Lecture 16: Microcirculation, Special Circulations

Question 1

Microcirculation

Answer 1

Arterioles, capillaries, venules

Question 2

Features of capillaries

Answer 2

Smooth single cell endothelium w/ basement membrane. Highest total cross-sectional area -> slowest flow velocity

Question 3

Metarterioles

Answer 3

Found before some capillaries. Have smooth muscle precapillary sphincters that can totally restrict flow

Question 4

4 Starling forces for net filtration pressure

Answer 4

1. Capillary hydrostatic P
2. Interstitial hydrostatic P
3. Osmotic force due to plasma proteins
4. Osmotic force due to interstitial proteins

Question 5

Net Filtration Pressure

Answer 5

Defines direction of bulk flow between blood and interstitum
NFP = P_cap + π_IF - P_IF - π_cap

Question 6

How does vasoconstriction/dilation affect NFP?

Answer 6

Vasoconstriction decreases capillary hydrostatic P (promotes reabsorption), dilation increases it (promotes filtration)

Question 7

Kwashiorkor

Answer 7

Protein malnutrition resulting in decreased capillary osmotic pressure -> edema

Question 8

Edema

Answer 8

Abnormal interstitial fluid accumulation due to an imbalance of Starling forces

Question 9

What is the lymphatic system?

Answer 9

A network of small organs (lymph nodes) + tubes (lymphatic vessels) through which lymph flows

Question 10

What is the purpose of the lymphatic system?

Answer 10

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.

Question 11

Lymphedema

Answer 11

Fluid accumulation due to damage or occlusion of lymphatics

Question 12

Roles of lymphatic smooth muscle

Answer 12

Provides intrinsic rhythmic pumping, working w/ lymphatic one-way valves. Respond to stretch and sympathetic innervation, also enhanced by sk. muscle + respiratory pumps

Question 13

When is moving interstitial fluid back to the blood important?

Answer 13

IF movement to the blood compensates hypotension; autotransfusion mechanism due to decreased hydrostatic capillary P increasing absorption. Also results in hematocrit decrease

Question 14

Crystalloids vs colloids

Answer 14

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)

Question 15

Angiogenesis

Answer 15

Growth/development of capillaries; stimulated by angiogenic factors e.g. VEGF

Question 16

Intercellular clefts, fused-vesicle channels

Answer 16

H2O filled channels in capillary walls that facilitate diffusion of materials

Question 17

Capillary leakiness

Answer 17

Depends on size of channels in capillary walls; varies between continuous, fenestrated, and sinusoidal

Question 18

What determines the transcapillary diffusion gradients?

Answer 18

Local metabolic rate; capillaries are constantly bringing in more nutrients and carrying away waste, so local consumption/excretion is the main variable

Question 19

Bulk flow

Answer 19

Movement of protein-free plasma across capillaries; determines distribution of ECF volume between plasma and IF.

Question 20

Causes for edema

Answer 20

1. Increase in net filtration P
2. Decrease in capillary oncotic P
3. Lymphatic blockage (no drainage)
4. Increase in hydraulic permeability (leakier capillaries)

Question 21

What typically causes a rise in circulation blood pressure?

Answer 21

Often a backup of pressure occurs due to venules, because venules have no smooth muscle compensation.

Question 22

Coronary special circulation

Answer 22

Extremely high capillary density in myocardium. Vessels are squeezed in systole, thus more flow occurs in diastole. More squeezing means more flow reduction.

Question 23

Coronary circulation autoregulation

Answer 23

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.

Question 24

Pulmonary special circulation

Answer 24

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

Question 25

Cutaneous circulation (acral)

Answer 25

Acral skin has AVAs; bypass capillaries, shunt to venous plexi very close to skin.
Hands, feet, lips, nose, ears.

Question 26

Cutaneous circulation (non-acral)

Answer 26

Symp. cholinergic stim. -> sweat glands: more sweating (all skin), more kallikrein -> kininogen -> kallidin -> bradykinin for vasodilation

Question 27

ANS innervation of skin

Answer 27

No parasymp.
α-2 receptors and NE affinity increase w/ lower temp -> more vasoconstriction when cold

Question 28

Skin circulation in prolonged cold exposure

Answer 28

Prolonged cold exposure leads to occasional vasodilation oscillation due to paralysis of noradrenergic transmission + prostacyclin release. Prevents starvation of tissue.

Question 29

Cutaneous autoregulation

Answer 29

Nutrient flow is regulated by pO2, pCO2, myogenic response.
Also profound, transient responses to emotional stimuli (blushing, pallor)