Microcirculation and Lymphatics

Question 1

microcirculation

Answer 1

•”business end” of the CV system •exchange of solutes and fluid between blood and tissue •slowest part of the circulation

Question 2

terminal arterioles

Answer 2

•immediately upstream of the capillaries •discontinuous smooth muscle •capillary recruitment

Question 3

larger arterioles

Answer 3

•completely enveloped with smooth muscle •”resistance” vessels •regulation of the distribution of cardiac output and MAP

Question 4

capillaries

Answer 4

•small tubes consisting of a single layer of endothelial cells surrounded by a simple collagen support matrix (the basement membrane) •well designed to enhance diffusive exchange -short distance (small capillary diameter) -thin microvascular wall -large number of capillaries close to the cells -large capillary surface area for exchange, relative to blood volume in capillaries -blood slows down as it traverses capillaries

Question 5

continous capillaries

Answer 5

• skeletal muscle, cardiac muscle, skin, lungs, adipose, CT and nervous system (BBB)
• endothelial cells overlap creating clefts with tight junctions that restrict solute exchange
• transport of lipid insoluble solutes (glucose) from blood to tissue ocurs through these clefts
• gases diffuse across the cell membrane

Question 6

fenestrated capillaries

Answer 6

• glomerulus, exocrine glands, intestinal mucosa, ciliary body, choroid plexus, synovial lining of joints and endocrine glands
• endothelial lining perforated by small circular windows containing thin diaphragms
• solute and fluid exchange is roughly 10x greater than across continous capillaries
• larger molecules such as salts can move in and out

Question 7

discontinous capillaries

Answer 7

• sometimes called sinusoids, liver, bone marrow and spleen
• discontinuities in the basement membrane as well
• large proteins and blood cells can move freely from blood to tissue (and vice versa) in these organs

Question 8

Fick’s Law of Diffusion

Answer 8

•governs the process of transcapillary solute exchange - solute molecules tend to move across the capillary wall from a region of higher concentration to a region of lower concentration

Js = PsS (Cc-Cl)

Js = solute flux

Ps= permeability coefficient

S = capillary surface area available for exchange

Cc = solute concentration within the capillary

Cl = solute concentration in the interstitial space

Question 9

The Starling Equation

Answer 9

• governs the process of transcapillary fluid exchange
• each day about 3 L of fluid are flitered from blood to tissue
• there are hydrostatic pressure forces tending to push fluid out of the capillary into teh tissue; there is also an oncotic pressure, exterted by the plasma proteins, that tends to suck fluid into the capillaries

JF = LP S [(Pc – Pt) - σ (Πc - Πi)]

Pc – Pi = Hydrostatic pressure difference

Πc - Πi = Oncotic pressure difference

σ = Protein reflection coefficient

Question 10

edema

Answer 10

• a term describing the accumulation of fluid in the interstitial space, occurs when the net fluid filtration from blood to tissue exceeds the lymphatic drainage

• arteriolar vasodilation
• long term sitting or standing
• liver failure
• malnutrition
• late term pregnancy
• impaired lymph drainage
• burns, inflammation
• snake bite

Question 11

effect of venous blood pressure on edema formation

Answer 11

• blood pressure in the capillaries can be increased by either increasing the blood pressure in the upstream arterioles or in the downstream venules
• much more influenced by a change in venules

Question 12

inflammatory swelling

Answer 12

• inflammation: rubor, calor, dolor, tumor and loss of function
• post exercise, joint sprains, arthritis, minor cuts and burns, immune

1. adhesion of PMN to microvasculature at wound site or infection
   - PMNs phagocytize injured cells or infectious agents
2. increase in microvascular permeability to proteins –> plasma exudes into tissue, carrying growth faactors that participate in the wound healing response
   - causes swelling and discomfort

Question 13

lymphatic architecture

Answer 13

• terminal lymph vessels permeate almost every tissue in the body
• endothelial cells in terminal lymphatic capillaries overlap and are not tight, “flaps” that serve as openings for interstitial fluid to freely enter
• not selective - everyone’s welcome!
• lymphatic capillaries –> lymph vessels –> lymph nodes –> venous vasculature
• lymph fluid is propelled by periodic compression of organs and by the smooth muscles in the lymph vessels that tend to contract when the vessel is distended - myogenic mechanism

Question 14

lymph function

Answer 14

1. return of excess filtered fluid
2. defense against disease
3. transport of absorbed fat
4. return of filtered protein

Question 15

lymphedema

Answer 15

• compromise of normal lymphatic function will lead to interstitial fluid accumulation (edema) and swelling
• depending which lymphatics are involved, the immune system may be involved

Question 16

venous architecture

Answer 16

•venous blood volume greater than arterial volume

• lots of veins
• larger than arterial counterparts (usually paired)
• larger and more of them –> less vasculature resistance to blood flow, little decrease in blood pressure as the CO returns through the venous circulation
• veins contain most of the blood volume in the CV system - blood “reservoir” or “capacitance” vessels
• thinner walls, less smooth muscle than arteries
• less elastic (collagen/elastin ratio greater) recoil less in response to increasing volume (less myogenic tone)
• one way valves

Question 17

venous return

Answer 17

•sympathetically induced venous contraction

• veins are not rich in vascular smooth muscle, but are richly innervated by sympathetic fibers –> more tone and constrict slightly
• mildly increases venous pressure but mobilizes venous blood –> increase venous return

•skeletal muscle activity

• muscle contraction
• Calf Muscle Pump

•respiratory activity

• during inspiration, pressure in teh chest cavity goes below atmospheric, distending venae cavae and increasing pressure gradient for venous return
• relfex increase in HR late in insporation as teh heart attempts to eject the increased venous return

•cardiac suction effect

• decrease in atrial pressure during atrial relaxation which increases the pressure gradient which also helps increase venous return
• during ventricular systole, the tendinous cords pull the AV valve cusps downward, slightly expanding the atrial space and creating a slight suction that draws blood from the venae cavae and pulmonary veins

Question 18

venous disease

Answer 18

*DVTs

•CVI (chronic venous insufficiency)

-DVTs often occur around venous valves due to increased opportunitiy for stasis behind valve leaflets

Question 19

central venous pressure

Answer 19

•blood pressure in the thoracic vena cava near the right atrium

Question 20

peripheral venous pressure

Answer 20

•blood pressure in a vein located in the periphery, outside the thorax

Question 21

venous return curve

Answer 21

• relationship between PVP and CVP
• CVP has to be less than PVP to maintain the pressure gradient that drives blood from veins to the heart
• venous blood volume

-if venous blood volume is increased, then PVP is increased and venous return increases

•venous tome

-when activation of teh sympathetic nervous system causes vasoconstriction, the venous blood pool is mobilized and PVP is modestly increased, increasing the venous return