Blood is a complex, specialised tissue containing a number of differentcell types suspended in a fluid matrix

When whole blood is spun in a centrifuge, theblood cells collect towards the lower half of

the tube leaving a pale, straw-coloured liquid above

There are between 5 and 10 thousand white bloodcells in every mm3 of blood

There are between 5 and 6.5 million red bloodcells in every mm3 of blood

The straw-coloured liquid is plasma, a fluid thatcontains 92% water in which are dissolved variousmolecules such as nutrients, salts, hormones and

plasma proteins

White blood cells collect at the top of thelower fraction and these include cell typesknown as granulocytes, lymphocytes and

monocytes

Red blood cells form the bulk of the lowerfraction and these cells occupy about 45% of

the total volume of the blood

Blood is a TissueBlood is a Tissue

Plasma is the fluid matrix and the cells can be classified as RED CELLS(ERYTHROCYTES), WHITE CELLS (LEUCOCYTES) and

PLATELETS (THROMBOCYTES)

Lymphocytes, monocytes and granulocytes are thedifferent types of white cell

This photomicrograph of a blood smear shows the appearance ofblood as viewed with the light microscope

Blood is a TissueBlood is a Tissue

This scanning electron micrograph (colour enhanced) showsa red cell and white cell on the surface of an epithelial tissue

The major function of white blood cells is to defend the body againstdisease-causing organisms (pathogens) and other foreign matter

White cells are classified as either GRANULOCYTES or AGRANULOCYTES

WHITE BLOOD CELLS

Granulocytesgranular cytoplasm;

lobed nucleus

Agranulocytesnon-granular cytoplasm;

kidney-shaped or round nucleus

LYMPHOCYTES(spherical nucleus)

MONOCYTES(kidney-shaped nucleus)

White Blood CellsWhite Blood Cells

GRANULOCYTE

MONOCYTE LYMPHOCYTE

Lobed nucleus

Granular cytoplasm

Kidney-shaped nucleus

Non-granular cytoplasm

Spherical nucleus

occupying alarge volume

of the cell

Non-granular cytoplasm

White Blood CellsWhite Blood Cells

Human red blood cells have

a life span of only 120 days

Mammalian red blood cells havethe shape of a biconcave disc

They have no nucleus allowing space for large amounts of

haemoglobin

The major functions of these cells is to transport oxygen from the alveoli of thelungs to the body tissues and carbon dioxide from the tissues to the alveoli

Each red cell contains about 270 million haemoglobin molecules each of which has a high affinity for molecular oxygen

The biconcave disc shape provides the red cells with a largesurface area to volume ratio

The large surface area to volume ratio ensures that each haemoglobinmolecule is close to the cell surface for rapid gas exchange

The large surface area to volume ratio ensures that diffusion ofoxygen into and out of the red cells occurs at a rate fast enough

to meet the metabolic needs of the organism

The biconcave centre also allows the cells to fold within

the smallest capillaries (facilitating passage)

Mammalian Red Blood CellsMammalian Red Blood Cells

Scanning electron micrograph of mammalian red blood cells showing thebiconcave disc shape that gives these cells a large surface area to volume ratio

Red Blood CellsRed Blood Cells

The blood vessels of the cardiovascular system, with the exception of the capillaries, have walls composed of three basic tissue layers

ALL BLOOD VESSELS, EXCEPT CAPILLARIES, ARE THEREFORE CLASSED AS ORGANS

Blood vessel walls are composed of the following plan

LUMEN(hole)

The differences betweenthe various blood vessels

are related to their function within thecirculatory system

Tunica Externa(the outer coat consistingmainly of collagen fibres)

Tunica Media(the middle coat

consisting of muscleand elastic tissue)

Tunica Intima(the inner coat consisting

mainly of endothelialcells and somecollagen fibres)

Blood VesselsBlood Vessels

Blood is pumped out of the heart into arteries which must be able to withstandthe pressure of the blood leaving the heart

Arteries possess large amounts of muscle tissue in themiddle layer

Arteries also possess large amounts of elastic tissue in the middle layer

The elastic tissue enables the artery walls to stretch to accommodate the blood ejected from the

heart as it contractsAs the heart relaxes,

the elastic tissue recoilsand exerts a pressure on

the blood

The arteries thusact as ‘subsidiarypumps’ helping tomaintain the blood

pressure and ensuringthe unidirectional flow

of blood

Tunica media(large amounts

of muscleand elastic

tissue)

ArteriesArteries

As blood is ejected from the heart into the arteries, they stretch to accommodatethe additional blood

As the heart relaxes, the artery walls recoil and propel blood onwards whilst theheart is at rest

The artery acts as a ‘subsidiary pump’ and is pulsatile

Elastic ArteriesElastic Arteries

TRANSVERSE SECTION THROUGH AN ARTERY

LUMEN

Connectivetissue

Smooth muscleand elastic

tissue

Endothelium

A rter io le s C a p illa r ie s

A rter ye .g . ren a l a r te ry

A o rta

H ea rt

O r g a n e .g . k id n ey

Arteriole means little artery

Major arteries branch intosmaller and smaller

arteries,eventually giving rise to

arteriolesArterioles subdivide into

capillaries where exchangeof materials between theblood and the tissue cells

takes place

NutrientsOxygen

Excretory productsCarbon dioxide

Blood from the capillariescollects into larger vessels,

called venules, which empty their contents into

the thin-walled veins

Arterioles control thevolume of blood flow

to a particularorgan

Ve n u les

Ve ine .g . ren a l v e in

Ve n a ca v a

ArteriolesArterioles

Tunica Externa(the outer coat consistingmainly of collagen fibres)

Tunica Media(very large amounts of

smooth muscle tissue butvery little elastic tissue)

Tunica Intima(the inner coat consisting

mainly of endothelialcells and some collagen fibres)

Arterioles, like arteries, are composed of three basictissue layers but they are about 100 times smaller

As arterioles approach the capillaries, their walls becomemore muscular and less elastic in composition

The diameter of the arterioles largely determines the amountof blood that flows to a particular organ

The degree of contraction of the smooth muscle in thearteriole walls determines their diameter and is controlled

by the sympathetic nervous system

ArteriolesArterioles

The large amount of smooth muscle in the walls of the arterioles isessential for controlling blood flow to different organs

During exercise there is an increased demand, by skeletal muscles, foradditional oxygen

This demand is met by redistributing blood flow from the less essential organsof exercise to the skeletal muscles

Blood flow to the skeletal muscles is increased when muscles in the arteriolewall become more relaxed and the arteriole diameter increases – a response

called vasodilationBlood flow to organs of less importance during exercise decreases as a result

of contraction of the arteriole muscle – a response called vasoconstriction

Arterioles and the Control of Blood FlowArterioles and the Control of Blood Flow

Veins transport blood from the various body organs to the heartCompared to arteries, veins have virtually no elastic fibres or smooth muscle

and are very thin-walled

VEIN ARTERY

Tunica Externa(the outer coat consistingmainly of collagen fibres)

Tunica Media(little elastic tissue

and smooth muscle;thin muscular walls)

Tunica Intima(the inner coat consisting

mainly of endothelialcells and somecollagen fibres)

One-wayvalves

VeinsVeins

This photomicrograph of a small artery and vein shows the difference inthickness of the walls of these two types of blood vessel

Vein(large lumen)

Artery(smaller lumen)

Blood flows from the capillary beds into vessels called venules, which empty theircontents into the thin-walled veins for return to the heart

Blood pressure within the veins is very low and blood flow is maintained by one-way valves within veins, the action of working skeletal

muscles and the pressure that is exerted during breathing

Vein

Lower valve

One-way valve

Skeletal muscle

Upper valve

Valve Action in VeinsValve Action in Veins

When skeletal musclesaround this section of vein

are relaxed, both setsof valves are closed

When the skeletal muscles contract,pressure in the section between the

valves increases and the uppervalve is forced open

Blood flows upwards throughthe open valve

Other skeletal muscles, below the level of those shownhere, also contract during which the lower valve thenfunctions as the upper valve for that section of vein

When the skeletalmuscles relax again,

the upper valve closesdue to the backflow

of blood

Pressure changes within a body cavity also squeeze the veins and aidvenous return to the heart

During inspiration the pressure in the thoracic cavity falls and the pressurein the abdominal cavity rises

Pressure withinthe abdomensqueezes on

the veins

Low pressurein the thorax

exerts a suckingaction onthe veins

These two pressures aid

the flow of bloodfrom the lowerorgans back to

the heart

Pressure in thethorax falls

Pressure in theabdomen rises

Blood Flow in VeinsBlood Flow in Veins

BloodVessel

DiagramBloodFlow

Size ofLumen

PressureWall

Structure

Artery

Blood flows away form the heart to

tissues and organs and to the

lungs

Relatively small

1 – 2.5 cm

HighDue to the

pumping action of the heart

Tunica media contains large

amounts of elastic tissue and smooth

muscle

Arteriole

Blood flows from arterioles into capillary beds;

the lumen diameter of

arterioles controls blood flow to the

organs

Relatively small

20 – 200 m

Wide Range – the greatest drop

in pressure occurs as blood travels through the arteriolesHigh at the

artery end and relatively low

approaching the capillaries

Tunica media contains very large amounts

of smooth muscle tissue but very little elastic tissue

Vein

Blood flows from the capillary beds into venules and then into veins.

Veins carry blood towards the heart

Relatively large

2 - 3 cm

LowOne-way valves, working skeletal

muscles and inspiration aid

the flow of blood in veins

Tunica media contains little elastic tissue and smooth

muscle tissue; thin muscular

wall

Summary of Arteries, Arterioles and VeinsSummary of Arteries, Arterioles and Veins

Acknowledgements

Copyright © 2003 SSER Ltd. and its licensors.All rights reserved. All graphics are for viewing purposes only.