Microcirculation, Venous Blood Flow and Venous Return

Question 1

fenestrated capillary

Answer 1

small lipophobic molecules

Question 2

sinusoid or discontinuous capillary

Answer 2

red blood cells and large lipophobic molecules

Question 3

continuous capillary

Answer 3

very slow diffusion; large lipophobic molecules
slow diffusion for small lipophobic molecules
very fast diffusion for gases and lipophilic molecules

Question 4

Interstitium

Answer 4

• Collagen and proteoglycan filaments

Question 5

Interstitial fluid

Answer 5

• Fluid trapped amongst filaments
• “Tissue gel”
• ~ 1% of water “free”
• Diffusion occurs in gel ~95-99% as rapidly in free fluid

Question 6

Crystalloids

Answer 6

– Low moleculoid solutes wt. solutes • e.g. Na+, Cl-, K+

Question 7

Colloids

Answer 7

Plasma proteins - albumin

Question 8

Diffusion

Answer 8

Net movement of nutrients, oxygen and metabolic end products

Question 9

Bulk flow

Answer 9

Distribution of extracellular fluid

Question 10

Oncotic pressure

Answer 10

the free generated by the colloids crystalloids

Question 11

Capillary wall is (generally) a barrier to proteins

Answer 11

– Readily permeable to water and most solutes – Not a perfect filter
• Permeability for albumin is 1/1000th that of water

Question 12

Oncotic pressure generated by plasma proteins

Answer 12

– ~28mmHg
– Predominately generated by albumin, lesser extent by
globulins

Question 13

Plasma oncotic pressure draws fluid in to capillaries

Answer 13

– Interstitial oncotic pressure is much lower (~5-8mmHg)

Question 14

Hydrostatic pressure

Answer 14

drop from one end of the capillary to the other

Question 15

Capillary hydrostatic pressure

Answer 15

– Forces fluid out of the capillaries and in to the interstitium
– Drops from arterial end to venous end
• Pressure at arterial end ~30-40mmHg
• Pressure at venous end ~10-15mmHg

Question 16

Interstitial hydrostatic pressure

Answer 16

– Forces fluid in to the capillary when positive

– Draws fluid in to the interstitium when negative

Question 17

Capillaries loose

Answer 17

more water than they gain

• Approx. 2-3L per day

Question 18

Lymphatic system

Answer 18

• Large, fenestrated walls of capillaries
• Drain via lymphatic vessels
• Pass through lymph nodes

Question 19

lymphatic system is important in controlling

Answer 19

• Concentration of proteins in interstitial fluids
• Volume of interstitial fluid
• Interstitial fluid pressure
• Also in immune response

Question 20

Systemic venous circulation - low pressure and high come system

Answer 20

low pressure - between 3-18mmHg

High volume - holds 60% of the total blood volume

Question 21

Inspiratory movements

Answer 21

– Diaphragm descends
• ↑abdominalpressure
• Transmittedpassivelyto intraabdominal veins
– ↓ Pressure in thorax
• ↓pressure in intrathoracic veins
and right atrium
– ↑pressure difference between peripheral veins and heart

Question 22

Sympathetic innervation of veins increases

Answer 22

venous return to the heart
 increases cardiac output
– Important in exercise, blood loss etc

Question 23

Standing completely still

Answer 23

– Pressure ↑ by 1mmHg for each 13.6mm below the surface
• By feet +90mmHg
– Mean arterial pressure at
level of heart ~100mmHg
– So, in feet ~190mmHg
– Leg oedema
• 10-20% of blood volume within 15-30min

Question 24

Postural changes in

hydrostatic pressure - Orthostatic (postural) hypotension

Answer 24

– Immediate effect in going from supine to upright
• Around 500 ml of blood from the upper body to legs
• ↓ venous return
–  ↓ cardiac output
–  ↓ blood pressure

– Reflex vasoconstriction in legs and lower abdomen

Question 25

Main points

Answer 25

• Fluid forces favour small amounts of loss into tissue space, reclaimed as lymph
• The venous system is high-volume, low pressure system
• Compliance of veins can be adjusted by sympathetic innervation
• Venous return limits cardiac output