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© Endeavour College of Natural Health endeavour.edu.au

BIOH122Human Biological Science 2

Session 5

Cardiovascular System 3–

Vasculature and Capillary

Exchange

Bioscience Department

© Endeavour College of Natural Health endeavour.edu.au 2

Session Plan

o Structure and function of blood vessels

• Anatomy of Blood Vessels

• Types of Blood Vessels

o Blood Distribution

o Capillary Exchange

• Diffusion

• Transcytosis

• Bulk Flow: Filtration and Reabsorption

• Dynamics of Capillary Exchange

• Net Filtration Pressure


Structure and Function of Blood

Vessels


Blood Vessels

o Angiogenesis: the growth of new blood vessels

It is an important process in the foetus and in postnatal

processes.

o Blood vessels: form a closed system of tubes that carry

blood away from the heart, transport it to the tissues of

the body, and then return it to the heart.

o Five main types:

• Arteries,

• Arterioles,

• Capillaries,

• Venules, and

• Veins


Blood Distribution


Blood Vessels

o Arteries: carry blood from the heart to the tissues.

o Arterioles: small arteries that connect to capillaries.

o Capillaries: are the site of substance exchange

between the blood and body tissues.

o Venules: connect capillaries to larger veins.

o Veins: convey blood from the tissues back to the heart.

o Vaso vasorum are small blood vessels that supply

blood to the cells of the walls of the arteries and veins.


Blood Vessels

http://www.baileybio.com/plogger/plog-print.php?id=462


Blood Vessels

o Structure of blood vessel wall: three major layers

o Tunica interna (intima)

• Epithelial layer known as endothelium

• basement membrane

• internal elastic lamina

o Tunica media

• circular smooth muscle and elastic fibers

o Tunica externa

• elastic and collagen fibers


Blood Vessels


Blood Vessels

o The functional properties:

• Elasticity:

• due to the elastic tissue in the tunica internal and

media,

• allows arteries to accept blood under great pressure

from the contraction of the ventricles and to send it on

through the circulation.

• Contractility:

• due to the smooth muscle in the tunica media,

• allows arteries to increase or decrease lumen size and

to limit bleeding from wounds.


Blood Vessels

o Sympathetic Innervation:

o Vascular smooth muscle is innervated by sympathetic nervous system

o increase in stimulation causes muscle contraction or vasoconstriction which decreases diameter of vessels

o decrease in stimulation or presence of certain chemicals causes vasodilation which increases diameter of vessels

• nitric oxide, K+, H+ and lactic acid all cause vasodilation


Arteries

o Characteristics:

o Tunica media : thick muscular-to-elastic

• Elastic arteries

• Muscular arteries

o High compliance: Arterial walls stretch easily or expand

without tearing in response to a small increase in

pressure.

o They are designed to hold low volume under high

pressure


Elastic Arteries

o Characteristics:

• Largest arteries

• Tunica media with more elastic fibers and less

smooth muscle

• Able to receive blood under pressure and propel it

onward

o Function:

• Conducting arteries: because they conduct blood

from the heart to medium sized muscular arteries.

• Pressure reservoir


Elastic Arteries


Muscular Arteries

o Characteristics:

• Medium-sized arteries

• Tunica media with more muscle than elastic fibers

• Walls are relatively thick

o Function:

o Distributing arteries: because they direct blood flow

o Capable of vasoconstriction and vasodilation to adjust

rate of flow


Anastomoseso Anastomoses: Union of 2 or more arteries supplying the

same body region

o Also occur between veins and arterioles and venules as

well.

o blockage of only one pathway has no effect on

circulation.

• Examples:

– Circle of Willis underneath brain

– Coronary circulation of the heart

– Gastrointestinal tract

o Collateral circulation: Alternate route of blood flow

through an anastomosis


Arterioles

o Characteristics:

• Very small, almost microscopic,

• Tunica media: Thin, few layers of muscle

• Tunica Interna: Thin that disappears distally.

o Function:

• Deliver blood to capillaries

• Resistance vessels: Regulate blood flow and arterial

blood pressure through vasoconstriction and

vasodilation


Arterioles

o Metarterioles:

• The terminal end of the arteriole

• form branches into the capillary bed

o Precapillary sphincters:

• At the metarteriole–capillary junction

• monitors the blood flow into the capillary

• Vasomotion: The intermittent contraction and

relaxation of sphincters that allow filling of capillary

bed 5-10 times/minute


Arterioles


Capillaries

o Characteristics:

• the smallest blood vessels, Microscopic

• Tunica media and a tunica externa: Absent

• Tunica interna: single layer of endothelial cells and a

basement membrane.

• Found near every cell in the body but more extensive

in highly active tissue (muscles, liver, kidneys and

brain)

only absent in epithelia, cornea and lens of eye and

cartilage


Capillarieso Function:

• Connect arterioles to venules

• Form Microcirculation

• Exchange of nutrients and wastes between blood and

tissue fluid


Types of Capillaries

o Continuous capillaries

• Characteristics:

Plasma membranes of endothelial cells form a

continuous tube

Interrupted by intercellular clefts - gaps between

neighboring endothelial cells

• Location:

Central nervous system, lungs, muscle tissue, and the

skin.



o Fenestrated capillaries

• Characteristics:

• Plasma membranes of the endothelial cells in these

capillaries have many holes.

• Location:

• Kidneys, villi of the small intestine, choroid plexuses of

the ventricles in the brain, ciliary processes of the eyes,

and most endocrine glands.



o Sinusoids

• Characteristics: endothelial cells have unusually large fenestrations

basement membrane incomplete or absent

have very large intercellular clefts that allow proteins and in some cases even blood cells to pass from a tissue into the bloodstream

• Location: Liver, bone marrow, spleen, anterior pituitary, adrenal glands and

parathyroid gland


Venules

o Characteristics:

• Post capillary venules:

– smallest veins collecting blood from capillaries

– very porous endothelium as significant sites of exchange of nutrients and wastes and white blood cell emigration

• muscular venules:

– Tunica media contains only a few smooth muscle cells and scattered fibroblasts

– have thicker walls across which exchanges with the interstitial fluid can no longer occur


Venules

o Function:

• drain the capillary blood and begin the return flow of

blood back toward the heart

• serve as excellent reservoirs for accumulating large

volumes of blood


Veins

o Characteristics:

o Tunica interna: thinner than arteries, lack internalelastic lamina

o Contain valves- thin folds of tunica interna- to prevent the backflow of blood

o Tunica media: much thinner than arteries, lack externalelastic lamina

o less elastic tissue and smooth muscle

o Tunica externa: thicker than arteries


Veins

o Flexibility: can adapt to variations in the volume and

pressure of blood but cannot withstand high pressure.

o They are designed to hold volume at low pressure

o Blood pressure: The average blood pressure in veins is

considerably lower than in arteries

o Vascular (venous) sinuses: veins with very thin walls with no smooth muscle to alter their diameters.

• Examples: brain’s superior sagittal sinus and the coronary sinus of the heart

o Function: Return blood to heart.


Varicose Veins

o Twisted, dilated superficial veins

• Cause: leaky venous valves

– congenital or mechanically stressed from

prolonged standing or pregnancy

• allow backflow and pooling of blood

– extra pressure forces fluids into surrounding

tissues

– nearby tissue is inflamed and tender


Varicose Veins

o common sites: in the oesophagus, superficial veins of

the lower limbs, and veins in the anal canal

(haemorrhoids).

o Deeper veins are not susceptible because of support of

surrounding muscles.

o Treatments: for varicose veins in the lower limbs include

• sclerotherapy, radiofrequency endovenous occlusion, laser

occlusion, and surgical stripping.


Blood Distribution and Capillary

Exchange


Blood Distribution

o Systemic veins and venules: contain 64% of blood

volume at rest

• functions as a blood reservoir

– veins of skin and abdominal organs (liver and

spleen)

• blood is diverted from it in times of need through

venoconstriction

– increased muscular activity

– haemorrhage

o systemic arteries and arterioles:13% of blood volume


Blood Distribution


Capillary Exchange

o Capillary Exchange: The continuous movement of

substances between the blood in systemic capillaries

and interstitial fluid.

o Three basic mechanisms:

• diffusion,

• transcytosis, and

• bulk flow


Capillary Exchange

o Diffusion:

• Nutrients and waste products including O2, CO2, glucose, amino acids, hormones, and others diffuse down their concentration gradients

• Most plasma proteins and blood cells cannot pass through capillary walls of continuous and fenestrated capillaries

o Diffuse through:

• fenestrations or intercellular clefts: Water-soluble substances such as glucose and amino acids

• lipid bilayer of endothelial cells: Lipid-soluble materials, such as O2, CO2, and steroid hormones

• intercellular clefts of sinusoids: larger, allow even proteins and blood cells to pass through


Capillary Exchange

• Blood brain barrier (BBB): blockade to movement of

materials into and out of brain formed by non-fenestrated

epithelium with tight junctions in brain capillaries

• Materials that can pass through BBB:

• A few water-soluble substances, such as glucose: active

transport.

• creatinine, urea, and most ions: slow movement

• lipid-soluble substances, such as oxygen, carbon dioxide,

alcohol, and most anaesthetic agents: through endothelial

plasma membrane

o Proteins and most antibiotic drugs—do not pass at all

from the blood into brain tissue.


Capillary Exchange

o Transcytosis:

o Passage of material across endothelium in tiny vesicles

by endocytosis and exocytosis

• large, lipid-insoluble molecules such as insulin or

maternal antibodies and proteins passing through

placental circulation to foetus


Capillary Exchange

o Bulk Flow: Filtration and Reabsorption

• Movement of large amount of dissolved or suspended

material in the same direction

• movement in response to pressure: from an area of

high pressure to an area of low pressure

• faster rate of movement than diffusion or osmosis

• regulates relative volumes of blood and interstitial

fluid


Capillary Exchange

• Filtration: movement of material into interstitial fluid

– promoted by blood hydrostatic pressure and

interstitial fluid osmotic pressure

• Reabsorption: movement from interstitial fluid into

capillaries

– promoted by blood colloid osmotic pressure

• Net filtration pressure: balance of the pressures that

promote filtration and reabsorption

– Whether fluids leave or enter capillaries depends

on net balance of pressures


Dynamics of Capillary Exchange

o Starling’s Law of the capillaries: The volume of fluid and solutes reabsorbed is almost as large as the volume filtered.

910


Oedema

o An abnormal increase in interstitial fluid if filtration

exceeds reabsorption

o Cause:

• Excess filtration:

– increased blood pressure (hypertension)

– increased permeability of capillaries allows plasma

proteins to escape


Oedema

• Inadequate reabsorption

– decreased concentration of plasma proteins lowers

blood colloid osmotic pressure

inadequate synthesis or loss from liver disease,

burns, malnutrition or kidney disease, blockage

of lymphatic vessels postoperatively or due to

filarial worm infection

o Often not noticeable until 30% above normal


Readings and Resources

o Tortora, GJ & Derrickson, B 2014. Principles of Anatomy and Physiology, 14th edn, Wiley.

o Harris, P, Nagy, S & Vardaxis, N 2010, Mosby’s Dictionary of Medicine, Nursing and Health Professions, 2nd edn, Mosby Elsevier.

o Guyton, AC & Hall, JE 2011, Textbook of Medical Physiology, 12th edn, Saunders Elsevier.

o Marieb, EN & Hoehn, K 2010, Human Anatomy and Physiology, 8th edn, Benjamin Cummings Pearson.

o Moore, KL, Dalley, AF & Agur, AMR 2010, Clinically Orientated Anatomy, 6th edn, Lippincott, Williams & Wilkins.


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