Blood Vessels, Blood Cells and Tissue Fluid
Arteries, Veins and Capillaries
Arteries carry blood away from the heart
all carry oxygenated blood except the pulmonary artery
have three layers:
tunica intima - layer of flattened epithelium (endothelium) and supporting connective tissue
tunica media - smooth muscle and elastic fibres
tunica adventitia (or externa) - connective tissue
the elastic fibres in the tunica media allow the artery to expand when blood is pumped through it and then return to its normal size by elastic recoil.
The elastic recoil also provides a pressure forcing the blood along the vessel
With distance from the heart the arteries get progressively smaller
smaller arteries have a greater proportion of smooth muscle in the tunica media then do larger ones. They have less capacity for elastic recoil but are able to undergo vasoconstriction causing the re-routing of blood
The final arteries are called arterioles
defined as less than 0.3mm diameter
no elastic tissue
Arterioles feed into capillaries
wall is single layer of flattened squamous epithelium
diameter 0.7µm - same diameter as an RBC
in regions of very high exchange (e.g. kidney tubule) there are pores between the endothelial cells facilitating movement across the wall
Capillaries drain into venules
Which feed into veins
carry deoxygenated blood back to the heart - except the pulmonary arteries
Veins have same layers as arteries
thinner walls
larger lumen
have valves to prevent backflow
larger veins found between panels of muscle in the limbs - contraction of muscles provides pressure to drive blood along
Components of Blood
cells suspended in fluid called plasma
plasma also contains proteins:
globulins - component of immune system
albumins - involved in clotting
fibrinogen - involved in clotting
Blood cells
White blood cells
granulocytes (have granules in the cytoplasm and a lobed nucleus [named for their staining characteristics]):
neutrophils - phagocytes
eosinophils - act against parasitic worms, also have a role in allergic response
agranulocytes (no prominent granules, nucleus not lobed)
monocytes - phagocytes
lymphocytes - secrete antibodies
Red blood cells
biconcave shape (hence lighter staining in central region)
no nucleus (in mature cells)
transport respiratory gases
Platelets
cell fragments
involved in clotting
Formation of Tissue Fluid
Tissue fluid bathes all cells
provides cells with nutrients and removes wastes
liquid and dissolved substances can pass through walls of capillaries
red blood cells and proteins are too large to pass through
Note WBC can alter shape to squeeze through the walls
Osmotic pressure provided by the proteins is 3.3kPa
pressure at arteriolar end is 5.3kPa - large enough to force liquid out
pressure at venular end is 1.3kPa so OP is greater so liquid moves back in
Not all liquid passes back into the capillaries - some drains off as lymph fluid
Lymph fluid returns to blood at lymphatic ducts