Ch 10.4 -- Capillaries -- VTPP 435

Ch 10.4 -- Capillaries -- VTPP 435

Capillaries

-sites for exchange of materials between blood & tissue cells
-exchange mostly by diffusion

Reasons why Capillaries are suitable for diffusion according to Fick's Law of Diffusion

1) Short distance between blood & surrounding cells
2) High distribution & surface area of capillaries
3) Slow blood flow through capillaries due to extensive branching

Pores

narrow, water-filled clefts that join endothelial cells in most capillaries

Types of Capillaries Porosity

1) Brain capillaries joined by tight junctions, no pores
2) Small, water & lipid-soluble substances can go thru pores or lipid bilayer
3) Fenestrations
4) Discontinuous -- big gaps

Fenestrations

-holes in capillaries of kidneys & intestines that extend thru the thickness of endothelial cells
-helps with nutrient absorption and urine formation

Precapillary Sphincters

-consist of a ring of smooth muscle around entrance to capillary it arises from metarteriole or arteriole
-stop cocks to control blood flow thru capillaries

Blood Flow thru a tissue is regulated by

1) Degree of resistance offered by arterioles in organ
2) Number of open capillaries

2 Ways Exchanges between Blood and Surrouding tissue si down

1) Passive Diffusion down concentration gradients
2) Bulk Flow -- movement in bulk of a protein-free plasma across capillary walls between blood & surrounding interstitial fluid

Ultrafiltration

net movement of a protein free plasma out of capillary into surrounding interstitial fluid

Reabsorption

-net movement of interstitial fluid into capillary
-when inward driving pressures > outward pressures across capillary wall

4 Forces Influence Fluid Movement Across Capillary Wall

1) Capillary Blood Pressure (Pc) -- out of capillary
2) Plasma-Colloid Osmotic Pressure (πp) -- into capillary
3) Interstital Fluid Hydrostatic Pressure (Pif) into capillary
4) Interstitial Fluid-Colloid Osmotic Pressure (πif) out of capillary

Lymphatic System

-one-way route of vessels to return fluid that leaks out from interstitial fluid to blood

Initial Lymphatics

-small, blind-ended terminal lymph vessels
-fluid enters between overlapping endothelial cells

Lymph

-interstitial fluid that picked up by lymphatic vessels and returned to venous system
-passes thru lymph nodes

Lymph Vessels

-formed from converging initial lymphatics
-empty into venous stem where blood enters right atrium

Lymph Moved Forward by 2 mechanicsm

(1) Smooth muscle on vessels rhythmically ocntract
(2) Lymph vessels are between skeletal muscles and so they are squeezed

Lymphatic System Functions

1) Return of excess filtered fluid
2) defense against disease
3) transport of absorbed fat
4) Return of filtered protein

4 Causes of Edema

1) Reduced Concentration of Plasma Proteins
2) Increased Permeability of Capillary Walls
3) Increased Venous Pressure
4) Blockage of Lymph Vessels