolic and diastolic blood pressure: ↳ Systolic blood pressure (SBP): the peak pressure during ventricular systole - average 120mmHg ↳ Diastolic blood pressure (DBP): the minimum blood pressure at the end of ventricular diastole - average 80mmHg · BP = 120/80 mmHg ↳ hypertension: abnormally high blood pressure (SBP >140mmHg) ↳ hypotension: abnormally low blood pressure (SBP < 90mmHg) · Pulse pressure: the difference between systolic and diastolic pressures · Mean arterial pressure (MAP): the average arterial pressure through one cardiac cycle ↳ used to calculate overall blood flow and perfusion ↳ in an adult MAP should be a minimum of 65 mmHg Factors Affecting Blood Pressure 1. Cardiac output 2. Resistance ↳ peripheral resistance: resistance to the flow of blood through small vessels ↳ peripheral resistance is determined by: i. Blood viscosity ii. Blood vessel length iii. Blood vessel diameter ↳ vasoconstriction and vasodilation Total Peripheral Resistance · Vascular resistance: friction between blood vessel wall, impacted by vessel length and diameter ↳ diameter influenced by constriction, dilation, and presence of an obstruction · Viscosity: the thickness of the fluid (will be constant in the absence of compensation or disease) · Turbulence: altered blood flow due to turbulence "swirling" flow ↳ irregular surfaces, sudden change in diameter, issues with valves Factors Affecting Blood Flow and Pressure Control of Blood Flow & Blood Pressure Autoregulation ↳ mechanisms that cause immediate, localized · Autoregulation · Central Regulation homeostatic adjustments ↳ Nervous System ↳ Endocrine · Vasodilation and vasoconstriction ↳ Vasodilation is when precapillary sphincters dilate and its called vasomotion - caused by low O2, elevated CO2, nitric oxide (NO), inflammatory mediators, and increased tissue temperatures ↳ vasoconstriction - vessel injury causes endothelin release, which stimulates vascular spasm and platelet aggregation - platelets and RBC release prostaglandin and thromboxane · if autoregulation is not sufficient, central regulation (neural and endocrine mechanisms) are used Central Regulation ① Neural Mechanisms · sense change in arterial pressure or blood gas levels at specific locations and adjust cardiac output and peripheral resistance to maintain blood flow to vital organs a. Cardiovascular (CV) Centers of the Medulla Oblongata 1. Cardiac Centers ↳ cardioacceleratory center → sympathetic activation (↑CO) ↳ cardioinhibitory center → parasympathetic activation (↓CO) 2. Vasomotor Center ↳ directs vasomotor responses in blood vessels; sympathetic input ↳ responsible for maintaining vasomotor tone at all times eg. Vasoconstriction: adrenergic neurons secrete NE, Central Regulation Overview ① Nervous System (neural) Control a. Cardiovascular centers in the medulla i. Cardiac center ii. Vasomotor center b. Reflex control i. Baroreceptors ii. Chemoreceptors ② Endocrine System Control a. Adrenal medulla - Ne and E b. ADH c. Aldosterone d. Angiotensin II e. Erythropoietin (EPO) f. Natriuretic Peptides stimulating smooth muscle contraction eg. Vasodilation: dilation of vessels in skeletal muscle when Ach at neuromuscular junction stimulates NO synthesis b. Reflex Control 1. Baroreceptor Reflex ↳ baroreceptors located in carotid sinus (internal carotid artery), aortic arch, and wall of RA ↳ monitor for stretch (BP)and communicate with CV - when BP increases, baroreceptors activity increases, leading to ↓CO and widespread peripheral vasodilation - When BP decreases, baroreceptor activity decreases, leading to ↑CO and vasoconstriction 2. Chemoreceptor Reflex ↳ monitors the chemical composition of arterial blood and CSF (CO2, O2, PH) - composition of arterial blood monitored by sensory neurons in carotid and aortic bodies - composition of CSF monitored by special receptors in medulla oblongata ↳ requires coordination of cardiovascular and respiratory activities → stimulation of cardio acceleratory and vasomotor centers → increased HR, increased CO, and increased BP → stimulation of respiratory center in medulla → increased respiratory rate and O2 intake ② Endocrine Mechanisms · hormones enhance short term neural changes and direct longer term changes · Adrenal Medulla ↳ stimulated by SNS - E and NE stimulate increased HR, CO, and vasoconstriction · Antidiuretic Hormone (ADH) ↳ released from posterior pituitary to decreased BP, increased solute concentration, or increased angiotensin II ↳ stimulates conservation of water by kidneys - elevated BP · Aldosterone ↳ released in adrenal cortex in response to low Na levels or increased angiotensin II ↳ Stimulates conservation of Na → retention of water → increased BP · Angiotensin II ↳ Renin angiotensin aldosterone system (RAAS system) - in response to decreased blood flow to kidneys, kidneys release renin, which starts a chain reaction - renin converts active angiotensin to angiotensin I - angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II ↳ functions of angiotensin II - stimulates aldosterone secretion (Na conservation) - stimulates ADH Secretion (water conservation) - stimulates thirst - stimulates increased cardiac output - stimulates vasoconstriction* · Erythropoietin ↳ released by kidneys in response to low BP or low O2 levels ↳ causes vasoconstriction, stimulates RBC production → increased O2 capacity · Natriuretic Peptides ↳ released by myocardial cells in response to excessive cardiac stretch ↳ functions of natriuretic peptides: - stimulate Na and water loss by kidneys → lowers BP - inhibits hormones that increase BP - inhibit thirst - stimulate vasodilation ↳ atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) Hypertension Hypotension · 2 types of hypertension: · occurs when there is too little blood to fill the · BP greater than 140/90 mmHg ↳ primary hypertension: cause typically unknown, dietary factors, family history, lifestyle, behaviours etc. ↳ secondary hypertension: usually occurs secondary to another known primary problem, or medications eg. Renal hypertension, endocrine hypertension · BP below 100/60 mmHg vessels or heart is too weak to drive the blood · Orthostatic (postural) hypotension: transient hypotensive condition resulting from insufficient compensatory responses to gravitational shifts in blood when a person moves from horizontal to vertical position ↳ drastic decrease in BP causes you to be dizzy when you stand up How does exercise impact blood flow and blood pressure? ↳ sympathetic activation causes ↑vasodilation, ↑tissue oxygenation, ↑CO, and ↑venous return How does blood loss impact blood pressure? ↳ causes a decrease in BP ↳ autoregulation vasospasm temporarily maintains BP by ↑CO and vasodilation What is the physiological response to blood loss? ↳ eventually too much blood loss will lead to hypovolemic shock and big ↓BP Edema · an abnormal increase in interstitial fluid that can occur when filtration exceeds reabsorption ↳ hydrostatic pressure > osmotic pressure · excess filtration may be due to hypertension, or increased permeability of capillaries (leaky blood vessels) · inadequate reabsorption may be due to decreased plasma proteins (eg. Albumin) Map of the Circulatory System · Pulmonary circuit: 1. as blood leaves the right ventricle, it exits via the pulmonary trunk which branches into the left and right pulmonary arteries (carrying deoxygenated blood toward the lungs) 2. oxygenated blood returning from the lungs to the heart does so via pulmonary veins which empty into the left atrium NOTE: the pulmonary circuit is the only time arteries carry deoxygenated blood while veins carry oxygenated blood!!! · Systemic circuit: 1. as oxygenated blood leaves the left ventricle, it does so via the ascending aorta. The left and right coronary arteries branch off the ascending aorta 2. The ascending aorta curves into the aortic arch, from which the brachiocephalic trunk (on the right), left common carotid, and left subclavian arteries arise. The brachiocephalic trunk divides into the right common carotid and the right subclavian artery 3. The descending aorta gives rise to the thoracic aorta and the abdominal aorta. 4. The abdominal aorta eventually branches into the left and right iliac arteries What are the three main branches of the aortic arch? ↳ brachiocephalic trunk, left common carotid, and left subclavian What areas of the body are supplied by these vessels? ↳ brain, arms, neck Effects of Aging · changes in the blood ↳ decreased hematocrit ↳ increased risk of clots ↳ increased pooling of blood in legs due to venous valve deterioration · changes in the heart ↳ reduced cardiac output ↳ increased incidence of alterations in conduction · changes in blood vessels ↳ reduced elasticity ↳ increased incidence of atherosclerosis ↳ increased risk of clots Fetal Circulation · child's lungs and digestive tract aren't working in utero ↳ respiratory and nutrition from placenta · umbilical artery → umbilical vein → inferior vena cava → heart · lungs are collapsed before birth ↳ blood bypasses lungs by foramen ovale (open in fetal dev. otherwise closed) ↳ in interatrial septum · ductus arteriosus ↳ acts as a conduit to bypass the lungs, connects the pulmonary trunk to the aorta · on first breath lungs expand and fill with blood ↳ ↑O2 causes constriction of ductus arteriosus and two systems become separated ↳ RA expands, foramen ovale closes (becomes fossa ovalis) · ligmentum arteriosum = ductus arteriosus