Blood Vessels & Circulation 

Blood vessels and Circulation

Pulmonary and systemic circulation

Arteries- Carry blood away from the heart

Capillaries- Site of exchange with tissues

Veins- Carry blood toward the heart

Tunics

• Tunica Intima- 4 Layers, most interior
• Tunica media- middle
• Tunica externa (adventitia)- most external

Arteries

Elastic or conducting arteries

• Largest diameters
• Pressure is high and fluctuates between systolic and diastolic
• more elastic tissue and less muscle tissue than other arteries
• thick tunica media and intima, thin tunica adventitia
• include most of the named arteries

Muscular Arteries

• Smooth muscle
• 25-40 layers of smooth muscle
• also called distributing arteries
• most of the smaller unnamed arteries

Arterioles

• transport blood from small arteries to capillaries
• Smallest arteries where the three tunics can be differentiated
• like small arteries, capable of vasodilation and vasoconstriction

Capillaries

Substances move through Capillaries through diffusion

Only site of exchange between the blood and interstitial spaces

• Lipid-soluble and small water soluble molecules pass through the plasma membrane
• Larger water-soluble molecules pass through fenestrae or gaps between endothelial cells

Endothelium

simple squamous epithelium resting on basement membrane

delicate layer of loose connective tissue

Pericapillary cells

scattered cells, closely associated with endothelial cells (fibroblasts, macrophages, and smooth muscle cells)

Types of Capillaries

Continuous

• no gaps
• no fenestrae
• less permeable to large molecules
• location: muscle and nervous tissue

Fenestrated

• Numerous fenestrae
• highly permeable
• location: intestinal villi, ciliary process of eyes, choroid plexus of CNS

Sinusoidal

• Large diameter with large fenestrae
• less prominent or absent basement membrane
• found in places where large molecules move into the blood
• location: endocrine glands, liver

Sinusoids

• Large diameter sinusoidal capillaries that allow the passage of large molecules and calls
• sparse basement membrane
• location: liver and basement membrane

Venus Sinuses

• similar in structure but even larger than sinusoids
• location: spleen

Capillary network

Arterioles> Metarterioles> Capillary network

Metarterioles- Vessels with isolated smooth muscle cells along their walls

thoroughfare channel: vessels within the capillary network that extend in a direct fashion from metarteriole to venule

Tissues with high metabolism have more capillary networks

capillary networks in the skin function in thermoregulation

Venules

Drain capillary network

Venules>small veins

Veins

Medium veins- collect blood from small veins and deliver it to large veins

Large veins- Tunica intima is thin; Tunica media has circularly arranged smooth muscle cells

Valves

found in all veins greater than 2 mm in diameter

Folds in intima form two flaps that overlap

**Function-**prevent backflow

more valves in veins of lower extremities than in veins of upper extremities

Other vessel systems

Portal veins

directly connect capillary networks

no pump between networks

3 portal vein systems in the human body

• Discussed in endocrine, digestive, and urinary systems

Vasa Vasorum

blood vessels that supply the walls of arteries and veins

Neural innervation of blood vessels

Sympathetic nerve fibers

parasympathetic nerve fibers

sensory nerve fibers

Aging of the Arteries

Arteriosclerosis

general term for degeneration changes un arteries making

Atherosclerosis

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Pulmonary Circulation

Dynamics of blood circulation

Factors that influence blood movement through vessels

Pressure

Measure of force exerted by blood against the wall

Blood moves through vessels because of blood pressure

Flow

The volume that passes a specific point per unit of time

Flow=(P1-P2/R)

Increase pressure gradient>increase flow

Flow of blood is opposed by resistance (increase resistance> decrease flow)

Laminar flow

Streamlined

no sound

Turbulent flow

interrupted

Resistance

Poiseuille’s Law

Resistance is effected by:

• Blood viscosity increased viscosity> increased resistance
• vessel length increased length> increased resistance (doesn’t really change in the human body)
• vessel diameter increased diameter> decreased resistance

  ## Viscosity

  Measure of resistance liquid to flow

  as viscosity increases, pressure required to force it to flow increases

  viscosity influenced largely by Hematocrit (the ratio of the volume of red blood cells to the total volume of blood)

• Laplace’s Law

  Force acting on blood vessel wall is proportional to diameter of the vessel times times blood pressure

  (F=DxP)

• Aneurysm

  Bulge that forms in a weakened part of a vessel wall

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Compliance

Vascular Compliance

Tendency for blood vessel volume to increase as blood pressure increases

easier to move and stretch to hold more blood

Physiology of the Systemic Circulation

Circulatory system maintains adequate blood flow to all tissues

determined by: Anatomy of circulatory system

Blood Volume, Mostly in Veins, and then capillaries and arteries

Cross-sectional area

area of one vessel multiplied by the number of vessels of that type

Aorta= Low cross sectional area, High velocity

Capillaries= high cross sectional area, low velocity

Capillary Exchange and regulation of interstitial fluid

Capillary exchange

The movement of substances in and out of capillaries

most important means of exchange: Diffusion

Movement of fluid from capillaries affected by

• Blood Pressure
• Capillary permeability

Osmosis (Water follows Solutes)

Net filtration pressure = Net hydrostatic pressure - net osmotic pressure

Arterial end

Venous end

hydrostatic pressure - pressure against the walls of the capillary

osmotic pressure - Blood colloid pressure; interstitial pressure

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