Lec 7 - microcirculation and the lymphatic system

what does microcirculation function to do?

Functions to transport nutrients to tissues and remove cell waste

what are the components of microcirculation

• Arterioles → highly muscular, controls blood flow to capillaries 

• Capillaries → thin-walled vessels, most blood-tissue exchange occurs 

  • Most cells are 50μm of a capillary

how do capillaries affect cross-sectional area

• Successive branching from artery to arterioles to capillaries  inc. total cross-sectional area 

  • Individual capillaries are small, but combining them → larger

how does cross-sectional area of capillaries affect velocity

• Velocity slows down (i.e. blood flow is slower) with inc. in area 

  • In order to max exchange so that it is more efficient

describe the structure of a capillary in the blood

• single layer of endothelial cells → surrounded by basement membrane (ex. Collagen, proteins etc.) 

• Intracellular clefts → between endothelial cells allow small solutes, ions and water to cross, not proteins though

• Caveole → facilitate transcytosis of large macrom/

What energy gradients influence exchange between blood and cells (gases and lipophilic substances) in peripheral tissues?

gas → partial pressure

lipophillic → conc. gradient

What part of the capillaries do gases and lipophilic substances cross?

• entire cap surface

• Lipophilic m/ are not charged, they can pass through CM, so they can pass through entire surface

What energy gradients influence exchange between blood and cells (small water soluble solutes including ions and glucose) in peripheral tissues?

• Concentration 

• Sometimes voltage, but capillaries are very thin and have lots of opening due to clefts, therefore voltage is not very different between inside and outside as such concentration drives exchange of small solutes

what part of the capillary do small water soluble solutes like ions and glucose cross?

intracellular clefts

What energy gradients influence exchange between blood and cells (water soluble proteins) in peripheral tissues?

• Rely on other mechanism for exchange 

What part of the capillary do water soluble proteins cross?

• Caveolae 

  • Proteins enter into space, pinch off to turn into vesicle, vesicle transported to other side of membrane

What energy gradients influence exchange between blood and cells (water) in peripheral tissues?

• Hydrostatic pressure and can rely on other mechanism for exchange (osmosis)

What part of the capillary does water cross?

• Through clefts 

  • Cant pass through cm (therefore not entire surface) 

  • Cant pass via caveolae 

what E gradients determine the movement of water across blood capillaries

hydrostatic pressure (P) and osmotic pressure (pi)

colloid osmotic pressure

osmotic pressure driven by water soluble plasma proteins

describe the two pressure gradients that affect movement of water across capillaries

• Hydrostatic pressure gradient 

  • Pc > Pif, drives H2O out (filtration)

• Colloid osmotic pressure gradient 

  • 𝛱p > 𝛱if, drives H20 in (reabsorption)

what does the magnitud and direction of water flow depend on, when water moves along blood capillaries?

• depend on P and π gradients

• there are hydrostatic pressures inside and outside of vessel (interstitial space) and same for osmotic pressure

net filtration pressure (NFP)

• driving force of water flow along capillaries

• NFP = Net hydrostatic pressure - Net osmotic pressure

  • = (Pc-Pif)-(πp - πif)

Net filtration rate

• = Kf x NFP

• Kf = capillary filtration coefficent

  • coefficent changes based on capillary bed

  • might be higher at sites of high gas exchange

  • example calculation attached in image*

    • for lecture explanation, go to 36:42 - oct 1st lecture

What determines magnitude and direction of net water flow between capillary and interstitial space?

• pressure gradients between hydrostatic and oncotic pressure

• hydrosttaic pressure in capillary pushes water out

• intersitial fluid pushes water in (weaker compred to hydrostaatic pressure)

• oncotic pressure inside capillary pulls water in

• intersistial oncotic pressure pulls water out (weaker)

• during a heart beat,

  • At the Beginning (Arterial End): bp from the heart is very high. The outward push completely beats the inward pull. Result: Water leaks out into the tissues to deliver nutrients.

  • At the End (Venous End): As blood moves along, the pushing pressure drops drastically because it is further from the heart. Now, the internal protein "sponges" win the tug-of-war. Result: Water gets sucked back in, carrying away waste.

  The total amount (magnitude) of water that moves depends on how strong these pressures are and how "leaky" or large the blood vessel walls are.

What determines the magnitude and direction of water flow from artery to vein?

• pushing gradient

  • hydrostatic pressure is higher at artery end

    • lower P at vein end

• pulling graident

  • water pulls inward bsed on osmotic pressure

  • pressure remains stable

• Direction: At the artery end, the pushing gradient (35) beats the pulling gradient (25), so water flows out into the tissue. At the vein end, the pushing gradient drops (18) and loses to the pulling gradient (25), so the flow reverses and water is sucked back into the blood.

• Magnitude: The amount of water that moves is directly determined by the size of the net pressure difference. Because the net pressure gradient is slightly stronger at the artery end (+10 mmHg) than the vein end (-7 mmHg), a little more water is pushed out than is pulled back in. (The leftover fluid is safely collected and returned to the blood by your lymphatic system).

how do blood capillaries differ in the brain?

• endothelial cells close together, intracellular clefts closed by tight junctions, allow small uncharged m/ to pass (ex. Water, O2, CO2, etc.) 

  • ions/proteins don’t move freely 

how do blood capillaries differ in the kidney

• Glomerular capillaries → very permeable 

  • Channels (fenestrae) in cells allow small m/ and ions to pass easily (high Kf and filtration rate)

how do blood capillaries differ in the liver?

• Intracellular clefts are wider, allow large macrom/ to pass 

  • Allows bigger nutrients to enter into liver 

how do capillaries change in high altitudes

• ↓10% per km in barometric pressure leading to drop in available O2

• ↓5-10°C per km

• Endotherms need to maintain high rate of body heat generation but are limited in available O2to support higher metabolism

What is the physiological response in animals that live in high altitudes to meet these demands?

• Animals at higher altitudes (deer mouse, bar headed goose, andean duck)  have more capillaries to maximize exchange to get available O2 (since O2 is low in high altitudes)

what is the function of the lymphatic vessel

• Because filtration exceeds absorption in most capillaries, ~3L/day of fluid leaves the circulatory system in humans

• Lymphatic vessels collect excess interstitial fluid and drain it into major veins near the heart

describe lymphatic capillaries

• highly permeable

• Junctions between adjacent endothelial cells can flap open acting like valves

  • Interstitial fluid pressure (Pif) drives everything in the fluid into lymph capillaries

• No colloid pressure gradient.

why is their no colloid pressure gradient in the lymphatic system?

• Proteins move freely through the gaps

describe the function of the lymphatic pump

• Valves exist all along lymphatic vessels to direct lymph fluid towards the heart 

• Lymphatic vessels contract, valves ensure its one way

• After contraction, anchoring filaments pull valves open again, reducing inside pressure and drawing fluid in

• Skeletal muscle contraction also squeezes the lymph vessels and helps pump lymph fluid

what is lymphatic edema?

• Excess extracellular fluid in body tissues that lymph can't filter out 

what are potential causes of lymphatic edema?

• Hypertension (inc. Pc) 

• Dec plasma protein (dec. IIp) 

• Endothelial dysfunction (inc. permeability) 

• Lymph vessel abnormality