Anatomy Ch 21

• vessels to the vessels
• tiny blood vessels in the tunica externa that supply blood to the tissues of the vessel

• 1. Arteries - large, then med, then small - elastic & muscular
• 2. Arterioles - BP control - vasodilation/ constriction
• 3. Capillaries - exchange of substances
• 4. Venules
• 5. Veins - blood reservoir via venodilation / constriction

• 1. Tunica Interna - innermost layer - direct contact with blood as it flows thorugh lumen - 3 layers
• - endothelium (simple squamous epithelium) that is continuous with the endocardium
• - basement membrane
• - internal elastic lamina (thin plate) - elastic fibers

• 2. Tunica media - middle - thickest- 3 layers
• - smooth muscle - circular arrangement
• - Elastic fibers
• - external elastic lamina

• 3. Tunica externa - external layer -
• - 2 layers of CT that contain elastic fibers and collagen fibers
• - provides structural support

The endothelium is part of the tunica interna - it is made of simple squamous epithelial tissue - it is continuous with the endocardium.

The tunica external provides structural support and helps anchor the vessels to surrounding tissues. - the vaso vasorium is in this layer.

The smooth muscle fibers are arranged in concentric layers.

The lumen is the interior opening of the vessel around which the tunicas are arranged.

Compliance is accomplished by the elastic fibers in the tunicas that allows vessels to expant to prevent rising pressure (that can cause rupture)

• Elastic artery - largest arteries - aorta & pulmonary trunk
• thinner walls with lots of elastic fibers
• conducting arteries - conduct blood to medium size muscular arteries
• *able to pump blood forward - expand as blood is pumped into them and recoil as the heart relaxes

• Muscular artery - medium size arteries - more smooth muscle and less elastic fibers in tuncia media
• - vadodialtion & vasoconstriction - controls rate of blood flow
• - distributing arteries - muscular arteries branch and distribute blood to all organs

The large elastic artery has a thinner turnica media that has lots of elastic fibers. The medium muscular artery has a thick layer of smooth muscle

• Arterioles regulate BP by changing diameter (regualtes resistance)
• Vasoconstriciton increases SVR (systemic vascular resistance) and thus increases blood pressure

Venoconstriction would also increase SVR and increase blood pressure

Microcirculation is the flow of blood from a metarteriole (the end of the arteriole that tapers into the capillaries) through the capillary bed and into a postcapillary venule.

A capillary bed is a network of 10- 100 capillaries that arise from a single metarteriole.

A capillary only has the tunica interna - a single layer of endothelial cells and a basement membrane

• capillaries are exchange vessels - they excange substances between the blood and interstitial fluid
• substances cross 1 layer of cells to enter intersticial space.

Precapillary sphincter - rings of smooth muscle fibers that control the flow of blood through the capillary - located at the metarteriole-capillary junction.

Metarteriole - terminal end of an arteriole that tapers into the thoroughfare channel - capillaries branch of the metarteriole and thoroughfare channel.

Throughfare channel - a direct route thorough the capillary bed - runs from the arteriole to the venule and bypasses the capillaries - precapillary sphincter but no muscle on the venule end

• At any given time blood flow through about 25% of the capillaries.
• This amount increases when a tissue is active such as when a muscle is contracting.

Precapillary sphincters - vasocontraction/ vasodilation -control the blood flow in the capillary - they alternate contraction and relaxation which is called vasomation. The metarterioles participate in the contraction and relaxation of vasomotion.

• 1. Continuous capillaries - most common - continuous uninterrupted lining with intercellular clefts between endothelial cells
• - permeable to water, small solutes, gases

• 2. Fenestrated capillaries - windows - oval pores in the endothelial cells
• - greater permeability of fluids and small solutes
• - located where absorbtion and filtration occur: kidney, choroid plexue (CSF)

• 3. Sinusoids - large opening - absent or incomplete BM (basement membrane)
• - allows large substances to pass
• - red bone marrow, liver

Portal system - when blood flows from one capillary network through a vein (called portal vein) and into another capillary network.

• hypophyseal portal system - pituitry gland
• hepatic portal system - liven

• Venules are very small as they leave the capillary bed and very pourous - oart of microcirculatory exchange - WBCs can exit
• -they have all three layers but the tunica media is very thin

as they continue they get larger and acquire more smooth mucle fibers - these are called muscular venules and exchanged with interstitial fluid doen't occur here.

• Veins have all three layers but all are thinner than in arteries
• - Tunica external is the thickest

Veins can only withstand low pressure because they have less smooth muscle to give support and they have no internal or external elastic lamina.

Veins have a larger lumen than arteries

Venous sinus - a vein with no smooth muscle - cannot alter is diameter

• Venous valve - cusp or flap of endothelium that points toward the heart
• - they are found mostly in the limbs and function to prevent back flow of blood (that might be caused by gravity) - they also help prevent low blood pressure and low venous return to the heart.

Vericose veins - caused by leaky venous valves - allows blood to backflow and pool which increses pressure and causes fluid to leak into the interstitial space - causes edema (extra fluid) in the interstitial space

Anastomosis - connecitons of blood vessels that are flowing to the same area - allow for collateral circualtion

Collateral circulation - alternate routes for blood to flow if one way becomes temporarily blocked

end artery - no anastomones - no collateral circuation - obstruction interrrupts the blood supply (causes death to that segment)

The blood resevoir is composed of systemic veins and venules - the larges percentage of blood is in these structure but blood can be moved quickly if necessary.

If blood is needed somewhere quickly (sympathetic input), the veins can venoconsrict which reduces blood in the veins and allows it to flow elsewhere.

• Blood distribution:
• 64% - veins and venules
• 13% - arteries
• 7% - capillaries
• 9% - pulmonary blood vessels
• 7% - heart

• 1. Diffusion - most commonly used - passive transport becuase substances move down the gradient
• - substances move through intercellular clefts or directly through endothelial cell (if lipid soluble)
• - large substances use fenestrated or sinusoid capillaries
• - Diffusion is NOT used in most of the brain - the BBB contins tight cell junctions that dont permit movement.

• 2. Transcytosis - active transport - only used by small quantity of material
• - substances are enclosed in vesicles and enter endothelial cells through endocytosis - travel across the cell and exit via exocytosis
• - used by large polar (hydrophillic) substances such as insulin and some antibodies

• 3. Bulk Flow - filtration and reabsorption- involves large amounts of substances that move down a pressure gradient (faster than diffusion)
• - balance the equlibrium between the blood and interstitial fluid (pressure determines wheather fluid enters or leaves the bloodstream)

• A. Filtration - driven by blood hydrostatic pressure
• - it is the flow of fluids out of the capillaries and into the interstitial fluid

B. Reabsorption - driven by blood colloid osmotic pressure - it is the flow of liquids out of interstitial fluid and back into capillaries

• Hydrostatic pressure (BHP) - generated by LV pumping
• - interstitial fluid hydrostatic pressure = 0 mm Hg
• - changes as blood flows from arterial end to the venous end of the capillary - 35 mmHg (arterial) to 16 mmHg (venous)

• Colloid osmotic pressure - controlled by substances that cant cross the capillary membrane (large proteins)-osmotic pressure does not change
• BCOP = 26 mmHg
• Interstital fluid osmotic pressure IFOP = 1 mmHg

• Filtration - reabsorption = net filtration pressure
• (BHP+IFOP) - (BCOP+IFHP) = Net filtration

Arterial end: 36 - 26 = 10 mm (filtration happens)

Venous end: 17 - 26 = -9 mm (reabsorption happens)

Edema is an increase in interstitial fluid (filtration > reabsoprtion)

Daily 20 liters of fluid are filtered and 17 are reabsorbed - the extra 3 liters ends up in the lymphatic capillaries and makes it way back to the blood stream

CO = amount of blood flow thought tissues in one min (when CO increases perfusion (blood flow) increases)

Blood flow to tissues depends on:

1. Pressure difference - blood flows from hight to low pressure - the greater the difference, the greater the flow

2. Resistance - SVR - greater resistance = less flow

Increased SVR slows down flow and increases blood pressure

• BP = CO x SVR (assuming a normal blood volume)
• CO (cardiac output) x systemic vascular resistance

CO = HR x SV

Blood pressure is about 35 mmHg in the arteries, arterioles and into capillaries

at the venous end of capillaries it is 16 mmHg

0 mmHg as blood flows into RV

since blood moves from high pressure to low pressure this pressure gradient help blood return to the heart.

Systolic blood pressure is the highest pressure attained in arteries during systole

Diastolic blood pressure is the lowest pressure in arteries during diastole

• Mean BP is the average blood pressure in arteries - it is about 1/3 of the way between the diastolic and systolic pressures
• MAP = diastolic BP + 1/3 (systolic BP- diastolic BP)

The pulse pressure is the difference between the systolic and diastolic pressures. (normally about 40 mmGh)

SVR = systemic vascular resistance - the opposition to blood flow (must be overcome for blood to flow)

• 1. lumen size - smaller the lumen - greater the resistance
• 2. Blood viscosity - higher viscosity - greater resistance
• 3. Blood vessel length - longer blood vessel - greater resistance

Polycythemia is when the % or RBC in blood is high - blood will be thicker (more viscous) and the SVR will increase.

Decreased vessel diameter will increase SVR and so will increased vessel length.

Aterioles, capillaries, and venules contribute to SVR (small vessels) but one of the functions of the arteriels is to control SVR by vasoconstriction/ vasodilation.

Venous return = volume of blood flowing back to the heart - caused by pressure difference between venuels (18 mmHg) and RA (0 mmHg)

• 1. Heart pump - contration of LV
• 2. Skeletal muscel pump (leg) - contration of leg muscles pushes blood through the proximal valve (while distal valve is closed to prevent back flow)
• 3. Respiratory pump - the diaphram moves down during inhilation which causes more pressure in the abdominal cavity and less pressure in the thoracic cavity - abdominal veins are constricted and blood moves to the lower pressure in the thoracic cavity

• Velocity of blood flow decreases as it is deverted into more branches
• velocity is greatest in the arteries and decreases as it moves to arterioles and then capillaries - slowest in capillaries which allows for exchange of materials - increases agian as branches unite - moving to venules and them to veins

• 1. Cardiocascular center - located in the medulla oblongata
• - Uses sympathetic and parasympathetic to control heart
• -- Increase in sympathetic stimulation
• **cardiac accelerator nerves - increase HR/ SV - beta adrenergic (EPI, NOR) receptors in heart
• **Vasomotor nerves - constriction/ dilation of atrioles/ venules - alpha adrenergic receptors (NOR, EPI) on blood vessles
• -- Increase in parasympathetic stimulation-
• **vagus nerve - decreases HR - muscarinic cholinergic (ACh) receptors on blood vessels
• -Receives input from:
• a. Higher brain centers - cerebral cortex, limbic system, hypothalamous
• 2. Neural Control - hypoththalamus (brain of ANS)
• b. Proprioceptors - joints - info on movement changes
• c. Baroreceptors - monitor pressure in blood vessels (sence degree of stretch) - located in the carotid artery and the aortic arch (↑ BP = more stretch - causes ↓CO - causes ↓ vessel diameter - ↓ BP).
• d. Chemoreceptors - monitor chamicals in blood vessels (CO2, O2, pH) - located in carotid bodies and aortic bodies
• -reflex responds to hypercpnia (↑ CO2), hypoxia (↓ O2) , and acidosis (↓ pH) which leads to ↑ in cardiac output, vasoconstriciton, and increased BP

• 3. Hormonal Control
• - a. RAA system - renin-angiotensin-aldosterone -
• renin - releasead by kidneys when BP is low
• angiotensin cenvering enzyme ACE - (lung when renin)
• angiotensin II - vasoconstrictor - stimulates release of aldosterone - increases blood volume (+BP) by resporbing water and sodium in kidneys
• -b. Adrenal gland - EPI, NOR secreted from adrenal medulla as response to sympathetic stimulation - increases HR, SV
• ** EPI, NOR stimulates vasoconstriciton in skin and abdominal organs
• **EPI, NOR stimulates vasodilation in cardiac and skeletal muscle
• c. ADH - pituitary gland (made in hypothal) in response to low blood volume
• **vasopressin = vasoconstriction
• ** causes H20 resorption in kidney
• d. Atrial Natriuretic Peptide ANP - made by atrail myocardial cells - increase in BP stretches RA and causes it to realese ANP
• **ANP stimulates vasodiation and sodium and water elimination in kidneys

• 4. Autoregulation - ability of tissue to adjust blood flow to meet metabolic demands
• - responds to local physical (warming=vasodilaiton, cooling=vasoconstriction)

Vasomotor tone - product of CV center in medulla oblongata - it is a constant state of partial vasoconstriction that sets the resting level of the SVR

• . Hormonal Control-
• a. RAA system - renin-angiotensin-aldosterone -
• 1. renin - releasead by kidneys when BP or volume is low
• 2. angiotensin cenvering enzyme ACE - (released from lung when renin is present)
• 3. angiotensin II - vasoconstrictor - stimulates release of aldosterone -
• 4. aldosterone - increases blood volume (+BP) by resporbing water and sodium in kidneys

• Adrenal gland - EPI, NOR secreted from adrenal medulla as response to sympathetic stimulation - increases HR, SV
• ** EPI, NOR stimulates vasoconstriciton in skin and abdominal organs
• **EPI, NOR stimulates vasodilation in cardiac and skeletal musclec.

ADH - pituitary gland (made in hypothal) in response to low blood volume**vasopressin = vasoconstriction** causes H20 resorption in kidneys.

• Atrial Natriuretic Peptide ANP - made by atrail myocardial cells - increase in BP stretches RA and causes it to realese ANP
• **ANP stimulates vasodiation and sodium and water elimination in kidneys

• Autoregulation - ability of local tissue to adjust blood flow to meet metabolic demands
• responds to local:
• 1. physical changes
• (warming=vasodilaiton, cooling=vasoconstriction)
• *myogenic response - smooth muscle in arteriole walls contracts more when stretched and relaxes when less stretch (↑ stretch = vasoconstriciton = less flow)

• 2. Local chemical changes -
• vasodilatin chemicals - lactic acid, H+, K+, adensine, NO
• vasoconstricting chemicals - thromboxane, serotonin
• hypoxia - in systemic circulation causes dilation to catch more O2
• in pulmonary circulation - vessels constrict in low O2 area so blood moves to better oxygenated areas of the lung

• Shock is the result of inadequate perfusion (not enough oxygen & nutrients, build-up of waste)
• 1. Hemodynamic = loss of blood
• 2. Cardiogenic = pump failure
• 3. Obstructive = no venous return to heart
• 4. Vasogenic = low SVR

• The body responds to increase perfusion pressure
• 1. ANS – sympathetic response –increases HR, SV, SVR to increase BP
• 2. Hormonal – RAA, ADH – to increase volume & SVR
• 3. Local – vasodilation of capillaries to increase perfusion

The local response is to vasodilate the smallest vessels to increase tissue perfusion

• 1. Low BP
• 2. High HR but weak pulse
• 3. Cold skin – less perfusion (skin is not a priority)
• 4. Low pH – increased lactic acid from anaerobic production of ATP
• 5. Delirium/ confusion – low perfusion to brain
• 6. Low urine output – will cause renal failure if shock is prolonged