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Blood Vessels
Chapter 20
Characteristic Artery Vein
Wall thickness thick thin
Shape in cross section round flattened
Thickest tunic media externa
Collagen and elastic fibers abundant sparse
Valves Absent present
BP range 40-over 100mmHg 20-0 mmHg
Blood flow Away from heart Towards heart
Volume of blood held 11% 54%
Velocity of blood flow fast slow
Summary of the Characteristics of
Arteries and Veins
Capillaries • Blood vessels so small
only one RBC can pass at a time, sometimes by folding.
• Supply somatic cells with nourishment and remove soluble waste.
• Precapillary sphincters of metarterioles control which capillary beds are perfused
– only 1/4 of the capillaries in the body are open at a given time.
Control of Capillary Bed Perfusion closing sphincters diverts blood from capillaries
Capillary Exchange Mechanisms • Simple Diffusion: lipid soluble materials like steroids
and gas
• Transcytosis: vesicles carry water soluble materials like electrolytes and sugars
• Filtration: large proteins and sometimes cells can pass through fenestrations within or between cells
Types of Capillaries • Continuous - occur in most tissues
– endothelial cells joined with tight junctions
– solutes moved by diffusion and transcytosis through the
capillary cells
• Fenestrated
– Found in organs that require rapid absorption or filtration
like the kidneys and small intestines
– endothelial cells have filtration pores called fenestrations
that pass through cells and may or may not be covered with
a basement membrane
– Fenestrations allow passage of small molecules
• Sinusoids – are extra large, elongated fenestrations that
allow large proteins and even blood cells to pass through
- found in filtering organs like liver, bone marrow, spleen
Fenestrated Capillary
• Opposing forces maintain tissue fluid balance
• blood (hydrostatic) pressure drives fluid out of capillary
• colloid osmotic pressure (COP) draws some fluid back into the capillary
• COP results from concentration of large plasma proteins, mostly albumin, that are not filtered out of the capillaries
• Oncotic Pressure = the difference between the COP of blood and tissue fluid: 28 in – 8 out = 20 in
• Onctoic Pressure tends to draw water back into the capillary by osmosis.
Capillary Filtration
Capillary Filtration and Reabsorption
filtration
reabsorption
Oncotic
Pressure
At a normal Oncotic Pressure, filtration and reabsorption
are in balance.
hydrostatic pressure
along a capillary
filtration
reabsorption
Oncotic
Pressure
hydrostatic pressure
along a capillary
If Oncotic Pressure increases, reabsorption increases and
tissues can dehyrate.
filtration
Oncotic
Pressure
hydrostatic pressure
along a capillary
reabsorption
If Oncotic Pressure decreases, reabsorption decreases
and can result in edema.
Edema = excessive interstitial fluid
• Increased capillary filtration results from:
– high capillary blood pressure
– poor venous return due to obstruction or
insufficient muscular activity
– kidney failure (water retention)
– histamine (makes capillaries more permeable)
• Decreased capillary reabsorption can result
from:
– hypoproteinemia (low blood albumin) due to
famine or liver cirrhosis
Ascites Fluid: fluid accumulated in the
peritoneal cavity
Kwashiorkor (KWASH-ee-OR-cor)
Severe dietary
protein deficiency
reduces the protein
content of the blood
plasma and the
plasma osmolarity
drops. The blood
looses more fluid to
the tissues than it
reabsorbs resulting
in excessive ascites.
Consequences of Edema
• Circulatory Shock
– excess fluid in tissue spaces causes low blood
volume and low blood pressure
• Tissue Necrosis
– Impaired oxygen delivery and waste removal
• Pulmonary Edema
– Suffocation due to fluid in and around lungs
• Cerebral Edema
– headaches, nausea, seizures and coma due to
pressure on the brain
Skeletal Muscle Pumping helps move blood
back into circulation against gravity
contracted skeletal muscle relaxed skeletal muscle
END