Lecture 26: Microcirculation I and II Flashcards
Microcirculation
Refers to the functions of the smallest blood vessels, capillaries, and neighboring lymphatic vessels
Two functions of microcirculation
- Exchange of nutrients and waste products in the tissue
2. Site of fluid exchange between vascular and interstitial compartments
Blood is delivered to the capillary bed via
arterioles
The degree of constriction or relaxation of the smooth muscle of the arterioles markedly affects
blood flow to the capillaries
Pre-capillary sphincters
Precede some capillaries and help to regulate blood flow
Structure of capillaries
Thin walled and are composed of a single layer of endothelial cells
Capillary pores
- Present between cell junctions
- Permit passage of water soluble substances
What can pass through the lipid bilayer
Lipid soluble substances like O2 and CO2
What must be transported by vesicular transport?
Large molecules such as protein hormones
Three types of capillaries
- Continuous
- Fenestrated
- Sinusoid
Where are continuous capillaries found
Most regions of the body
Fenestrated capillary structure
Contain pores that permit small molecules to filter through without having to pass between clefts
Fenestrated capillaries are found where
In endocrine organs, the glomerulus of the kidneys, and the absorptive areas of intestinal tract
Sinusoid capillary structure
Gaps between adjacent endothelial cells which permit the free exchange of water and solutes as large as plasma proteins
Sinusoid capillaries are found where?
Liver and bone marrow
Are precapillary sphincters innervated?
Nope
What are precapillary sphincters sensitive to?
Local metabolic changes
Do capillaries actively participate in the regulation of blood flow?
Nope, they have no smooth muscle
When a muscle becomes more active,
a greater percentage of pre-capillary sphincters relax simultaneously, which opens up more capillary beds
Concurrent arteriolar vasodilation increases
Blood flow to organs
As a result of more blood flowing through more open capillaries,
The total volume and sa available for exchange increases and the diffusion distance between the cells and an open capillary decreases
Two things that regulate the blood flow through a particular tissue
- Degree of resistance offered by the arterioles in the organ which is controlled by sympathetic activity and local factors
- Number of open capillaries controlled by the action of the same local metabolites on smooth muscle of terminal arterioles and precapillary sphincters
Capillary exchange is accomplished two ways
- Passive diffusion
2. Bulk flow
Passive diffusion
- Down concentration gradient
- Primary mechanism for exchange of individual solutes
Bulk flow
- Movement in bulk of protein-free plasma across capillary walls between blood and surrounding interstitial fluid
- Encompasses ultrafiltration and reabsorption
Starling pressures
- differences in the hydrostatic and osmotic pressures between the plasma and interstitial fluid
- Fluid movement across a capillary wall is driven by this
Hydraulic conductance (Kf)
- Filtration rate of fluid per mmHg
- Influenced by the size of the water filled pores
Hydraulic conductance determines
How much fluid movement will be produced for a given pressure difference
Capillary hydrostatic pressure (Pc)
- Hydrostatic pressure extered on the inside of the capillary walls by the blood
- Tends to force fluid out of the capillaries into the interstitial fluid (filtration)
Capillary hydrostatic pressure is more affected by changes in
venous pressure than changes in arterial pressure
Capillary hydrostatic pressure is highest and lowest where?
Highest at the arteriolar end of the capillary and lowest at the venous end
Interstitial hydrostatic pressure (Pi)
- Fluid pressure exterted on the outside of the capillary wall by the interstitial fluid
- Pressure tends to force fluid into the capillaries
Normal interstitial hydrostatic pressure
Very low, nearly zero
Capillary osmotic pressure (3.14c)
- Forces opposing filtration
- Effective osmotic pressure of capillary blood due to the presence of plasma proteins
- Determined by the protein concentration of capillary blood
Interstitial osmotic pressure (3.14i)
- Force favoring filtration
- Determined by the interstitial fluid protein
- Normally there is little loss of protein in the capillaries, meaning this value is close to 0
A positive net exchange pressure
- Outward pressure exceeds inward pressure
- Filtration pressure
Negative net exchange pressure
- Inward pressure exceeds outward pressure
- Reabsorption pressure
Two pressures that tends to move fluid out of the capillaries
- Capillary pressure
2. Interstitial pressure
Two opposing pressures that tend to move fluid into the capillaries
- Capillary osmotic pressure
2. Interstitial hydrostatic pressure
Once interstitial fluid enters a lymphatic vessel, it is called
lymph
Are lymphatic openings bigger or smaller than pores in blood capillaries?
Bigger
The lymphatic system is responsible for
returning interstitial fluid and proteins to the vascular compartment
What directs the flow of lymph towards its venous outlet in the chest?
Smooth muscle and one way valves
