biol 2304 fall 2008chapter 191 chapter 19 – vascular system

BIOL 2304 Fall 2008 Chapter 19 1

Chapter 19 – Vascular System


A. categories and general functions:

1. arteries - carry blood

away from heart

2. capillaries - allow

exchange of materials

between blood and

tissue fluid

3. veins - return blood

to heart


B. wall structure

most vessel walls have 3 layers

lumen = space inside vessel

1. tunica intima / tunica interna

endothelium –

simple squamous e.

subendothelial layer –

loose c.t. (collagen)


2. tunica media

a. smooth muscle - cells circularly arranged• controlled by ANS and chemical factors• constriction decreases blood flow and increases

systemic blood pressure• dilation increases blood flow and decreases

systemic blood pressure

b. elastic c.t.


3. tunica adventitia / tunica externa

• c.t. attaches vessel to surrounding structures• vasa vasorum nourish outer part of vessel wall


C. arteries

1. elastic (conducting) - large arteries near heart (aorta and major branches)

* conduct blood to muscular arteries

* low resistance

* tunica media = circular elastin sheets with few smooth m. cells

* recoil maintains blood pressure during diastole


2. muscular - middle-sized arteries, distal to elastic arteries

* distal to elastic arteries

* tunica media very thick; much smooth m. and some elastin

* regulate blood flow to organs

* have an internal and an external elastic lamina


3. arterioles - smallest arteries

* tunica media contains smooth m. only

* diameter controlled by ANS and chemical messengers

* diameter determines blood flow and blood pressure


D. capillaries

* wall consists of endothelium and basal lamina (no tunica media or externa)

* 8 to 10 m in diameter

* join and branch to form capillary beds

* cells are joined at spots around perimeter by tight junctions and desmosomes

* intercellular clefts are spaces between cells


1. types

a. fenestrated capillaries (high permeability)

* have fenestra (openings) in endothelial cells

* some fenestra are covered by a membrane, others are not

* also have intercellular clefts

* found in small intestine, synovial joints, kidney


b. continuous capillaries

* intercellular clefts but no fenestra

* most common type

c. sinusoids

* wide, leaky capillaries, usually fenestrated

* fewer cell junctions

* allow passage of large particles

* found in bone marrow, spleen, liver


d. low-permeability capillaries

* complete tight junctions

* no fenestra, no clefts

* restricted transport vesicles

* can transport specific molecules in or out

* found in brain (blood-brain barrier)


2. capillary beds

precapillary sphincter - smooth m. cell wrapped around origin of capillary

* controls blood flow through capillaries

* sphincter controlled by autoregulation

* when sphincter is closed, blood is diverted to thoroughfare channel


E. veins

• have thinner walls than arteries for the same diameter (larger lumen)

• act as capacitance vessels - store extra blood (65%)• low pressure• tunica adventitia thicker than tunica media• venous valves prevent backflow


F. vascular anastomosis• occurs when vessels join midstream• anastomoses provide alternate pathways (collateral

channels)


G. circulatory routes

1. pulmonary circuit: right ventricle to lungs to left atrium

* pressure supplied by right ventricle

* low pressure system

* takes low oxygen blood (75% saturated) to lungs and brings high oxygen (98%

saturated) blood

back to heart


right ventricle

pulmonary semilunar valve

pulmonary trunk

left and right pulmonary arteries

lobar arteries (3 R, 2 L) take blood to lung lobes

pulmonary capillaries

pulmonary veins (superior and inferior, L and R)

left atrium


2. systemic circuit: left ventricle to body to right atrium

* pressure supplied by left ventricle

* high pressure system

* takes O2 to tissues and removes CO2

* distributes nutrients from digestive tract to body

* collects wastes and takes them to kidney for excretion


a. circulatory pathways of the brain (cerebral arterial circle, circle of Willis)


anterior communicating a.

anterior cerebral a.

internal carotid a.

posterior communicating a.

posterior cerebral a.

basilar a.

vertebral a.


cross section of neck:


b. hepatic portal circulation

portal system = two capillary beds in series, joined by veins or arteries


capillaries of small intestine, part of large intestine and stomach

superior mesenteric vein

capillaries of spleen, stomach and pancreas

splenic vein

capillaries of distal large intestine and rectum

inferior mesenteric vein


hepatic portal vein

liver

liver sinusoids

hepatic veins

inferior vena cava


c. fetal circulation

placenta = organ formed from extra-embryonic membranes and endometrium to exchange materials between fetal and maternal blood

the umbilical veins take high-oxygen and high-nutrient blood from the placenta to the fetus

the umbilical arteries take low-oxygen, high-waste blood back to the placenta

umbilical vein from placenta fetal liver hepatic portal vein ductus venosus inferior vena cava right atrium foramen ovale left atrium right ventricle left ventricle pulmonary trunk ductus arteriosus aorta lungs

internal iliac a.

umbilical a. to placenta



adaptations:

1) to bypass the fetal liver

ductus venosus - allows some umbilical vein blood to go through the fetal liver but diverts most of it directly into the inferior vena cava

2) to bypass the non-functional fetal lungs

foramen ovale – opening in interatrial septum that allows blood to go from the right atrium directly into the left atrium

ductus arteriosus – vessel that connects pulmonary trunk and aorta