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