Download File - Dr. Jerry Cronin

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Structure and Function of Veins


Collect blood from capillaries in tissues and organs

Return blood to heart

Are larger in diameter than arteries

Have thinner walls than arteries

Have lower blood pressure

Structure and Function of Veins

Vein Categories


Very small veins

Collect blood from capillaries

sized veins

Thin tunica media and few smooth muscle cells

Tunica externa with longitudinal bundles of elastic fibers

Large veins

Have all three


Thick tunica externa

Thin tunica media

Structure and Function of Veins

Venous Valves

Folds of tunica intima

Prevent blood from flowing backward

Compression pushes blood toward heart

Structure and Function of Veins

Figure 19

6 The Function of Valves in the Venous System

Blood Vessels

The Distribution of Blood

Heart, arteries, and capillaries


35% of blood volume

Venous system



1/3 of venous blood is in the large venous networks of the liver, bone
marrow, and skin

Blood Vessels

Figure 19

7 The Distribution of Blood in the Cardiovascular


Blood Vessels

Capacitance of a Blood Vessel

The ability to stretch

Relationship between blood volume and blood

Veins (
capacitance vessels
) stretch more than

Blood Vessels

Venous Response to Blood Loss

Vasomotor centers stimulate sympathetic nerves

Systemic veins constrict (

Veins in liver, skin, and lungs redistribute

Pressure and Resistance

Figure 19

8 An Overview of Cardiovascular Physiology

Pressure and Resistance

Pressure (P)

The heart generates P to overcome resistance

Absolute pressure is less important than pressure gradient

The Pressure Gradient (


Circulatory pressure = pressure gradient

The difference between

Pressure at the heart

And pressure at peripheral capillary beds

Pressure and Resistance

Force (F)

Is proportional to the pressure difference (


Divided by R

Pressure and Resistance

Measuring Pressure

Blood pressure (BP)

Arterial pressure (mm Hg)

Capillary hydrostatic pressure (CHP)

Pressure within the capillary beds

Venous pressure

Pressure in the venous system

Pressure and Resistance

Circulatory Pressure

∆P across the systemic circuit (about 100 mm Hg)

Circulatory pressure must overcome total
peripheral resistance

R of entire cardiovascular system

Pressure and Resistance

Total Peripheral Resistance (R)

Vascular R

Due to friction between blood and vessel walls

Depends on
vessel length

vessel diameter:

adult vessel length is constant

vessel diameter varies by vasodilation and vasoconstriction:

R increases exponentially as vessel diameter decreases

Pressure and Resistance


R caused by molecules and suspended materials in
a liquid

Whole blood viscosity is about four times that of

Pressure and Resistance


Swirling action that disturbs smooth flow of liquid

Occurs in heart chambers and great vessels

Atherosclerotic plaques cause abnormal

Pressure and Resistance

Pressure and Resistance

Pressure and Resistance

Pressure and Resistance

An Overview of Cardiovascular Pressures

Systolic pressure

Peak arterial pressure during ventricular systole

Diastolic pressure

Minimum arterial pressure during diastole

Pulse pressure

Difference between systolic pressure and diastolic pressure

Mean arterial pressure (MAP)

MAP = diastolic pressure + 1/3 pulse pressure

Pressure and Resistance

Abnormal Blood Pressure

Normal = 120/80


Abnormally high blood pressure:

greater than 140/90


Abnormally low blood pressure

Pressure and Resistance

Elastic Rebound

Arterial walls

Stretch during systole

Rebound (recoil to original shape) during diastole

Keep blood moving during diastole

Pressure and Resistance

Figure 19

9 Relationships among Vessel Diameter, Cross
Sectional Area,
Blood Pressure, and Blood Velocity.

Pressure and Resistance

Pressures in Small Arteries and Arterioles

Pressure and distance

MAP and pulse pressure decrease with distance from

Blood pressure decreases with friction

Pulse pressure decreases due to elastic rebound

Pressure and Resistance

Figure 19

10 Pressures within the Systemic Circuit

Pressure and Resistance

Venous Pressure and Venous Return

Determines the amount of blood arriving at right
atrium each minute

Low effective pressure in venous system

Low venous resistance is assisted by

Muscular compression of peripheral veins:

compression of skeletal muscles

pushes blood toward heart
way valves)

The respiratory pump:

thoracic cavity action

inhaling decreases thoracic pressure

exhaling raises thoracic pressure

Pressure and Resistance

Capillary Pressures and Capillary Exchange

Vital to homeostasis

Moves materials across capillary walls by




Pressure and Resistance


Movement of ions or molecules

From high concentration

To lower concentration

Along the concentration gradient

Pressure and Resistance

Diffusion Routes

Water, ions, and small molecules such as glucose

Diffuse between adjacent endothelial cells

Or through fenestrated capillaries

Some ions (Na
, K
, Ca

Diffuse through channels in plasma membranes

Pressure and Resistance

Diffusion Routes

Large, water
soluble compounds

Pass through fenestrated capillaries

Lipids and lipid
soluble materials such as O

and CO

Diffuse through endothelial plasma membranes

Plasma proteins

Cross endothelial lining in sinusoids

Pressure and Resistance


Driven by hydrostatic pressure

Water and small solutes forced through capillary

Leaves larger solutes in bloodstream

Pressure and Resistance


The result of osmosis

Blood colloid osmotic pressure

Equals pressure required to prevent osmosis

Caused by suspended blood proteins that are too large to cross
capillary walls

Pressure and Resistance

Figure 19

11 Capillary Filtration

Pressure and Resistance

Interplay between Filtration and Reabsorption

Hydrostatic pressure

Forces water

of solution

Osmotic pressure

Forces water


Both control filtration and reabsorption through capillaries

Pressure and Resistance

Net Hydrostatic Pressure

Is the difference between

Capillary hydrostatic pressure (CHP)

And interstitial fluid hydrostatic pressure (IHP)

Pushes water and solutes

Out of capillaries

Into interstitial fluid

Pressure and Resistance

Net Colloid Osmotic Pressure

Is the difference between

Blood colloid osmotic pressure (BCOP)

And interstitial fluid colloid osmotic pressure (ICOP)

Pulls water and solutes

Into a capillary

From interstitial fluid

Pressure and Resistance

Net Filtration Pressure (NFP)

The difference between

Net hydrostatic pressure

And net osmotic pressure





Pressure and Resistance

Capillary Exchange

At arterial end of capillary

Fluid moves

of capillary


interstitial fluid

At venous end of capillary

Fluid moves



of interstitial fluid

Transition point between filtration and reabsorption

Is closer to venous end than arterial end

Capillaries filter more than they reabsorb

Excess fluid enters lymphatic vessels

Pressure and Resistance

Figure 19

12 Forces Acting across Capillary Walls

Pressure and Resistance

Fluid Recycling

Water continuously moves out of capillaries, and back into
bloodstream via the lymphoid system and serves to

Ensure constant plasma and interstitial fluid communication

Accelerate distribution of nutrients, hormones, and dissolved
gases through tissues

Transport insoluble lipids and tissue proteins that cannot cross
capillary walls

Flush bacterial toxins and chemicals to immune system tissues

Pressure and Resistance

Capillary Dynamics


Reduces CHP and NFP

Increases reabsorption of interstitial fluid (recall of fluids)


Increases BCOP

Accelerates reabsorption

Increase in CHP or BCOP

Fluid moves out of blood

Builds up in peripheral tissues (