Capillaries Flashcards
How are capillaries involved in exchange?
Diffusion e.g. gases
Filtration - water
What are the components of capillaries?
Mostly all endothelium.
Small amount of collagen in the basement membrane.
No smooth muscle so cannot constrict themselves.
What is the size of capillaries?
Very small.
Some cells passing through are bigger than the capillary.
Red blood cells take up lots of the capillary.
Larger cells are deformable, so can get through the small capillary without clogging the vessel up.
What is the structure of continuous capillaries?
Continuous, no significant gaps.
Surrounded by pericytes - wrap around epithelial cell to regulate blood flow and for barrier function.
Continuous basement membrane.
Most common type.
What is solute transfer in continuous capillaries?
Paracellular
Transcellular - active, passive, transcytosis.
Transcytosis is where things are carried by vesicles.
What is the location of continuous capillaries?
Lungs, muscle, adipose tissue.
Central nervous system - including the blood brain barrier (neurovascular unit).
What is the neurovascular unit?
Maintains integrity of the blood brain barrier.
Endothelial cell with a very tight tight junction.
Pericytes also prevent movement out the vessel.
Astrocytes have foot processes and sit around the capillary.
Intercellular clefts are also very small.
What are the pores of capillaries?
Capillaries are permeable to small molecules at the points where the endothelial cells meet - intercellular clefts.
What is the structure of fenestrated capillaries?
Openings in their endothelium - fenestrations.
60-100nm diameter, any items smaller than this can pass through.
Although the basement membrane still acts as a barrier.
What is the solute transfer in fenestrated capillaries?
Not selective in what passes through, as long as it is small.
Water, ions, small molecules.
What is the location of fenestrated capillaries?
Organs where exchange is critical - kidneys, intestines, endocrine glands.
What is the structure of discontinuous capillaries?
Large gaps in the endothelial layer and the basement membrane.
Provide little resistance to movement.
What is the solute transfer in discontinuous capillaries?
Water, ions, small molecules and large molecules.
Cells can get through.
What is the location of discontinuous capillaries?
Liver, spleen, bone marrow, adrenal glands, endocrine glands.
In the bone marrow, RBCs are produced, so need to get into the blood through sinusoids.
Why does the liver have discontinuous capillaries?
The liver is important for detoxification (of bacteria and toxins) and metabolism of various compounds, it has direct access to plasma.
Hepatocytes produce albumin, so direct contact with blood allows for efficient secretion.
It also provides additional compartment for cell-cell interactions, protecting from shear stress.
What is the space of Disse?
In between the sinusoid vessel and the hepatocyte is the space of Disse.
This allows direct access to the plasma without significant filtration before it.
How are the capillaries the bulk of the circulatory system?
Total surface area is 6000m^2.
So provides a very large surface area for exchange.
What is the relationship of cross sectional area in capillaries?
Flow = velocity x cross sectional area
Q = u x A
Velocity decreases in capillaries, and cross sectional area increases, so flow decreases.
Why are the capillaries the bulk of the circulatory system?
Slowing down the blood in the capillaries is very good for filtration and diffusion.
It increases transit time, so there is more time for exchange.
How is blood flow regulated in capillaries?
Arterioles are the main regulators of capillary blood flow.
Capillaries can also autoregulate blood flow using pre-capillary sphincters at their proximal end.
What are meta-arterioles?
Meta-arterioles come off arterioles, then capillaries branch off meta-arterioles.
Meta-arterioles allow shunting of blood from one side to the other id the capillaries are closed.
What nutrients can be exchanged in the capillaries?
Plasma proteins cannot leave unless they have a specific transporter, or it is a discontinuous capillary, or the capillary is damaged.
Proteins with specific receptors on the vessel wall side can go into the tissues.
Small water soluble substances can pass through intercellular clefts.
Lipid soluble molecules can pass straight through the endothelium.
What are the mechanisms of capillary exchange?
Diffusion
Bulk flow
Vesicular transport
Active transport
What is diffusion?
Movement of a molecule from an area of high concentration to an area of low concentration.