Characteristics of Blood Vessels

Blood Vessel System

• artery → arterioles → capillaries

• capillaries → venules → veins

Arteries

ARTERIES

1. large arteries near the heart have thick walls containing smooth muscle and many collagen and elastic fibres

   1. this allows them to stretch and not rupture under the high pressure created by the heart

2. these elastic fibres undergoes elastic recoil, returning the pressure to the blood helping to maintain a steady flow through the smaller vessels

3. even tho the flow of the blood is pulsatile, it more continuous and smooth then it would be compared to a system of rigid pipes

1. small arteries further away from the heart posses more smooth muscle fibres - contriction or dilation

Control of Blood Flow Distribution

• ‘shunt vessel's' act as shortcuts between arterioles and venules as they bypass the capillary beds- these constrict and dilate to regulate quantity of blood which flows thru capillaries according to the needs of the body

• precapillary sphincters; rings of smooth muscle where shunt vessels are connected with the capillaries - contract and close off entrance to capillaries

• fight/flight - vasoconstriction - increasing blood pressure and speed, while arterioles leading to the heart/muscle dilate ensures increased delivery of oxygen and glucose for increased activity.

• vasodilation/constriction for thermoregulation, hot day dilation, cold day constriction esp in extremities, which is how frostbite arises, body diverts heat toward major organs

Veins and Venules

• goes back to the heart, carries deoxygenated blood along with other cellular wastes

• venules → veins

• diameter of lumen of veins is larger than that of arteries, to prevent less resistance to blood flow which is already under low pressure

• thinner, less muscular and more expandable walls then the arteries

How do veins and venules return blood back to the heart

1. negative pressure created in the chest area during inhaling

2. skeletal muscle contraction during movement, exerting pressure on veins and squeezes them flat, forcing blood through them

3. semi lunar valves - ensure blood only flows toward the heart

Capillaries

• smallest vessel

• exchange between blood and tissue fluid occurs over the capillary walls

1. thin; only made of endothelium reducing diffusion distance

2. narrow; rbc must pass very close to the capillary wall, reducing diffusion distance, and flow o rbc is slow enough to allow time for gas exchange to occur

3. vast network of capillaries ensure all body cells are close to a capillary

4. permeable to water and dissolved substances

Methods with which exchange of materials between the blood and interstitial fluid can occur

1. by simple/facilitated diffusion through plasma membrane of endothelial cells

   1. lipid soluble substances + gases - simple diff

   2. glucose,lactate,ions - faciliated

2. through fenestrations within plasma membrane of endothelial cells

   1. very small gaps between adj endothelial cells - water, salts and small charged molecules which dont diffuse thru plasma membrane

proteins, rbc stay in the blood

Why do capillaries have the lowest pressure even though they have the smallest diameter

1. this is because the arterioles branch into huge number of capillaries

2. so the total csa of capillaries is actually larger then arterioles, so pressure drops

   1. allows more diffusion time

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

• tunica intima

• tunica media

• tunica externa

Capillary is made only of endothelium why is this good

• allows molecules to diffuse through

• allows filtration through the pores between the cells

Varicose Veins

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

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.

when Hydrostatic pressure is lower then coillodial osmotic pressure - tissue fluid is reabsorbed and water returns to the capillary

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