Chapter 19 – Blood Vessels and Circulation

Types of blood vessels• Arteries – Carry blood away from the heart– Branch into smaller vessels called arterioles

• Capillaries – Smallest vessels – Form beds in tissues/organs– Where diffusion occurs

• Veins – Carry blood back to heart – Venules – small veins from capillary beds

• Converge to form larger veins

Vessel walls • With the exception of capillaries, vessels have 3 layers/tunics

surrounding the central lumen • Tunica intima/interna

– Endothelium – simple squamous epithelium continuous with the endocardium of the heart

– Tends to be convoluted in large arteries • Tunica media

– Smooth muscle and elastin fibers • Can alter lumen size

– Vasodilation – muscle relaxes/lumen enlarges– Vasoconstriction – muscle contracts/lumen decreases

– Thicker in arteries • Tunica externa or adventitia

– Loose connective tissue and collagen – Large vessels have small vessels within externa called vasa vasorum

Vessel walls

Arterial system • Elastic/conducting arteries

– Largest in diameter • Aorta and major branches

– Large amount of elastic fibers• Expand and recoil

• Muscular arteries – Relative to size, largest tunic

media – Ability to vasocontrict

• Arterioles – Few elastic fibers – Diameter controls amount of

blood delivered to capillary beds

Capillaries • Consist only of tunica intima

and basement membrane – Continuous – endothelium is

complete • Prevents loss of blood cells and

proteins – Fenestrated – contains pores

• Allows for movement of small proteins

• Choroid plexi, GI tract for absorption, kidneys, endocrine system

• Forms beds – network of capillaries

Capillary microcirculation • Blood from arteriole to venule • Terminal arteriole →

metarteriole → true capillaries → thoroughfare channel → post capillary venule

• Precapillary sphincter – Band of smooth muscle at

metarteriole/capillary junction – When contracted, blood

bypasses capillary bed – Blood goes directly from

metarteriole to thoroughfare channel

Venous system • Venules – Very porous, like capillaries

• Veins – Walls are thinner and

lumens are larger as compared to same size artery

– Relatively little smooth muscle • Large veins tend to collapse

in histological preparation – Externa is the largest layer

Venous system cont • Since veins are exposed to

relatively low pressure:– Veins contain valves

• Prevents backflow of blood due to gravity

– Skeletal muscles “milk” veins • Contraction of muscles close to veins

help move blood through the vessel – Abdominal pressure differences

due to breathing • Abnormalities

– Varicose veins • Poor circulation causes valves to

become leaky• Veins stretch out and become floppy

– Usually superficial veins that have little support from underlying tissues

• Hemorrhoids – varicose veins of anal veins

Vascular anastomoses

• Where multiple vessels unit

• Arterial anastomoses– Arteries that supply same area merge – Provides alternate blood supply

• Venous anatomoses – More common than aterial

Disorders

• Atherosclerosis – Lipid deposits on arterial

walls – Can cause restriction or

blockage of blood flow • Arteriosclerosis – Decreased elasticity

“hardening of arteries”– Affects proper blood

flow

Circulation physiology

• Blood flow – Volume of blood that passes through a specific point

of a vessel in a specific time • ml/min

• Blood pressure– Force per unit exerted by blood on vessel wall

• mm Hg– Usually refers to arterial pressure – Pressure gradient – pressure difference is required for

blood to flow

Circulation physiology cont • Resistance

– Opposition to flow; measures friction the blood encounters– Influenced by:

• Blood viscosity • Blood vessel length

– Longer the vessel = greater resistance • Blood vessel diameter

– Blood next to the wall flows more slowly

• F = ∆P/R– F = flow; ∆P = pressure difference; R = resistance – As pressure increases, flow increases – As pressure decreases, flow decreases

Systemic blood pressure • Vessel pressure is highest in the

aorta and decreases to zero in right atrium – Blood flows from region of high

pressure to low • Blood pressure is measured in

arteries – exposed to highest pressure

• Systole – ventricular contraction – ~120mm Hg

• Diastole – ventricular relaxation – ~80mm Hg

Maintaining blood pressure

• Hormonal control (discussed during endocrine system)

• Baroreceptors – detect pressure changes – Located in aortic arch, carotid arteries, and large

head/neck arteries – When relaxed, the medulla oblongata sends signals to

vessels for vasoconstriction, which increases pressure – When stretched, medulla doesn’t send signal – causes

vasodilation, which decreases pressure

Baroreceptors

Maintaining blood pressure cont

• Chemoreceptors – detect changes in respiratory gases– Increases in carbon dioxide is detected by aortic

arch and carotid sinuses • Vasocontriction causes an increase of blood pressure,

which increases blood flow – Gets to respiratory system to unload carbon dioxide more

quickly

– Decrease of carbon dioxide causes vasodilation – causes slower flow

Chemoreceptors

Pulse

• Surges of pressure in an artery

• Pulse rate should equal the heart rate (beats per minute)– Apical pulse – actual

heartbeat count – BIG difference = pulse

deficit • Felt with fingers (do not

use thumb)

Blood pressure • Pressure from blood against the vessel walls (arteries)

– Reported as diastolic pressure over systolic pressure • Measured with a sphygmomanometer

– Inflation blocks bloodflow through brachial artery – Pressure of cuff gradually reduced to allow partial flow

• Sounds of Korotkoff – tapping sound of blood flowing back into artery– First appears at systole

• Blood spurting into artery – Sound disappears when artery is no longer compressed

• Diastole

Blood pressure alteration

• Hypertension – >140 systole; >90

diastole – To pump against

increased pressure, heart works harder• Myocardium increases

(especially LV), heart ultimately weakens

• Hypotension – <100 systole

Circulatory shock

• Blood vessels inadequately filled; abnormal blood flow

• Hypovolemic – Significant blood loss

• Vascular – Poor circulation due to extreme vasodilation

• Anaphylactic – allergies; septic – infection

• Cardiogenic – Heart not pumping (myocardial infarction)

Arteries

Arteries

Arteries

Veins

Veins

Veins