Structure and Function Of Blood Vessels

Blood Vessels

• Tubes that transport blood throughout the body

• Tissues have an

  • arterial supply: carry O2 and nutrients to tissue

  • venous drainage: take away waste and deoxygenated blood

• Different types of vessels, each with a different function

Characteristics Of Vessels

• Resilient: Vessels must be able to stretch and recoil to accommodate blood flow

• Flexible: adapt to pressure changes

• Always remain open to allow blood to flow

  • thrombosis/DVT/Clots block veins

Tunica Intima:

• Innermost layer

• endothelium - simple squamous

  • only layer found in capillaries

• Basal lamina: connective tissue supporting endothelium (aka basement membrane)

• Sub-endothelial connective tissue (below basal lamina)

  • tiny blood vessels that supply endothelium with nutrients

Tunica Media:

• Middle Layer

• Smooth muscle fibres in loose connective tissue

  • amount varies depending on vessel

• Many elastic fibres

  • depends on the pressure and flow rate of blood in vessels

Tunica Externa/ Adventita:

• Outermost Layer

• Consists of connective tissue

  • layers merge with surrounding connective tissue

  • help adhere blood vessels to adjacent structures

• May contain vast vasorum - tiny blood vessels that supply the walls of thick walled blood vessels with oxygen

• Provides support and protection

Arteries

• Carry blood away from the heart at high pressures to the rest of the body

• Features

  • thicker walls

  • smaller lumen

  • maintain its shape

  • more resilient

  • no valves

Elastic Conducting Artery

• e.g aorta, common carotid and brachiocephalic

• Large diameter - up to 2.5 cm

• Can withstand pressure changes in the cardiac cycle and ensure a continuous blood flow

  • due to elastic fibres in tunica media

Structural Adaptations of Elastic (Conducting) Arteries:

• Structural Adaptations

  • Thick tunica media

  • Many elastic fibres

  • Few Smooth Muscles

  • Flexible - Able to expand and recoil

Muscular (Distributing) Arteries:

• e.g Brachial (arms) and femoral (legs) vessels

• Diameter: 0.5mm-0.4cm

• Distributes blood to muscle and organs

• Able to vasodilator and vasoconstriction to control rate of blood flow

Structural Adaptations Of Muscular (Distributing) Arteries:

• Lots of smooth muscles cells in tunica media

• Distinct internal (IEL) and external (EEL) elastic laminae

  • structural support

• Thick tunica externa

Arterioles:

• Resistance vessels

• Able to vasoconstrict and vasodilate - more than muscular

• Controls blood flow to organs - involved in blood pressure control

• Diameter: Less than or equal to 30 μm

Structural Adaptations of Arterioles:

• 1-2 layers of smooth muscle cells in tunica media

• Poorly defined tunica externa

Capillaries

• Connect venues and arterioles - microcirculation

• Site of gas exchange

• Thin walls: Facilitate diffusion

• Slow blood flow - one RBC through at a time

• Structure allows 2 way exchange

• Diameter: 8μm

• Found near almost every cell

Continuous Capillaires:

• Most type of capillaries

• Found in skeletal and smooth muscle, CT and lungs

• No gaps - molecules can’t get in/ out

• Diffusion of gasses is possible

Fenestrated Capillaries:

• Pores in endothelial lining

• Rapid exchange of water/ large solutes

• Used in absorption

  • found in kidney: chloride plexus and endocrine glands

Sinusoidal Capillaries:

• Has open spaces between endothelial cells

• Lots of gaps -more blood can flow through

• Incomplete/ absent basement membrane

• Involved in exchange of large solutes - plasma proteins

• Blood moves through slowly

• Found in liver - specialised lining

Capillary beds:

• Capillaries organised into groups

• Found in lungs, liver, intestines and kidney

Metarterioles:

• Supplies a single capillary bed

• Each continues as a through fare channel, directly leading to a vein

  • has numerous capillaries leading off it

• Constriction of these can reduce the flow of blood to the whole capillary bed

Pre-Capillary Sphincter:

• Guards entrance to each capillary and further regulation of blood flow

• Constriction narrows entrance - blood flow decreases

• Relaxation dilates entrance - increases blood flow

Arterio-venous Anastomoses:

• Direct communication between arteriole and venule

• When dilated, blood passes capillary bed and flows directly to venous circulation

Venules:

• Collect blood from capillary beds and deliver it to small veins

• Diameter varies average 20μm

Structural Adaptations Of Venules:

• Small endothelium on basement membrane

• Large n.o smooth muscle located outside endothelium

Veins:

• Carry blood under low pressure towards the heart from the rest of the body

  • low pressure system

• Classified by size:

  • Small: >2mm diameter

  • Medium: 2-9mm diameter

  • Large: <9mm diameter

Structural Adaptations Of Veins:

• Thin walls (collapsed, but open when blood slows down)

• Wide lumen

• Many Valves - prevent back flow of blood

  • No valves above the heart, as blood flows down with gravity

• Large tunica externa

Valves and Musculovenous Pump:

• When skeletal muscles contract, veins are squashed, propelling blood upwards

• When muscles relax - valves close

• When muscles contract - valves above muscle open to prevent back flow of blood