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The Microcirculation & Lymphatic System

Blood Vessel Structure & Function

The major types of blood vessels are

Arteries

The large distributing vessels that bring blood to the body

Arterioles bring blood to the capillaries

Capillaries

The tiny vessels that distribute blood to the cells

Venules drain blood from the capillaries

Veins

The large collecting vessels that bring blood back to the heart

Capillary Beds

A capillary bed is a network of the body’s smallest vessels that run throughout almost all tissues, especially the loose connective tissue

This flow is also called a microcirculation

Structure of the Microcirculation

Capillary System

• Metarterioles

• Precapillary sphincter

• True capillaries

The microcirculation

This is transport of nutrients to the tissues and removal of cell excreta.

The small arterioles control blood flow to each tissue area

local conditions in the tissues in turn control the diameters of the arterioles.

Each tissue controls its own blood flow in relation to its individual needs

The walls of the capillaries are thin, constructed of single-layer, highly permeable endothelial cells

Therefore, water, cell nutrients, and cell excreta can all interchange quickly and easily between the tissues and the circulating blood

Structure of the Microcirculation and Capillary System

Each artery entering an organ branches six to eight times before the arteries become small enough to be called arterioles

Then the arterioles themselves branch two to five times, reaching diameters of 5 to 9 micrometers at their ends where they supply blood to the capillaries.

Metarteriole and precapillary sphincter

The metarterioles do not have a continuous muscular coat, but smooth muscle fibers encircle the vessel at intermittent points

At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary

This is called theprecapillary sphincter

This precapillary sphincter. can open and close the entrance to the capillary

Capillary Beds

When the precapillary sphincters are relaxed, blood flows through the true capillaries and takes part in exchanges with tissue cells

Capillary Beds

When the precapillary sphincters are contracted, blood flows through the shunts and bypasses the tissue cells

Arterioles

The diameter of each arteriole is regulated in two ways:

Local factors in the tissues signal the smooth musculature to contract or relax, thus regulating the amount of blood sent downstream to each capillary bed

Sympathetic nervous system adjusts the diameter of arterioles throughout the body to regulate systemic blood pressure

Structure of the Capillary Wall

The wall is composed of endothelial cells and is surrounded by basement membrane

The total thickness of the capillary wall is only about 0.5 micrometer

The internal diameter of the capillary barely large enough for red blood cells and other blood cells to squeeze through

Two passageways connecting the interior of the capillary with the exterior

1. intercellular cleft

2. plasmalemmal vesicles

The cleft normally with a width of about 10 nanometers, slightly smaller than the diameter of an albumin protein molecule

The rate of thermal motion of water molecules, ions and small solutes is so rapid that all of these diffuse with ease between the interior and exterior of the capillaries through these the intercellular clefts

Pores" in the Capillary Membrane

Flow of Blood in the Capillaries-Vasomotion

Blood usually flows intermittently, turning on and off every few seconds or minutes

The cause of this intermittency is the phenomenon called vasomotion, which means intermittent contraction of the metarterioles and precapillary

Exchange of water and substances between the blood and interstitial fluid

The most important means by which substances are transferred between the plasma and the interstitial fluid is diffusion

Diffusion results from thermal motion of the water molecules and dissolved substances in the fluid

Lipid-Soluble Substances Can Diffuse Directly Through the Cell Membranes of the Capillary Endothelium

If a substance is lipid soluble, it can diffuse directly through the cell membranes of the capillary without having to go through the pores.

Such substances include oxygen and carbon dioxide

Non-Lipid-Soluble Substances Diffuse Only Through Intercellular "Pores" in the Capillary Membrane

Oxygen, CO2, small solutes, nutrients move across capillaries primarily through diffusion

Diffusion through Capillary Membrane

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Molecular Weight (Daltons)

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Water

Albumin

The relative permeability of skeletal muscle capillary pores to different sized molecules substances through capillary membrane decreases roughly with the molecular size

NaCl

Urea

Glucose

Sucrose

Inulin

The capillaries in different tissues have extreme differences in their permeabilities

The membrane of the liver capillary sinusoids is so permeable that even plasma proteins pass freely through these walls, almost as easily as water and other substances

The membrane of the renal glomerular is permeable to water and electrolytes, but this is not true for the plasma proteins

Capillaries are grouped according to their “leakiness”

Most common intestine, kidneyLiver sinusoids

10-15 nm holes 50-80 nm holes 100-1000 nm holes

The Interstitium and Interstitial Fluid

The spaces between cells are called the interstitium

The fluid in these spaces is the interstitial fluid

It contains two major types of solid structures:

(1) collagen fiber bundles

(2) proteoglycan filaments

The collagen fiber bundles are extremely strong and therefore provide most of the tensional strength of the tissues

The proteoglycan filaments, however, are extremely thin coiled molecules

Fluid filtration across capillaries is determined by hydrostatic and colloid osmotic pressure

The hydrostatic pressure in the capillaries tends to force fluid through the capillary pores into the interstitial spaces

Osmotic pressure caused by the plasma proteins (called colloid osmotic pressure) tends to cause fluid movement by osmosis from the interstitial spaces into the blood

Also important is the lymphatic system, which returns to the circulation the small amounts of excess protein and fluid that leak from the blood into the interstitial spaces

Four Primary Hydrostatic and Colloid Osmotic Forces Determine Fluid Movement Through the Capillary Membrane

Starling forces are:

The capillary pressure (Pc), which tends to force fluid outward through the capillary membrane

The interstitial fluid pressure (Pif), which tends to force fluid inward through the capillary membrane when Pif is positive but outward when Pif is negative.

The capillary plasma colloid osmotic pressure (Πp), which tends to cause osmosis of fluid inward through the capillary membrane.

The interstitial fluid colloid osmotic pressure (Πif), which tends to cause osmosis of fluid outward through the capillary membrane

If the sum of these forces, the net filtration pressure, is positive, there will be a net fluid filtration across the capillaries

If the sum of the Starling forces is negative, there will be a net fluid absorptionfrom the interstitial spaces into the capillaries. The net filtration pressure (NFP) is calculated as:

NFP = Pc – Pif – Πp + Πif

The rate of fluid filtration in a tissue is also determined by the capillary filtration coefficient (Kf) which is a measure of the capacity of the capillary membranes to filter water for a given NFP and is usually expressed as ml/min per mm Hg net filtration pressure

The rate of capillary fluid filtration is therefore determined as:Filtration = Kf X NFP