Download 1.Blood and Vessels

Blood is a Tissue
Blood is a complex, specialised tissue containing a number of different
cell types suspended in a fluid matrix
When whole blood is spun in a centrifuge, the
blood cells collect towards the lower half of
the tube leaving a pale, straw-coloured liquid above
The straw-coloured liquid is plasma, a fluid that
contains 92% water in which are dissolved various
molecules such as nutrients, salts, hormones and
plasma proteins
White blood cells collect at the top of the
lower fraction and these include cell types
known as granulocytes, lymphocytes and
monocytes
There are between 5 and 10 thousand white blood
cells in every mm3 of blood
Red blood cells form the bulk of the lower
fraction and these cells occupy about 45% of
the total volume of the blood
There are between 5 and 6.5 million red blood
cells in every mm3 of blood
Blood is a Tissue
This photomicrograph of a blood smear shows the appearance of
blood as viewed with the light microscope
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 the
different types of white cell
This scanning electron micrograph (colour enhanced) shows
a red cell and white cell on the surface of an epithelial tissue
White Blood Cells
The major function of white blood cells is to defend the body against
disease-causing organisms (pathogens) and other foreign matter
White cells are classified as either GRANULOCYTES or AGRANULOCYTES
WHITE BLOOD CELLS
Granulocytes
granular cytoplasm;
lobed nucleus
Agranulocytes
non-granular cytoplasm;
kidney-shaped or round nucleus
LYMPHOCYTES
(spherical nucleus)
MONOCYTES
(kidney-shaped nucleus)
White Blood Cells
Lobed nucleus
Granular
cytoplasm
MONOCYTE
GRANULOCYTE
Kidney-shaped
nucleus
Spherical
nucleus
occupying a
large volume
of the cell
Non-granular
cytoplasm
Non-granular
cytoplasm
LYMPHOCYTE
Mammalian Red Blood Cells
Mammalian red blood cells have
the shape of a biconcave disc
Human red blood
cells have
a life span of only
120 days
They have no nucleus
allowing space for
large amounts of
haemoglobin
The biconcave centre also
allows the cells to fold within
the smallest capillaries
(facilitating passage)
The major functions of these cells is to transport oxygen from the alveoli of the
lungs 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 large
surface area to volume ratio
The large surface area to volume ratio ensures that each haemoglobin
molecule is close to the cell surface for rapid gas exchange
The large surface area to volume ratio ensures that diffusion of
oxygen into and out of the red cells occurs at a rate fast enough
to meet the metabolic needs of the organism
Red Blood Cells
Scanning electron micrograph of mammalian red blood cells showing the
biconcave disc shape that gives these cells a large surface area to volume ratio
Blood Vessels
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
Tunica Externa
(the outer coat consisting
mainly of collagen fibres)
Tunica Media
(the middle coat
consisting of muscle
and elastic tissue)
Tunica Intima
(the inner coat consisting
mainly of endothelial
cells and some
collagen fibres)
LUMEN
(hole)
The differences between
the various blood vessels
are related to their
function within the
circulatory system
Arteries
Blood is pumped out of the heart into arteries which must be able to withstand
the pressure of the blood leaving the heart
Arteries possess large amounts of muscle tissue in the
middle 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 contracts
Tunica media
As the heart relaxes,
(large amounts
the elastic tissue recoils
of muscle
and exerts a pressure on
and elastic
the blood
tissue)
The arteries thus
act as ‘subsidiary
pumps’ helping to
maintain the blood
pressure and ensuring
the unidirectional flow
of blood
Elastic Arteries
As blood is ejected from the heart into the arteries, they stretch to accommodate
the additional blood
As the heart relaxes, the artery walls recoil and propel blood onwards whilst the
heart is at rest
The artery acts as a ‘subsidiary pump’ and is pulsatile
TRANSVERSE SECTION THROUGH AN ARTERY
Connective
tissue
Smooth muscle
and elastic
tissue
LUMEN
Endothelium
Arterioles
Arteriole means little artery
Major arteries branch into
smaller and smaller arteries,
eventually giving rise to
arterioles
Heart
Vena cava
Aorta
Arterioles subdivide into
capillaries where exchange
of materials between the
Artery
blood and the tissue cells
e.g. renal artery
takes place
Organ e.g. kidney
Blood from the capillaries
collects into larger vessels,
called venules, which
empty their contents into
the thin-walled veins
Nutrients
Oxygen
Arterioles
Arterioles control the
volume of blood flow
to a particular
organ
Vein
e.g. renal vein
Capillaries
Excretory products
Carbon dioxide
Venules
Arterioles
Arterioles, like arteries, are composed of three basic
tissue layers but they are about 100 times smaller
As arterioles approach the capillaries, their walls become
more muscular and less elastic in composition
The diameter of the arterioles largely determines the amount
of blood that flows to a particular organ
The degree of contraction of the smooth muscle in the
arteriole walls determines their diameter and is controlled
by the sympathetic nervous system
Tunica Media
(very large amounts of
smooth muscle tissue but
very little elastic tissue)
Tunica Intima
(the inner coat consisting
mainly of endothelial
cells and some collagen fibres)
Tunica Externa
(the outer coat consisting
mainly of collagen fibres)
Arterioles and the Control of Blood Flow
The large amount of smooth muscle in the walls of the arterioles is
essential for controlling blood flow to different organs
During exercise there is an increased demand, by skeletal muscles, for
additional oxygen
This demand is met by redistributing blood flow from the less essential organs
of exercise to the skeletal muscles
Blood flow to the skeletal muscles is increased when muscles in the arteriole
wall become more relaxed and the arteriole diameter increases – a response
called vasodilation
Blood flow to organs of less importance during exercise decreases as a result
of contraction of the arteriole muscle – a response called vasoconstriction
Veins
Veins transport blood from the various body organs to the heart
Compared to arteries, veins have virtually no elastic fibres or smooth muscle
and are very thin-walled
Tunica Externa
(the outer coat consisting
One-way
mainly of collagen fibres)
valves
Tunica Media
(little elastic tissue
and smooth muscle;
thin muscular walls)
Tunica Intima
(the inner coat consisting
mainly of endothelial
cells and some
collagen fibres)
VEIN
ARTERY
This photomicrograph of a small artery and vein shows the difference in
thickness of the walls of these two types of blood vessel
Vein
(large lumen)
Artery
(smaller lumen)
Valve Action in Veins
Blood flows from the capillary beds into vessels called venules, which empty their
contents 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
Upper valve
One-way valve
Skeletal muscle
Lower valve
When skeletal muscles
around this section of vein
are relaxed, both sets
of valves are closed
When the skeletal
muscles relax again,
the upper valve closes
due to the backflow
of blood
When the skeletal muscles contract,
pressure in the section between the
valves increases and the upper
valve is forced open
Blood flows upwards through
the open valve
Other skeletal muscles, below the level of those shown
here, also contract during which the lower valve then
functions as the upper valve for that section of vein
Blood Flow in Veins
Pressure changes within a body cavity also squeeze the veins and aid
venous return to the heart
During inspiration the pressure in the thoracic cavity falls and the pressure
in the abdominal cavity rises
Pressure within
the abdomen
squeezes on
the veins
Low pressure
in the thorax
exerts a sucking
action on
the veins
These two
pressures aid
the flow of blood
from the lower
organs back to
the heart
Pressure in the
thorax falls
Pressure in the
abdomen rises
Summary of Arteries, Arterioles and Veins
Blood
Vessel
Artery
Arteriole
Vein
Diagram
Blood
Flow
Size of
Lumen
Pressure
Blood flows away
form the heart to
tissues and
organs and to the
lungs
Relatively
small
1 – 2.5 cm
High
Due to the
pumping action
of the heart
Tunica media
contains large
amounts of
elastic tissue
and smooth
muscle
Blood flows from Relatively
arterioles into
small
capillary beds;
20 – 200
the lumen
mm
diameter of
arterioles
controls blood
flow to the organs
Wide Range –
the greatest drop
in pressure
occurs as blood
travels through
the arterioles
High at the
artery end and
relatively low
approaching the
capillaries
Low
One-way valves,
working skeletal
muscles and
inspiration aid
the flow of blood
in veins
Tunica media
contains very
large amounts
of smooth
muscle tissue
but very little
elastic tissue
Blood flows from
the capillary beds
into venules and
then into veins.
Veins carry blood
towards the heart
Relatively
large
2 - 3 cm
Wall
Structure
Tunica media
contains little
elastic tissue
and smooth
muscle tissue;
thin muscular
wall
Acknowledgements
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