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Cardiovascular Physiology Lecture 7 (Objective 2: Understand how capillary…
Cardiovascular Physiology Lecture 7
Objective 1:
Know and understand the anatomy of the microcirculation and factors that influence capillary blood flow
Key materials transported through the blood and exchanged at capillaries
Capillaries
:
Absence of vascular smooth muscle and elastic tissue reinforcement facilitates exchange
-One cell-thick layer of endothelial cells on basal lamina
Capillary beds
Microcirculation
:
Collateral arteries ->
Arteriole wall is smooth m. ->
Metarterioles can act as bypass channels when precapillary sphincters constrict ->
Precapillary sphincters can close off capillaries in response to local signals
->
Capillaries ->
Small venule ->
Venule ->
Vein
Objective 2: Understand how
capillary fluid exchange
occurs and how this process is influenced by changes in fluid filtration and absorption
Low velocity of blood flow through the capillaries facilitates exchange of materials
Total cross-sectional area is large
Velocity
= Flow rate/
Cross-sectional area
Flow rate is not the same as velocity of flow
The narrower the vessel, the faster the velocity of flow.
Plasma and cells exchange materials across thin capillary walls
Capillary density in a given tissue is related to the metabolic activity of cells
-High metabolic rate= higher oxygen requirement
->more capillaries per unit area
Body Fluid Compartments
ECF and ICF in osmotic equilibrium but very different composition
ICF
(Intracellular fluid) is 2/3 of the total body water volume
ECF
:
1/3 of total body water volume. Consists of:
Interstitial fluid
(between the circulatory system and the cells)
Blood Plasma
: is the liquid matrix of blood
Capillary exchange
Bulk Flow
(endothelial cell junctions that are leaky) : H20, electrolytes, small molecules transfer between two cells in the interstitial fluid
-Mass movement as a result of hydrostatic or osmotic pressure gradients
Hydrostatic pressure:
force of blood or fluid against walls of CV system
Osmotic pressure
: tendency of solute to move from high to low concentration (also resistance it would take to stop water movement)
Absorption
: fluid movement into capillaries
-Net absorption at venous end
-Caused by colloid osmotic pressure
Venuole end: Osmotic pressure > Hydrostatic pressure
FAVORS ABSORPTION
Filtration
: fluid movement out of capillaries
-Caused by hydrostatic pressure
-Net filtration at arterial end
Arteriole end: Osmotic pressure < Hydrostatic pressure
FAVORS FILTRATION
Lymph vessels
pick up excess water and solutes that filter out of the capillary and return it to circulation
Lymphatic System
-Returns fluid and proteins filtered out of the capillaries to circulatory system
-Picks up fat absorbed at small intestine and transfers it to circulatory system
-Serves as filter to capture and destroy foreign pathogens
Vesicular Transport
: larger solutes and proteins (macro-molecules)
In most capillaries, large molecules (including selected proteins) are transported by transcytosis
Diffusion
: of O2, CO2, lipid-soluble substances, depending on concentration gradient
Fenestrations
Fenestrated capillaries have large pores
-> more permeable pores and leaky junctions in kidneys and intestines
Objective 3: Identify how alterations in capillary exchange can produce
tissue edema
Blood Capillary filtration that exceeds absorption
Decrease in plasma protein concentration
-Plasma proteins are responsible for colloid osmotic pressure
-Ex. severe malnutrition
Increase in interstitial proteins
-Excessive leakage of proteins out of blood decreases colloid osmotic pressure gradient and increases net filtration
Increase in capillary hydrostatic pressure
-usually indicative of elevated venous pressure
-Ex. Heart failure
Inadequate drainage of lymph
If hydro. pressure is above osmotic pressure the whole time -> little absorption
-> increased fluid filtered out, accumulation of fluid