Lecture 16: Microcirculation, Special Circulations Flashcards
Microcirculation
Arterioles, capillaries, venules
Features of capillaries
Smooth single cell endothelium w/ basement membrane. Highest total cross-sectional area -> slowest flow velocity
Metarterioles
Found before some capillaries. Have smooth muscle precapillary sphincters that can totally restrict flow
4 Starling forces for net filtration pressure
- Capillary hydrostatic P
- Interstitial hydrostatic P
- Osmotic force due to plasma proteins
- Osmotic force due to interstitial proteins
Net Filtration Pressure
Defines direction of bulk flow between blood and interstitum
NFP = P_cap + π_IF - P_IF - π_cap
How does vasoconstriction/dilation affect NFP?
Vasoconstriction decreases capillary hydrostatic P (promotes reabsorption), dilation increases it (promotes filtration)
Kwashiorkor
Protein malnutrition resulting in decreased capillary osmotic pressure -> edema
Edema
Abnormal interstitial fluid accumulation due to an imbalance of Starling forces
What is the lymphatic system?
A network of small organs (lymph nodes) + tubes (lymphatic vessels) through which lymph flows
What is the purpose of the lymphatic system?
Provides a route for interstitial fluid to return to circulation; drains excess interstitial fluid and is also a path for GI-absorbed fat to the blood.
Lymphedema
Fluid accumulation due to damage or occlusion of lymphatics
Roles of lymphatic smooth muscle
Provides intrinsic rhythmic pumping, working w/ lymphatic one-way valves. Respond to stretch and sympathetic innervation, also enhanced by sk. muscle + respiratory pumps
When is moving interstitial fluid back to the blood important?
IF movement to the blood compensates hypotension; autotransfusion mechanism due to decreased hydrostatic capillary P increasing absorption. Also results in hematocrit decrease
Crystalloids vs colloids
Crystalloids = low MW penetrating solutes e.g. Na+, K+, Cl- through capillary pores
Colloids = nonpenetrating solutes i.e. plasma proteins (v. low concentration in IF)
Angiogenesis
Growth/development of capillaries; stimulated by angiogenic factors e.g. VEGF
Intercellular clefts, fused-vesicle channels
H2O filled channels in capillary walls that facilitate diffusion of materials
Capillary leakiness
Depends on size of channels in capillary walls; varies between continuous, fenestrated, and sinusoidal
What determines the transcapillary diffusion gradients?
Local metabolic rate; capillaries are constantly bringing in more nutrients and carrying away waste, so local consumption/excretion is the main variable
Bulk flow
Movement of protein-free plasma across capillaries; determines distribution of ECF volume between plasma and IF.
Causes for edema
- Increase in net filtration P
- Decrease in capillary oncotic P
- Lymphatic blockage (no drainage)
- Increase in hydraulic permeability (leakier capillaries)
What typically causes a rise in circulation blood pressure?
Often a backup of pressure occurs due to venules, because venules have no smooth muscle compensation.
Coronary special circulation
Extremely high capillary density in myocardium. Vessels are squeezed in systole, thus more flow occurs in diastole. More squeezing means more flow reduction.
Coronary circulation autoregulation
Robust myogenic response maintains flow for small vessels penetrating the tissue; large epicardial conducting vessels have no myogenic response.
Symp. stim. is a major extrinsic controller of the heart.
Pulmonary special circulation
Function and regulation are reversed: pick up gases and deliver metabolites.
-Local hypoxia, acidosis -> vasoconstriction
-High O2, low CO2 -> dilation
Directs blood to well ventilated areas
Very low P system favoring reabsorption, very little ANS innervation
Cutaneous circulation (acral)
Acral skin has AVAs; bypass capillaries, shunt to venous plexi very close to skin.
Hands, feet, lips, nose, ears.
Cutaneous circulation (non-acral)
Symp. cholinergic stim. -> sweat glands: more sweating (all skin), more kallikrein -> kininogen -> kallidin -> bradykinin for vasodilation
ANS innervation of skin
No parasymp.
α-2 receptors and NE affinity increase w/ lower temp -> more vasoconstriction when cold
Skin circulation in prolonged cold exposure
Prolonged cold exposure leads to occasional vasodilation oscillation due to paralysis of noradrenergic transmission + prostacyclin release. Prevents starvation of tissue.
Cutaneous autoregulation
Nutrient flow is regulated by pO2, pCO2, myogenic response.
Also profound, transient responses to emotional stimuli (blushing, pallor)
