Microcirculation & Capillary Filtration Flashcards
What vessels make up the microcirculation?
Terminal arterioles
Capillaries
Post-capillary venules
What occurs in the microcirculation?
Exchange of gases, fluids, nutrients and waste materials
What do terminal arterioles do?
Control flow through capillaries
What stimulates the smooth muscle in terminal arterioles?
Local factors
In which vessels does exchange occur in the microcirculation?
Capillaries
Post-capillary venules
How long is a typical capillary?
500 - 1000 um (<1mm)
How wide is a typical capillary?
4 - 8 um
Where is the smooth muscle in the microcirculation?
Terminal arterioles
What can alter the lumen diameter of continuous capillaries?
Pericytes
What do lymphatic vessels do?
Absorb fluid and protein and return them to the blood
Take up and transport micro-organisms to lymph nodes
What is vasomotion?
Terminal arterioles constrict and relax periodically due to transient and intermittent depolarisation of smooth muscle cells
What does heterogeneity in perfusion mean?
Blood flow is not uniform in all vessels in a bed
How can uniformity in blood flow through a bed be increased?
Decreased arterial tone/vasodilatation
How does vasodilatation affect transit time?
Decreased
How is smooth muscle arranged around the vessel lumen?
Concentrically
How are endothelial cells arranged in capillaries?
Aligned with direction of flow due to shear stress/laminar flow
Why is the arrangement of endothelial cells in capillaries beneficial?
Helps stimulate production of local mediators (for regulation)
Describe the structure of a continuous capillary
Continuous thin layer of 1-3 endothelial cells surrounded by a basement membrane
Gap and tight junctions
Pericytes
Why is exchange efficient in capillaries?
Short transcapillary diffusion distance ~0.3um
Where are continuous capillaries found? (6)
Lungs
Skeletal muscle
Myocardium
Skin
Connective tissue
Fat
Describe the structure of a fenestra
Basal lamina and basal membrane of endothelial cells come together to form fenestrae
Bridged by a fenestrae diaphragm due to continuous basal lamina
How large are fenestrae?
50-60nm
What are fenestrated capillaries for?
Rapid movement of macromolecules and fluid
Where fenestrated capillaries found?
Kidneys
Intestinal mucosa
Some endocrine glands
Joints
Describe the structure of a discontinuous capillary
Endothelial gaps over 100nm wide
Disrupted basal lamina
What are discontinuous capillaries for?
Allow free movement of macromolecules and cells
Where are discontinuous capillaries found?
Liver
Spleen
Bone marrow
How do lipophilic molecules permeate the capillary wall?
Easily diffuse across endothelial cells
How do small lipophobic molecules permeate the capillary wall?
Fenestrae
Intercellular gaps
How do large lipophobic molecules permeate the capillary wall?
Wide intercellular gaps formed during inflammation
Transcytosis
Trans-endothelial channels
What substances move across the capillary wall the fastest?
Gases
What percentage of water passes through intercellular gaps and water channels?
90% intercellular gaps
10% water channels
What is filtration?
Movement of water across capillary wall
How much plasma is pumped through capillaries each day?
4000L
What net volume of plasma leaves the circulation each day?
8L
How is the volume of plasma that leaves the circulation returned to the bloodstream?
Lymphatic system
What is the primary force driving plasma filtration?
Hydrostatic pressure gradient
In which direction does the hydrostatic pressure gradient tend to drive water?
Out of capillaries
How do you calculate the hydrostatic pressure gradient?
Pcap - Pint
What is the primary force retaining fluid within capillaries?
Oncotic osmotic pressure gradient
What is the oncotic osmotic pressure gradient caused by?
Large plasma proteins
How do you calculate the osmotic pressure gradient?
σ(πcap - πint)
What is σ in relation to the osmotic pressure gradient?
Reflection coefficient for plasma which corrects for endothelium as an imperfect barrier
What is the range of values for σ of plasma?
0.8-0.95
In which direction does the oncotic osmotic pressure gradient tend to drive water?
Into capillaries
What is Starling’s equation for the net fluid of movement?
Jv ∝ (Pcap - Pint) - σ(πcap - πint)
If the Jv calculated is positive, which direction is water moving?
Out of capillaries
If the Jv calculated is negative, which direction is water moving?
Into capillaries
Which pressure gradient does not normally change along a capillary and why?
Oncotic osmotic
Proteins cannot leave capillaries
What happens when you have been standing for a long time?
Increased Pcap in lower extremities due to gravity
Filtration transiently increased
Reflexes quickly cause arteriole constriction
Pcap reduced
What happens in the splanchnic circulation during exercise? (same as following major tissue injury and blood loss)
Local arteriolar constriction to reduce Pcap
Reduced filtration
Local net fluid absorption
Describe the flow in lymphatic vessels
Unidirectional (larger and blunted vessels)
Driven by periodic compression
Describe the structure of a lymphatic capillary
Continuous, overlapping endothelium
Interrupted basal lamina
Anchoring filaments including elastin
Why does fluid readily move into lymphatic capillaries?
Lots of protein in lymphatic vessels = high internal oncotic osmotic pressure
What prevents backflow in lymphatic capillaries?
Valves
Where in the circulatory system do the lymphatic ducts drain?
Where subclavian and internal jugular veins meet
When does oedema occur?
When amount of fluid leaving microcirculation increases
Why might the amount of tissue fluid formed increase?
Increased capillary hydrostatic pressure
Decreased capillary oncotic osmotic pressure
Blocked lymphatic system
What may cause an increased capillary hydrostatic pressure gradient and how?
Congestive heart failure
Increased CVP due to decreased cardiac output and/or fluid retention
Systemic and pulmonary Pcap increases
Increased filtration overwhelms lymphatic clearance
What may cause a decreased oncotic osmotic pressure gradient and how?
Inflammation (histamine)
Local vasodilatation and increased vascular permeability allows proteins to leave
Increased πint so decreased oncotic osmotic pressure gradient
What may cause a blockage in the lymphatic system and how?
Elephantiasis:
Nematode worm/filariasis blocks nodes
