Cardiovascular The System: Blood Vessels and Hemodynamics
Structure and function of blood vessels Hemodynamics Major circulatory routes
Anatomy of Blood Vessels Closed system of tubes that carries blood Arteries carry blood from heart to tissues Capillaries are thin enough to allow exchange Venules merge to form veins that bring blood back to the heart Vasa vasorum
Arteries Tunica interna (intima) Tunica media Tunica externa
Sympathetic Innervation Vascular smooth muscle is innervated by sympathetic nervous system
increase in stimulation causes muscle contraction or vasoconstriction injury to artery or arteriole causes muscle contraction reducing blood loss
(vasospasm)
decrease in stimulation or presence of certain chemicals causes vasodilation
Elastic Arteries
Largest-diameter arteries have lot of elastic fibers in tunica media
Muscular Arteries Medium-sized arteries with more muscle than elastic fibers in tunica media
Arterioles
Small arteries delivering blood to capillaries Metarterioles form branches into capillary bed
to bypass capillary bed, precapillary sphincters close & blood flows out of bed in thoroughfare channel Capillaries form Microcirculation
Microscopic vessels that connect arterioles to venules Found near every cell in the body but more extensive in highly active tissue
(muscles, liver, kidneys & brain)
Function is exchange of nutrients & wastes between blood and tissue fluid Structure is single layer of simple squamous epithelium and its basement
membrane Types of Capillaries
Continuous capillaries Fenestrated capillaries Sinusoids
Venules
Small veins collecting blood from capillaries Tunica media contains only a few smooth muscle cells & scattered fibroblasts Venules that approach size of veins more closely resemble structure of vein
Veins
Proportionally thinner walls than same diameter artery Still adaptable to variations in volume & pressure Valves are thin folds of tunica interna designed to prevent backflow Venous sinus has no muscle at all
Anastomoses Union of 2 or more arteries supplying the same body region
blockage of only one pathway has no effect
Alternate route of blood flow through an anastomosis is known as collateral circulation Alternate routes to a region can also be supplied by nonanastomosing vessels
Systemic Circulation
All systemic arteries branch from the aorta All systemic veins drain into the superior or inferior vena cava or coronary sinus
to return to the right-side of heart Circulatory Routes
Systemic circulation is left side heart to body & back to heart Hepatic Portal circulation is capillaries of GI tract to capillaries in liver Pulmonary circulation is right-side heart to lungs & back to heart Fetal circulation is from fetal heart through umbilical cord to placenta & back
Arterial Branches of Systemic Circulation
All are branches from aorta supplying arms, head, lower limbs and all viscera
with O2 from the lungs
Aorta arises from left ventricle (thickest chamber)
4 major divisions of aorta Aorta and Its Superior Branches
Aorta is largest artery of the body
ascending aorta
2 coronary arteries supply myocardium
arch of aorta -- branches to the arms & head
brachiocephalic trunk branches into right common carotid and right subclavian left subclavian & left carotid arise independently
thoracic aorta supplies branches to pericardium, esophagus, bronchi, diaphragm, intercostal & chest muscles, mammary gland, skin, vertebrae and spinal cord Coronary Circulation
Right & left coronary arteries branch to supply heart muscle
anterior & posterior interventricular aa.
Subclavian Branches
Subclavian aa. pass superior to the 1st rib
gives rise to vertebral a. that supplies blood to the Circle of Willis on the base of the brain
Become the axillary artery in the armpit Become the brachial in the arm Divide into radial and ulnar branches in the forearm
Common Carotid Branches
External carotid arteries
supplies structures external to skull as branches of maxillary and superficial temporal branches
Internal carotid arteries (contribute to Circle of Willis)
supply eyeballs and parts of brain Abdominal Aorta and Its Branches
Supplies abdominal & pelvic viscera & lower extremities
celiac aa. supplies liver, stomach, spleen & pancreas superior & inferior mesenteric aa. supply intestines renal aa supply kidneys gonadal aa. supply ovaries and testes
Splits into common iliac aa at 4th lumbar vertebrae
external iliac aa supply lower extremity internal iliac aa supply pelvic viscera Visceral Branches off Abdominal Aorta Celiac artery is first branch inferior to diaphragm
left gastric artery, splenic artery, common hepatic artery
Superior mesenteric artery lies in mesentery
pancreaticoduodenal, jejunal, ileocolic, ascending & middle colic aa.
Inferior mesenteric artery
descending colon, sigmoid colon & rectal aa
Arteries of the Lower Extremity
External iliac artery become femoral artery when it passes under the inguinal
ligament & into the thigh
femoral artery becomes popliteal artery behind the knee Veins of the Systemic Circulation
Drain blood from entire body & return it to right side of heart Deep veins parallel the arteries in the region Superficial veins are found just beneath the skin All venous blood drains to either superior or inferior vena cava or coronary sinus
Major Systemic Veins All empty into the right atrium of the heart
superior vena cava drains the head and upper extremities inferior vena cava drains the abdomen, pelvis & lower limbs coronary sinus is large vein draining the heart muscle back into the heart
Veins of the Head and Neck External and Internal jugular veins drain the head and neck into the superior vena cava Dural venous sinuses empty into internal jugular vein Venipuncture
Venipuncture is normally performed at cubital fossa, dorsum of the hand or
great saphenous vein in infants Hepatic Portal System
Subdivision of systemic circulation
Formed by union of splenic, superior mesenteric & hepatic veins
Pulmonary Circulation
Carries deoxygenated blood from right ventricle to air sacs in the lungs and
returns it to the left atria
Vessels include pulmonary trunk, arteries and veins Differences from systemic circulation
Fetal Circulation
Oxygen from placenta reaches heart via fetal veins in umbilical cord.
bypasses liver
Heart pumps oxygenated blood to capillaries in all fetal tissues including lungs. Umbilical aa. Branch off iliac aa. to return blood to placenta.
Blood Distribution
60% of blood volume at rest is in systemic veins and venules
blood is diverted from it in times of need
15% of blood volume in arteries & arterioles
Capillary Exchange Movement of materials in & out of a capillary
diffusion (most important method)
substances move down concentration gradient all plasma solutes except large proteins pass freely across
through lipid bilayer, fenestrations or intercellular clefts blood brain barrier does not allow diffusion of water-soluble materials
(nonfenestrated epithelium with tight junctions)
transcytosis
passage of material across endothelium in tiny vesicles endocytosis by and exocytosis
large, lipid-insoluble molecules such as insulin or maternal antibodies passing
through placental circulation to fetus
bulk flow
Bulk Flow: Filtration & Reabsorption
Movement of large amount of dissolved or suspended material in same direction
move in response to pressure
from area of high pressure to area of low
faster rate of movement than diffusion or osmosis
Most important for regulation of relative volumes of blood & interstitial fluid
filtration is movement of material into interstitial fluid
promoted by blood hydrostatic pressure & interstitial fluid osmotic pressure
reabsorption is movement from interstitial fluid into capillaries
promoted by blood colloid osmotic pressure
balance of these pressures is net filtration pressure
Dynamics of Capillary Exchange Starling's law of the capillaries is that the volume of fluid & solutes reabsorbed is almost as large as the volume filtered Net Filtration Pressure
Whether fluids leave or enter capillaries depends on net balance of pressures
net outward pressure of 10 mm Hg at arterial end of a capillary bed net inward pressure of 9 mm Hg at venous end of a capillary bed
Edema
An abnormal increase in interstitial fluid if filtration exceeds reabsorption
result of excess filtration result of inadequate reabsorption
Not noticeable until 30% above normal
Hemodynamics Factors affecting circulation An interplay of forces result in blood flow
Velocity of Blood Flow Speed of blood flow in cm/sec is inversely related to cross-sectional area
blood flow is slower in the arterial branches
Blood flow becomes faster when vessels merge to form veins
Circulation time is time it takes a drop of blood to travel from right atrium back to right atrium Volume of Blood Flow
Cardiac output = Other factors that influence cardiac output
Blood Pressure
Pressure exerted by blood on walls of a vessel
caused by contraction of the ventricles highest in aorta
120 mm Hg during systole & 80 during diastole
If heart rate increases cardiac output, BP rises Pressure falls steadily in systemic circulation with distance from left ventricle
35 mm Hg entering the capillaries 0 mm Hg entering the right atrium
If decrease in blood volume is over 10%, BP drops Water retention increases blood pressure
Resistance
Friction between blood and the walls of vessels
average blood vessel radius blood viscosity (thickness) total blood vessel length
Systemic vascular resistance is the total of above
Venous Return
Volume of blood flowing back to the heart from the systemic veins
depends on pressure difference from venules (16 mm Hg) to right atrium (0 mm Hg) tricuspid valve leaky and buildup of blood on venous side of circulation
Skeletal muscle pump Respiratory pump
Control of Blood Pressure & Flow
Role of cardiovascular center
help regulate heart rate & stroke volume specific neurons regulate blood vessel diameter
Input to the Cardiovascular Center
Higher brain centers such as cerebral cortex, limbic system & hypothalamus
anticipation of competition increase in body temperature
Proprioceptors Baroreceptors Chemoreceptors
Output from the Cardiovascular Center Heart
parasympathetic (vagus nerve) sympathetic (cardiac accelerator nerves)
Blood vessels
sympathetic vasomotor nerves
continual stimulation to arterioles in skin & abdominal viscera producing vasoconstriction (vasomotor tone) increased stimulation produces constriction & increased BP Neural Regulation of Blood Pressure
Baroreceptor reflexes
carotid sinus reflex aortic reflex
If feedback is decreased, CV center reduces parasympathetic & increases
sympathetic stimulation of the heart
Innervation of the Heart
Speed up the heart with sympathetic stimulation Slow it down with parasympathetic stimulation (X) Sensory information from baroreceptors (IX)
Chemoreceptor Reflexes
Carotid bodies and aortic bodies
detect changes in blood levels of O2, CO2, and H+ (hypoxia, hypercapnia or
acidosis )
causes stimulation of cardiovascular center increases sympathetic stimulation to arterioles & veins vasoconstriction and increase in blood pressure
Also changes breathing rates as well
Hormonal Regulation of Blood Pressure
Renin-angiotensin-aldosterone system
decrease in BP or decreased blood flow to kidney
Epinephrine & norepinephrine ADH causes vasoconstriction ANP (atrial natriuretic peptide) lowers BP
causes vasodilation & loss of salt and water in the urine Local Regulation of Blood Pressure
Local factors cause changes in each capillary bed
autoregulation is ability to make these changes as needed by demand for O2 & waste removal
important for tissues that have major increases in activity (brain, cardiac & skeletal muscle)
Local changes in response to physical changes Vasoactive substances released from cells alter vessel diameter (K+, H+, lactic
acid, nitric oxide) Shock and Homeostasis
Shock is failure of cardiovascular system to deliver enough O2 and nutrients
inadequate perfusion cells forced to switch to anaerobic respiration lactic acid builds up cells and tissues become damaged & die
Types of Shock
Hypovolemic shock due to loss of blood or body fluids (hemorrhage, sweating, diarrhea)
venous return to heart declines & output decreases
Cardiogenic shock caused by damage to pumping action of the heart (MI, ischemia, valve problems or arrhythmias) Vascular shock causing drop inappropriate vasodilation -- anaphylatic shock, septic shock or neurogenic shock (head trauma) Obstructive shock caused by blockage of circulation (pulmonary embolism)
Homeostatic Responses to Shock Mechanisms of compensation in shock attempt to return cardiac output & BP to normal
activation of renin-angiotensin-aldosterone
secretion of antidiuretic hormone activation of sympathetic nervous system release of local vasodilators
If blood volume drops by 10-20% or if BP does not rise sufficiently, perfusion may be inadequate -- cells start to die Signs & Symptoms of Shock
Rapid resting heart rate (sympathetic stimulation) Weak, rapid pulse due to reduced cardiac output & fast heart rate Clammy, cool skin due to cutaneous vasoconstriction Sweating -- sympathetic stimulation Altered mental state due to cerebral ischemia Reduced urine formation -- vasoconstriction to kidneys & increased aldosterone
& antidiuretic hormone
Thirst -- loss of extracellular fluid Acidosis -- buildup of lactic acid Nausea -- impaired circulation to GI tract
Evaluating Circulation
Pulse is a pressure wave
alternate expansion & recoil of elastic artery after each systole of the left ventricle pulse rate is normally between 70-80 beats/min
tachycardia is rate over 100 beats/min/bradycardia under 60
Measuring blood pressure with sphygmomanometer
Korotkoff sounds are heard while taking pressure
systolic blood pressure from ventricular contraction diastolic blood pressure during ventricular contraction
provides information about systemic vascular resistance
pulse pressure is difference between systolic & diastolic normal ratio is 3:2:1 -- systolic/diastolic/pulse pressure