44. Endothelium

Question 1

What is the basic structure of endothelium?

Answer 1

Endothelium refers to the
simple squamous epithelium
that is found lining organs,

blood vessels and body cavities.

This single layer of cells lies on top
of a basement membrane,

and in small arteries,
arterioles and glands it is
also in proximity to smooth muscle.

Endothelial structure varies according to
its site and function.

The major types include:
> Continuous endothelium: 
It consists of a continuous basement
membrane with endothelial cells 
anchored together via tight junctions.

It has a low permeability and is found
in the blood–brain barrier and the lung.

> Fenestrated endothelium:

It has pores (fenestrae) within the endothelium.
It is very permeable and is found lining renal glomeruli.

> Discontinuous endothelium:

It has large gaps between the endothelial
cells and basement membrane.
It is the most permeable of all types and
is found in the liver and spleen.

Question 2

What are the functions of endothelium?

Answer 2

Endothelium is a highly sophisticated

and

cellularly active tissue with essential roles

in coagulation, inflammation and vasomotor tone. Its functions
are varied and include the following:

1
> Diffusion:

The endothelial lining at the
alveolar–capillary interface plays an
important role in the diffusion of gases
(e.g. O2 and CO2) and
lipid-soluble agents (e.g. anaesthetic drugs).

The diffusion of such substances follows
Fick’s law.

2
> Osmosis:

The endothelium lining the 
smaller blood vessels forms a
semi-permeable membrane, 
which allows the formation 
of interstitial fluid,
based on Starling’s forces.

3
> Filtration:

The pores present within the 
endothelium of the Bowman’s
capsule permit the passage of 
fluid and electrolytes via bulk flow and
thereby enable blood to be filtered.

4
> Barrier:

In the blood–brain barrier the 
tightly anchored endothelial cells
form a relatively impermeable 
barrier that protects the central nervous
system.

5
> Vasomotor tone:

Endothelium releases several
vasoactive substances
that are crucial in regulating
vascular smooth muscle tone.

It produce nitric oxide (NO)
from L-arginine in a reaction
catalysed by nitric oxide
synthetase.

NO activates guanylate cyclase
to produce cGMP, which
causes vasodilatation.

Endothelin-1 is another vasoactive
substance produced by endothelial
cells that causes vascular smooth muscle
vasoconstriction.

6
> Inflammation:

Vascular endothelium can synthesise several
prostaglandins with prostacyclin (PGI2) 
being the major derivative. 
PGI2 promotes vasodilatation 
and inhibits platelet adhesion 
and therefore when vascular endothelium 
is damaged (e.g. atherosclerotic plaques)
 these vessels become 
prone to vasospasm and thrombosis

7
> Coagulation:

Endothelial damage exposes
blood to tissue factor, which
initiates the activation of the extrinsic clotting cascade.

8
> Secretion:

Vascular endothelium,
especially that lining the lungs,

is rich in angiotensin-converting enzyme
(ACE) that catalyses the conversion
of angiotensin I to angiotensin II.

This forms an important step in the
renin– angiotensin–aldosterone system regulating blood pressure, sodium and water.