Lecture 12: Capillaries, Lymphatics, and veins

Question 1

What are the 3 different types of capillaries?

Answer 1

1) continuous: found in the blood-brain barrier, lots of tight junctions - allow gases to diffuse through
2) fenestrated: some small fenestrations (windows/openings)
3) sinusoidal: large gaps between cells, allows cells to exit/enter circulation

Question 2

What are the three types of capillary exchange, and describe each

Answer 2

1) Diffusion: based on Fick’s Law: J=PA([ ]out-[ ]in)
2) Filtration (starling forces): Filtration Pressure=ΔP-π, π is oncotic pressure, ΔP is hydrostatic pressure (Pcap-Pinterstitial, Pi = 0). π=δRT (Ci-Co)
3) Vesicular transport: stack of fused vesicles across cell (pore)

Question 3

Describe what happens when net filtration balance is disturbed

Answer 3

Can be disturbed by pregnancy (creates higher increase in BV than increase in plasma proteins), burns/inflammation (causes more gaps in capillaries), capillary injury (proteins escape blood). Standing leads to increased hydrostatic pressure, and hypertension also leads to increased hydrostatic pressure. This all leads to increased filtration due to lower oncotic pressure and lower amounts of plasma protein concentrations. Usually goes to the lymph system, but can be inactivated by lack of motion, tumours, removal of glands, which would then lead to edema.

Question 4

Describe how lymph drains

Answer 4

From the right head and neck, goes into right subclavian vein, from the left head and neck, goes to thoracic duct, everywhere else goes to left subclavian vein

Question 5

Describe the two phases of lymphatic flow

Answer 5

1) Expansion phase - hydrostatic pressure from the interstitium pushes lymph into the lymph vessels
2) Compression phase - lymph gets compressed and pushed down the vessel through 1-way valves

Question 6

Describe the three methods of lymphatic flow regulation

Answer 6

1) Interstitial pressure: more efflux from capillaries, more interstitial pressure, more lymphatic flow
2) compression: due to skeletal muscle
3) myogenic tone: stretch causes VSM contraction

Question 7

Describe active and passive capacitance

Answer 7

Active capacitance is due to neural regulation (sympathetic)
Passive capacitance is due to changes in venous volume
in the peripheral circulation, only passive capacitance can be seen. constriction of arterioles causes loss of flow into veins, veins deform/collapse, less venous reserve volume, increased VR, increased cardiac output
In the splanchnic circulation, you can see some active capacitance

Question 8

Describe the two types of responses for capacitance

Answer 8

peripheral: entirely passive. constriction of arterioles, veins collapse (decrease BP), leads to increased VR and increased CO due to length-tension relationship and Starling’s law.
splanchnic: 10% active. constriction of venules means more blood returned.