Aims Regulation of vascular tone. Capillaries Readings; Sherwood, Chapter 10.

Aims

• Regulation of vascular tone.

• Capillaries

• Readings; Sherwood, Chapter 10

Sherwood’s Human Physiology 10-10

Vascular Tone• The state of arteriole resistance that is established

by the arteriolar smooth muscle.– Normally under a slightly constricted state

• Smooth muscle cells undergo self-induced contractile activity due to ___________________ potentials.


Vasoconstriction• Increased contraction of circular smooth muscle cells in

arteriolar wall.• Results in:

– Increased resistance.

– __________________________ flow.

• Due to:– Increased myogenic activity.

– Increased oxygen.

– Decreased carbon dioxide.

– Increased sympathetic stimulation.

– Vasopressin, Angiotensin II

– Cold


Vasodilation• Decreased contraction of circular smooth muscle cells in

arteriolar wall.• Results in:

– Decreased resistance.

– _________________ flow.

• Due to:– Decreased myogenic activity.

– Decreased oxygen.

– Increased carbon dioxide.

– Decreased sympathetic stimulation.

– Histamine release

– Heat

Regulation of Vascular Tone

• Intrinsic (local) Controls– Important in distribution of cardiac output

• Chemical influences• Physical influences

• Extrinsic Controls– Important in blood pressure regulation

• Neural influences• Hormonal Influences


• Intrinsic (local) Controls• Chemical Influences

• Oxygen

• Carbon Dioxide

• Blood pH

• Blood Osmolarity

• Adenosine

• Prostaglandins

• Histamine


• Intrinsic (local) Controls• Physical Influences

– Heat or Cold application– Myogenic response to stretch


• Extrinsic Controls• _____________________ Nervous System

– Mainly vasoconstrictor nerve fibers (norepinephrine).

• Most potent in kidneys, gut, spleen, and skin.• Less potent in skeletal muscle and brain.

– Very few vasodilator fibers (epinephrine).• Unlikely to play an important role in normal

circulation.• Fainting due to emotional distress.


• Extrinsic Controls• Hormonal System

– Norepinephrine and epinephrine from the adrenal medulla

• Support sympathetic nervous stimulation.

– Vasopressin and Angiotensin II• Potent vasoconstrictors

• Play important role in maintaining blood pressure during hemorrhage.


• Extrinsic Controls• Adrenergic Receptors

1 (vasoconstriction) 2 (vasodilation)

Sherwood’s Human Physiology Table 10-4

Response to Exercise and Fright

• Stimulate sympathetic system and inhibit parasympathetic system.– Almost all arterioles are constricted. (increase peripheral resistance)

• Increases arterial pressure.

– Veins are constricted.• Displaces blood from large peripheral blood vessels toward the heart.• Result in heart beating stronger. (increasing stroke volume)

– Autonomic nervous system stimulates heart rate.

Response to Exercise and Fright

• Exercise causes local O2 depletion.– Resulting in local

vasodilation

• Elevated BP and local vasodilation = great supply for skeletal muscle.


Summary of Regulation


Sherwood’s Human Physiology 10-28 (10-27 6th Edition)

Veins are Volume Reservoirs

• Veins serve as ________ resistance passageways to return blood to the heart.

• Additionally, they serve as a blood reservoir.– Thinner walls and less

smooth muscle than arteries.


Veins Have Valves

• Valves prevent backward blood movement.

• When valves no longer function properly you get varicose veins.– Allows for the pooling

of blood and swelling of the vein.

Guyton’s Textbook of Medical Physiology 15-10

Venous Pump• Gravity forces blood to pool in the lower

extremities.– Thus pressure in the lower veins is greater

than that of the higher veins.

• The venous pump or muscle pump pushes blood up from the lower extremities.– Utilizing muscle contraction and valves to

ratchet blood upward.

• Thus, when walking the blood pressure in leg is 25mm instead of 90mm

CapillariesCapillaries make up most of the total cross-sectional area of blood vessels (600 m2).

Every cell is within 0.01 cm of a capillary.

Contain only about 5% of total blood volume at any given time.

Very slow velocity of blood flow (distance/time).


Blood Flow through Capillaries

Same volume flows from A1-B1 as flows from A2-B2 in 1 minute (= flow rate), but the distance from A1-B1 > A2-B2 (velocity of flow).



Capillary lumen size

• Capillaries have lumens about 7 m but can be as small as 3.5m.

• Barely big enough for erythrocytes (about 8 m) to squeeze through.


Capillary Regulation of Blood Flow

• Capillaries branch off of metarterioles.

• At this location the capillary is circled by a smooth muscle cell forming the precapillary sphincter.

• Regulates whether the capillary is open or closed.

• O2 Dependent.


Capillaries are the site of fluid and nutrient exchange

• Capillaries prime function is _____________ exchange

• Intercellular clefts.

• Plasmalemmal vesicles.

• Vesicular channels

• Fenestrations


Diffusion Through Membranes• Lipid soluble substances pass right through the endothelial cell.

– O2 & CO2


Intercellular Clefts

• Intercellular cleft– Water filled pore.– 6-7 nm wide

• Small water-soluble substances pass through these pores.– Ions Na+ and K+

– Glucose– Amino acids


Plasmalemmal Vesicles

• Lipid insoluble proteins are moved across by vesicular transport.– Non-plasma

proteins.– Plasma proteins

are usually unable to leave the circulation.

Adopted from Ganong’s Review of Medical PhysiologySimilar to Guyton’s Textbook of Medical Physiology 16-2

Vesicular Channels

• Vesicles coalesce to form channels that go completely through the endothelial cell.

Adopted from Ganong’s Review of Medical PhysiologySimilar to Guyton’s Textbook of Medical Physiology 16-2

Specialized Capillaries

• Fenestrations– Numerous small oval

windows which pass through the endothelial cell.

– Found in the kidney.– Responsible for

filtration in the glomeruli.

Specialized Capillaries

• In the liver sinusoids the intercellular clefts are very large.– Resulting in almost all dissolved substances

including plasma proteins can pass from the blood to the liver tissue.

• In the _________________ the intercellular clefts do not exist due to tight junctions between endothelial cells.– Resulting in only small molecules such as

water, CO2 and O2 can pass from the blood to this tissue.

Sherwood’s Human Physiology 10-21 5th Edition only & Guyton’s Textbook of Medical Physiology 16-4

Interstitial Fluid

• Makes up 80% of the ECF while the plasma makes up the remaining 20%.

• Gel of _________________ and Proteoglycans.

• Collagen fiber bundles.

• Free fluid vesicles.

• Free fluid.


Exchange of Solutes

• Solutes will diffuse __________________ their concentration gradients during capillary-cell exchange.

• Also leave capillaries via bulk flow which is most important in maintaining plasma volume.

Guyton’s Textbook of Medical Physiology 16-5

Forces Influencing Bulk Flow• Capillary hydrostatic pressure

– Forces fluid out of the capillaries.

• Interstitial fluid hydrostatic pressure– Usually forces fluid into the capillaries.

• Plasma colloid osmotic pressure– Causes osmosis driving fluid into the capillary.

• Interstitial fluid colloid osmotic pressure– Causes osmosis driving fluid out of the capillaries.


Exchange of Solutes

Next Time

• Lymphatic system.

• Blood composition– Erythrocyte production

• Reading; Sherwood, Chapter 10

Objectives

1. Describe the function and properties of blood vessels and their regulation.

1. Distensibility and Compliance

2. Pulsatility

3. Vascular Tone (vasoconstriction & vasodilation)

2. Describe the structure and function of veins.

Aims Regulation of vascular tone. Capillaries Readings; Sherwood, Chapter 10.

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Transcript of Aims Regulation of vascular tone. Capillaries Readings; Sherwood, Chapter 10.