Characteristics of Blood Vessels

Blood Vessel System

• artery → arterioles → capillaries

• capillaries → venules → veins

Arteries and Arterioles

• Heart pumps blood into major arteries

• muscular arteries

• microscopic arterioles

• thicker walls then veins

  • arterial system is higher pressure - round and thick walls

• more smooth muscle

• more elastic tissue

  • allows them to dilate as blood pumps

Veins and Venules

• microscopic venules

• veins

• venae cava

• thinner and less elastic

  • because venous system is a lower pressure system

• valves prevent backflow

• appear flattened

Capillaries

• microscopic vessels which branch out from arterioles

• connect arteries to veins

• made only of a tunica intima (Only endothelium)

• in the lungs they absorb o2 and release co2

• everywhere else capillaries absorb co2 and release o2

3 layer structure of vessels → arteries/oles veins/venules

• tunica intima

• tunica media

• tunica externa

Tunica Intima

• thin layer

• squamous epithelium → endothelium

• small amount of connective tissue

→ minimizes friction as blood is passing through lumen

Tunica Media

• made of smooth involuntary muscle and elastic fibers

• controls vasodilation and constriction

  • hence influences blood flow and pressure

• this layer is thicker in arteries

Tunica Externa

• layer of inelastic collagen fibres

• connective tissue

• anchors vessel to surrounding tissue

• protects and reinforces blood vessels

Compare the Tunics in Arteries and Veins

Capillary is made only of endothelium why is this good

• allows molecules to diffuse through

• allows filtration through the pores between the cells

Arteries

• larger arteries closed to the heart

  • thick artery walls

  • elastic middle layer → allows them to recoil/contract with blood flow (Smoother)

  → allow them to support high pressure

• arteries further away have more smooth muscle

• have neurons → regulate diameter of the artery

• smaller arteries are less elastic but thick smooth muscle layer prevents them from bursting

Arterioles

• endothelium wrapped with muscle fibres at intervals

• where the arteriole and capillary meet there are pre-capillary sphincters

  • muscles which adjust blood flow

• vasoconstriction (contraction of smooth muscle) → higher bp

• vasodilation (relaxation of smooth muscle) → lower bp

Capillaries

• so many capillaries and branching means that every cell is within 100 micrometers of a capillary

• thin layer of cells- short diffusion distance

• narrow lumen - rbc close to cells, shorter diffusion dist

• membrane is permeable to water and dissolved substances

• blood moving through capillaries slows down → more time for exchange of substances

• loses pressure and velocity

Venules and Veins

• same tissue layers as arteries but thinner smooth muscle layer

  • due to the fact they have a lower pressure

• venule wall is made of inelastic and tough collagen fibres

• greater lumen than artery

• moves in one direction - venous valves

Varicose Veins

• caused when the venous valves no longer work and blood pools in the veins

effect of skeletal muscle pump on venous blood flow

• venous/muscle pump returns the blood back to the heart from the legs

• skeletal muscle contracts - upper valve opens and blood moves upward while the lower valve closes (So contracting blood moves up)

• muscles relax lower valve opens allowing veins to fill, upper valve closes preventing backflow

Comparison of Arteries, Veins and Capillaries

Pulsatile and Continuous Blood Flow

• pulsatile flow; rhythmic, intermittent propagation of blood flow

• found only in arteries

• from great arteries to capillaries → flow becomes continuous

• normal resting heart rate is 60-100bpm

Define Pressure

• measure of the force that the blood exerts against the vessel wall as it moves through it

Blood Flow

• volume of transported fluid per unit time and over a given arterial cross section

Blood Velocity

• speed of a moving particle of blood

Velocity and Pressure in Arteries and Veins and Capillaries

• The heart pumps blood at high pressure into the arteries.

• Blood in arteries flows fast and under the highest pressure.

• As blood moves from arteries → arterioles → capillaries → venules → veins, the pressure gradually decreases.

• In capillaries, pressure and speed are lowest (to allow exchange).

• From capillaries to veins, total cross-sectional area decreases, so velocity increases slightly again.

Vasoconstriction and Vasodilation

Vasoconstriction

• contraction of smooth muscle

• increase bp

• maintain healthy blood flow

• keep body temp from getting too cold

Vasodilation

• relaxation of circular smooth muscle cells

• decreased bp

• increase blood volume when theres low o2 or low nutrients

• also prevents increases in temperatrure

vasodilators are prescribed to those with hyper tension