Capillary Types
General Overview of Capillaries
There are three distinct types of capillaries that students must be familiar with: continuous, fenestrated, and sinusoidal.
While structurally different, all three types share the primary physiological function of exchange between the blood and surrounding tissues.
Continuous Capillaries
Definition and Structure:
Continuous capillaries are the most common type of capillary in the human body.
They are characterized by endothelial cells that surround the lumen and are connected to one another via tight junctions.
These tight junctions are continuous throughout the capillary, though they occasionally feature small breaks or gaps.
Permeability Characteristics:
They are the least permeable of the three types.
Constituents of the blood plasma can exit through the occasional small breaks in the tight junctions.
Crucially, blood cells (formed elements) generally cannot pass through these tight junctions.
The Phenomenon of "Cupping":
The lumen of these capillaries often measures approximately in diameter.
A standard red blood cell measures approximately in diameter.
To fit through the smaller capillary space, red blood cells perform a behavior known as "cupping."
The biological advantage of cupping is that it causes the membrane of the red blood cell to rub directly along the surface of the capillary wall. This maximizes the available surface area for the diffusion of gases from the blood into the surrounding tissue.
Common Locations:
Skin.
Muscle tissue.
Blood-brain barrier (BBB).
Specialization (Blood-Brain Barrier):
At the blood-brain barrier, continuous capillaries are further reinforced by astrocyte cells.
Astrocytes send out processes that cover the capillary entirely.
These astrocyte processes are also "sewn together" via tight junctions.
The consequence of this structure is that nothing can pass from the blood into the cerebrospinal fluid without being actively transported through the cytosol of the cells.
Fenestrated Capillaries
Definition and Structure:
Fenestrated capillaries are substantially more permeable than continuous capillaries.
In addition to occasional breaks in the tight junctions, these capillaries possess "fenestrae," which are tiny pores or windows located within the endothelial cell membrane.
They also feature intercellular pores or clefts.
Function and Role in Secretion/Absorption:
These are primarily found in sites of active secretion or absorption.
The increased permeability allows for the movement of chemicals, toxins, or waste products into the bloodstream, or the filtration of chemicals out of it.
The pores allow for the passage of ions, very small particles, and small proteins.
Large elements like red blood cells are still restricted from exiting through these pores.
Common Locations:
Intestines (absorption).
Kidneys (filtration/secretion).
Endocrine glands (adding hormones to the bloodstream).
Basement Membrane:
Like continuous capillaries, fenestrated capillaries possess a basement membrane, which is a transparent layer composed of glycoproteins from both the capillary and the surrounding tissues.
Sinusoidal Capillaries
Definition and Structure:
Sinusoidal capillaries are the most permeable type of capillary.
They are characterized by "sinusoids," which are giant openings or gaps in the capillary wall.
The structure is so porous that entire epithelial cells may be missing from the wall, and tight junctions are almost entirely absent.
Functional Significance:
These capillaries allow for the passage of whole blood cells and very large proteins.
They allow large blood cells produced in the bone marrow, such as monocytes (identifiable by their U-shaped nuclei), to enter the circulation.
They allow megakaryocytes to send cytoplasmic extensions into the lumen to be sheared off (forming platelets).
They facilitate the entry or exit of large liver-produced proteins, such as angiotensinogen and various apolipoproteins like HDL (high-density lipoprotein) and LDL (low-density lipoprotein).
Common Locations and Specific Organ Functions:
Bone Marrow: Used to add newly created blood cells into general circulation.
Liver: Used to add large protein components into the bloodstream.
Spleen:
Blood cells exit circulation through the sinusoidal openings.
They are pushed through a filter of reticular tissue.
Older red blood cells that have lost their flexibility (the ability to "flex and bend and do the twist") are caught in this filter and destroyed.
These destroyed cells are then "gobbled up" by surrounding white blood cells.
Specialized Capillary Barriers and Filtration
Reinforcement Barriers:
Aside from the blood-brain barrier, there are several other locations where capillaries are reinforced with specialized cells to control exchange:
The Blood-thymus barrier.
The Blood-testes barrier.
The Kidney Filtration Membrane:
In the kidney, an additional epithelial cell called a podocyte helps form a specialized filtration membrane.
Podocytes have "fingers" or "foot processes" that interlace with the processes of adjacent podocytes.
The space between these interlocking fingers is filled with a thin membrane called a slit diaphragm.
Together, the foot processes and slit diaphragms create a restrictive filter for the blood passing through the kidney.