Biology 2402 A&P II       Lecture Notes 
Blood Vessels
Dr. Weis

Circulatory System : Blood Vessels

Function of circulation B> to serve the need of the tissues to maintain life & fxn of cells.

Blood Vessels: lie side by side, form a closed system from superficial to deep as follows :

Blood Vessels : three major types

Artery : transport blood UNDER PRESSURE to tissue, efferent vessels away from heart.

Capillary : tissue exchange, epithelial layer + Basement membrane

Vein : return blood back to heart, afferent vessels, under low pressure

Others : arterioles : smallest arterial branch

  Metarterioles : last arterial branch, also known as the pre-capillary sphincter

  Venules : smallest veins

  Vaso vasorum : blood vessels found in the outer layers of larger vessels, to supply their smooth muscle

  Sinus : thin walled vein

Blood vessel structure : wall of vessels formed by three tunics (layers)

                                      Internal opening (space) within the vessels is called the lumen

Outer wall :       Tunica Adventitia or Tunica Externa

Consists of connective tissue collagen fibers, some elastic fibers

Functions to protect vessel and anchor it to the surrounding structures; will also have nerves and lymphatic vessels.

Middle wall :     Tunica Media

Consists of smooth muscle in concentric layers.  Will also have some degree of Elastic fibers.  Regulated by the Sympathetic Nervous System to maintain muscle tone and influence blood flow and pressure by changing the lumen diameter.  This is called vasodilation or vasoconstriction.

Inner Wall:     Tunica Intima or Tunica Interna

Consists of endothelium (simple squamous epithelium with it=s basement membrane and loose C.T. with elastic fibers).   Forms a flat, slick surface lining that is continuous with the endothelial lining of the heart (endocardium).

     Problems : aneurysm = sac formed by dilation of the wall of an artery, vein, or heart.

                                         Named for the location involved, cause, or area of wall involved.

I. Arteries : Three (3) groups :: Elastic, Muscular, Arteriole

1. Elastic Artery            conducting arteries, large thick walled tunica media that contain more elastic fibers in the smooth muscle.  Examples: Aorta, Carotid arteries.
Can expand and recoil passively in response to volume in order to maintain continuous flow.   This creates a pressure wave called PULSE and reflects the Heart rate

PULSE monitoring :      head @ temporal, facial, carotid

Problems : hardening of the arteries = arteriosclerosis, causes intermittent flow of blood.

2. Muscular Artery       distributing arteries, most common type. 
Delivers blood to specific Body organs.  Tunica media is very thick and prominent and consists of smooth muscle with a small amount of elastic tissue. TM functions in vasoconstriction / vasodialation.

3. Arterioles                 smallest arterial vessels.  Some will have all three tunic layers, but the Tunica media is smaller.  Some will have two tunics with only one layer of tunica media.  FXN : to control the amount of blood delivered to the capillaries.  This is mainly the job of the metarterioles, the terminal arteriole, which forms the precapillary sphincter.

II. Capillaries :  Have only Tunica Intima (Interna) Simple squamous epithelium with a Basement membrane and small cells called pericytes to help stabilize the wall. Lumen opening is large enough for a RBC to pass.

FXN of capillaries : to exchange fluid, electrolytes, nutrients, hormones, waste by way of pressure gradients, membrane permeability, and vesicle transport within the cells. Vesicles are also found within the cytoplasm of the cells lining the capillaries to allow for transport of substances through the cell. 
There are three types of capillaries:

1. Continuous : Endothelium in uninterrupted, joined by tight junctions

2. Fenestrated : incomplete endothelial lining, have gaps/pores in cell

3. Sinusoidal : extremely leaky capillaries. Have pores in cell AND

Capillary beds form an interweaving network of vessels collectively called microcirculation
Arrangement of network can be classified as one of two types:

III. Venous System :     efferent vessels, directs blood back to the heart.

1. Venules        formed when capillaries unite. Starting with the smaller venules that have only two layers (tunica interna and tunica media), the larger venules will have all three layers, but in smaller amounts.

2. Veins            large thin walled blood reservoirs, formed when venules unite.  Tunica media is thin and has a little smooth muscle and elastic fibers, therefore this can cause the veins to collapse.  Tunica adventitia is the thickest and has elastic and collagen fibers, may also have a small amount of smooth muscle.

Veins act as reservoirs since they contain almost 2/3 blood volume at any given time.

The venous system is under low pressure and partially filled with blood due to their larger lumen.  Some veins have valves to prevent back flow of blood.  These valves are formed from folds of the tunica intima (interna). Examples are seen in the lower limbs, since blood has to be forced up to the heart against gravitational pull.

Venous sinus : Found as the dural sinuses and the coronary sinus. Very thin walled vein, only endothelium with a little C.T., under the lowest pressure.

Vein Problems :: 

84% of the entire blood volume (~5 liters) is in the systemic circuit = 4 L in Systemic, 1 L in pulm.

64% in veins + 13% in arteries + 7% in arterioles and capillaries (the systemic circuit)

7% in heart and 9% in pulmonary vessels (the pulmonary circuit)

Total blood volume = 84% + 7% + 9% = 100%

Vascular Anastomoses :   where vascular channels unite that normally do not to provide alternate route.

1. Arterial anastomoses             provided alternate arterial blood pathway for supply to tissues

2. Arteriovenous anastomoses bypass the bed of true capillaries

Circulatory pathways :: Pulmonary Circuit and Systemic Circuit

Pulmonary Circuit : rt ventricle B> pulmonary trunk B> rt, lf pulmonary arteries B> lobar

arteries (three for the right and two for the left) B> pulmonary arterioles B> pulmonary capillaries

(surround the alveoli) B> pulmonary venules B> pulmonary veins (two right, two left) B> left atrium.

This circuit is short, goes to one organ, and is under low pressure (15 mm Hg)

Physiology of Circulation ::

Flow through a vessel is determined by the pressure difference between two ends of the vessel and any impediment to blood flow.

Definitions

1). Blood viscosity        thickness of blood created by the formed elements of plasma proteins.  The greater percentage of cells or proteins.  The more cells the more the friction and friction determines the viscosity.  The greater the viscosity, the less easily the molecules will slide past one another.  With lowerviscosity, the blood will flow at the lower pressure due to less friction.

2) Length          the longer the vessel, the greater the resistance

3) Diameter      slight changes in the diameter of a vessel can cause tremendous changes in the ability to conduct blood.  Conduction of the blood flow for a given pressure difference will increase in proportion to the fourth power of the diameter, that is    Conduction  is proportional to  Diameter  

Also, due to laminar blood flow, the blood near the wall is slower, and the blood near the center of the vessel moves faster, therefore, in the smaller lumen vessels affect all of the fluid, so all of the blood is considered to be near the wall, and exposed to frictional forces. Irregularities in the diameter of the vessel can upset the smooth flow of blood. Instead of a streamline flow, it can cause a turbulence (considered a fourth factor of resistance) which is the swirling of blood and the sound of this turbulent fluid can be detected and is called Bruitt.

Relating all these factors created a formula known as Poiseulle=s Law ::

Q =     Δ  P r /8n l

Q is blood blow

P is pressure difference

r is the radius ( r   B> small changes can affect flow)

n is viscosity

l is vessel length

Another formula that you are responsible for is another relationship and a modified version

of Ohm=s Law : I = V/R (current = voltage divided over resistance)

Q =  Δ P / W              Some books write this formula as     F =   Δ P / R                                                                                       

that is to say, blood flow is proportional to the pressure difference delta P, (P1 - P2) and divided by (therefore indirectly proportional to) the resistance.

Remember that all blood vessels are distensible :

If the pressure in arterioles is increased, this dilates the diameter and decreases the resistance, therefore, blood flow is increased because of an increased pressure and a decreased resistance.

This allows for accommodation of pulsated output of the heart, to average out these pulsations to provide completely smooth continuous flow of blood through the tissues.

Veins are more distensible than arteries and will provide a reservoir function of the storage of large quantities of blood.

Systemic gradients are pressure gradients created by the left ventricle to allow blood flow from high pressure to low pressure.

Systemic Blood Pressure :: determined by distensibility and volume as fluid flows from high to low pressure.

Therefore the blood pressure is given with the systolic (S) over the diastolic (D) = 120 / 80

Further manipulations with these values for systole and diastole can give us :

1. Mean Average Arterial Pressure

the pressure that propels the blood to the tissues. 
It is calculated by the following formula :   MAP = D + 1/3 (S - D)

We can=t take the average of Systolic to Diastolic since diastole takes longer, so more blood is stored, creating pressure.

2. Pulse pressure is the difference between the two pressures (S- D), in the above example of 120/80, the pulse pressure is 120 - 80 = 40. 

Pulse pressure is affected by stroke volume and compliance (how well a vessel can be distended).

Blood Pressure Factors ::

Blood pressure is also related to cardiac output (blood flow for the whole body), blood volume and peripheral resistance.

and is controlled by ANS, hormonal influence, and venous return

Blood volume is usually 5 liters, but is affected by venous return.  Since the veins are under low pressure, adequate return cannot be maintained even with valves to prevent back flow.   Other Apumps@ are needed to help with venous return.

1. Skeletal muscle pump : skeletal activity helps force blood to return to the heart.

2. Respiratory pump : changes in pressure during inhalation allows venous expansion and return of blood to the right atrium

Peripheral resistance is primarily due to the changes in the arterial diameter, since the resistance (R) varies inversely to the vessel radius (r).  Remember that the radius is half the diameter.   This is shown in the formula by the formula :: R = 1 / r  to the 4th power

So, the relationship of blood pressure to the above factors gives us another formula ::

Blood pressure = Cardiac output X peripheral resistance (BP = CO x PR )
             In order to maintain adequate tissue profusion

Regulation of Blood Pressure : neural, chemical, renal.  
Regulation is based on to modifying CO, PR, & Blood volume.

I. Nervous :: Short term regulation

A. Vasomoter fibers :: sympathetic N. system efferents to smooth muscle in arterioles

Skeletal muscles can be affected two ways :

B. Vasomotor centers :: areas involved in the ANS and brainstem

The sympathetic neurons in the medulla keeps the arterioles in a constant state of partial constriction, called Vasomotor Tone.

The nervous system can be modified by negative feedback from:

a. Baroreceptors :: mechanoreceptors that respond to changing pressures.  They function for short term changes such as posture.  They are of no importance for long term regulation as their threshold can be reset.  Baroreceptors are located in the larger arteries B> aortic, carotid sinus Will affect vasomotor center for arteriole tone and the cardioinhibitory center (vagus nerve) and the cardioacceleratory center (sympathetic nerves). 
Therefore these adjust cardiac output (ANS input) and peripheral resistance (vasomotor tone).

b. Chemoreceptors :: will also stimulate the vasomotor centers in response to O2, CO2, pH.  These receptors are found in the aortic and carotid bodies.  Also are regulatory for respiration rate and depth.

c. Higher Brain Centers :: pons, diencephalon, cortex, hypothalamus has sympathetic ties, affects the vasomotor center, especially in response to exercise & increase in temperature.

II. Chemical Controls (Hormonal), act on smooth muscle or the vasomotor center

2. Atrial Natriuretic Peptide (ANP) :: peptide hormone from the right atrium of the heart.

3. ADH B> some vasoconstriction when pressure severely decreased, affected (inhibited) by alcohol

4. Endothelium B> peptide hormone from the endothelium to cause vasoconstriction, via Ca++ entry

5. Nitric Oxide [NO]B> vasodilation on a local and systemic level

6. Inflammation B> vasodilation due to mediators

III. Renal

A. Direct :: remove or conserve H2O in relation to blood pressure, decreased blood pressure will conserve H2O, increased blood pressure will allow H2O excretion which is in response to ADH

B. Indirect :: renin-Angiotensin system that affects the release of aldosterone

decreased blood pressure B> renin from the kidneys B> renin B> liver B> Angiotensinogen to angiotensin I B> angiotensin I B> lungs B> angiotensin II B>

C. Erythropoietin :: from the kidneys in response to low oxygen and pressure, stimulates the bone marrow RBC production to increase blood cellular volume & O2 capabilities.

Blood Pressure Changes

To identify changes, need to measure pulse and pressure

Cuff is used to occlude artery, usually use the brachial artery since it is near the lower level of the heart and listen for tuburlence sounds.

Cuff is pump up until artery is totally occluded, no sounds are heard from auscultation with a stethoscope.  Pressure is released, first sounds of blood through the artery is the systolic pressure.  More pressure is let of the cuff until no more sounds are heard, at this point the diastolic pressure is determined.

Problems with Blood pressure

Chronic :: changes in the heart B> work harder, enlarges tears in the endothelium, accelerated hardening and decreases blood flow to the tissues. Sustained increased arterial blood pressure of 140/90 or higher.  An increase in Diastolic pressure may indicate an occlusion or hardening.

Factors :: genetic, diet, stress, obesity, cholesterol levels (LDL)

Treatment :: weight loss with diet & exercise

        Drugs : diuretics, Ca++ channel blockers, ACE inhibitors.

Blood Flow

The need for blood flow changes, the greater the degree of metabolism, the greater the blood flow.  At rest, the blood flow to the various tissues is as follows :

Brain 13%, heart 4%, kidneys 20%, abdomen 20%, skeletal muscle 20%.

Blood flow allows oxygen / nutrients to be delivered,  removes wastes, Kidney filtration

Velocity is INVERSELY related to the cross sectional area of the vessel, so it is fastest when the total cross sectional area is the least.

Local Blood flow can occur in two phases ::

Regulation of Blood flow :: modify the vessel diameter and is stimulated by O2, ions, nutrients.

Two theories of how blood flow is regulated

1) Metabolic ::  in response to low O2, tissue secretes vasodilator substances

2) Myogenic :: O2 demand theory, O2 is necessary to maintain vascular smooth muscle

Contraction (vasomotor tone).  As O2 decreases, the vessels dilate locally possibly due to the decreased nutrients involved with generating ATP.

Regulation of Circulation :: Humoral

a. Vasoconstrictors ::

1. Norepinephrine is very powerful, epinephrine is less powerful, some dilation

Both released due to stimulation of the adrenal medulla

2. Angiotensin : constricts small arterioles, powerful

    Caused by release of renin from the kidneys

3. Vasopressin (ADH) from the Hypothalamus, stored in the Posterior Pituitary

H2O reabsorbed in the kidney tubules

Very powerful vasoconstrictor

B. Vasodilators

1. Kinin : polypeptides

Bradykinin B> arteriole dilator, increased permeability

2. Serotonin B> from abdominal tissues & platetlets can vasodilate or constrict

3. Histamine B> from mast cells & tissue damage arteriole dilator, increased Capillary permeability

4. Prostaglandins B> from every tissue, local vascular dilation

CAPILLARY DYNAMICS *************************************************************

Blood flow through the capillaries is intermittent due to the contraction of metarterioles and capillary sphincters.  This phenomenum is called VASOMOTION.  Regulated by O2 concentrations for local tissue conditions.  With so many capillaries present in the tissue, the blood flow is averaged.

The exchange of gases and nutrients is primarily by DIFFUSION between the blood plasma and the interstitial fluid and occurs along a concentration gradient from high to low.  Remember, we can also have active transport occurring by the endothelial cells in specialized areas to move substances across in vesicles.

Diffusion : of lipid soluble molecules can diffuse directly across the cell membrane, thereby increasing the rate of transport.  This occurs for O2 and CO2 of H2O soluble occurs through pores due to the velocity of motion of the molecules, but it depends on the pore size and therefore determines the permeability of that sized molecule.

This occurs with H2O, Na+, Cl-, glucose.

FYI :: spaces between cells are called the interstitium and fluid in these spaces is the interstitial fluid. The insterstium structure is collagen fiber bundles for strength, proteoglycan filaments to create a thin mat to trap fluid.  Fluid is derived from filtration of the capillaries, and is almost like plasma, but has less protein.  The fluid trapped in the mat of the proteoglycan filaments will help from a tissue gel so that substances must diffuse through.

EDEMA develops when free fluid expands the interstitium.

Four primary forces will determine fluid movement through the capillary membrane ::

The force exerted by a fluid against a wall in the capillaries creates the capillary pressure which then forces fluid through the capillary walls.  This pressure is higher on the arterial end than the venous end of the capillaries.

In the interstitial fluid, there is pressure forcing the fluid back into the vessel.  This force can vary in pressure and when it is negative (which is most of the time), it acts like a suction to enhance the movement of fluid out of the capillaries.

These two pressure forces are called Hydraulic (old name Hydrostatic) pressure and happens in two areas which are located inside the blood vessel (capillary) and outside in the tissue spaces (interstitial):

The next two forces deal with osmotic pressure.  The pressure of large NONDIFFUSIBLE molecules (i.e. plasma proteins) causes a higher solute concentration and therefore a higher osmotic pressure.  The pressure created by the movement of these molecules is called Colloid Osmotic Pressure [COP] or oncotic pressure and it will encourage osmosis, therefore water moves toward and into the vessel.  SO, it becomes the pressure that moves fluid back into the capillaries [Normally net osmotic pressure is around 25 mmHg since COP = 26 and IOP = -1], unless there is more positive oncotic pressure created in the interstitium, due to the loss of large proteins into the tissue spaces preventing the fluid from re-entering the veinous end of the capillary bed and allowing more fluid to move out into the interstitium.

Lets see how this works :

So hydraulic pressure will dominate on the arterial end, osmotic forces dominate on the venous end of the capillary.  Not all fluid is returned back to the capillaries.  This extra fluid and proteins in the tissue spaces are picked up by the lymphatic vessels and returned to the venous system.

Problems with circulation ::

EDEMA :: fluid collected in the interstitium

SHOCK :: low blood pressure, inadequate peripheral blood flow

Treatment :: replace fluids/pressure, sympathetic drugs, O2, glucocorticoids, place head at lower level.

Aging Problems ::

Blood : decreased RBC #, thrombus formation, pooling, Edema

Heart : decreased fibrous skeleton=less elastic, decreased conduction, arrhythmia,

Arteriosclerosis, damage to heart is repaired as scar tissue

Vessels : arteriosclerosis, stiff, inflexible, Ca++ deposits / plaque, aneurysms, thrombus

Blood Vessel Routes:

Make sure you know the major blood supply (arterial) and return (venous) for the body.

See text and class/lab notes and web links for vessels.

            Arterial Circulation overview

            Venous Circulation overview

NOTE: You must know the following circulatory pathway in detail:

In the brain, there is an arterial anastomoses called the Circle of Willis, found encircling the pituitary and the optic chiasm at the base of the brain. 
These vessels will unite the anterior and posterior cerebral supply with communicating branches anteriorly and posteriorly. 

Vessels Include:

Anterior Cerebral artery (via internal carotids)

 Middle cerebral artery (via internal carotids)

 Posterior cerebral artery (via the basilar a. from the joining of vertebral arteries)

The cerebral arteris are then connected by segmental arteries, the anterior and posterior communicating arteries to form the arterial anastomosis that provides alternate blood supply to the brain. This might happen if the carotid or vertebral a. is blocked.

Most stroke victims are affected after the anastomosis and usually at distal branches of the middle cerebral artery supplying specific regions of the brain.