Tissue fluid

Tissue Fluid and Edema

Tissue Fluid Functions Formation Filtration Reabsorption Edema Types Pitting & Non-pitting

1 Definition of Tissue Fluid

Tissue fluid or interstitial fluid is the medium in which cells are bathed.

ECF:

It forms about 20% of ECF.

2 Functions of Tissue Fluid

  • Because of the capillary membrane, there is no direct contact between blood and cells.
  • Tissue fluid acts as a medium for exchange of various substances between cells and blood in the capillary loop.

Exchange of Substances

Oxygen and Nutritive Substances

Diffuse from the arterial end of capillary → through tissue fluid → reach the cells.

Carbon Dioxide and Waste Materials

Diffuse from cells → into the venous end of capillary → through tissue fluid.

3 Formation of Tissue Fluid

Formation of tissue fluid involves 2 processes:

  1. Filtration
  2. Reabsorption

4 Filtration

  • Tissue fluid is formed by the process of filtration.
  • Normally, blood pressure, also called hydrostatic pressure, at the arterial end of the capillary is about 30 mm Hg.
  • This hydrostatic pressure is the driving force for filtration of:
    • Water
    • Other substances
  • These are filtered from blood into tissue spaces.

Starling Hypothesis and Starling Forces

Starling hypothesis states that the net filtration through the capillary membrane is proportional to:

  • Hydrostatic pressure difference across the membrane
  • Minus oncotic pressure difference.

These pressures are called Starling forces.

Determination of net filtration pressure is based on Starling hypothesis.

5 Reabsorption

  • Fluid filtered at the arterial end of capillaries is reabsorbed back into blood at the venous end of capillaries.
  • The pressure gradient plays an important role.

At the Venous End of Capillaries

15 mm Hg Hydrostatic pressure
25 mm Hg Oncotic pressure
10 mm Hg Pressure gradient
  • Due to the pressure gradient of 10 mm Hg, the fluid is reabsorbed along with waste materials from the tissue fluid into the capillaries.
  • About 10% of filtered fluid enters the lymphatic vessels.

Filtration at the arterial end of capillaries → helps in formation of tissue fluid.

Reabsorption at the venous end → helps to maintain volume of tissue fluid.

6 Formation of Tissue Fluid — Pressure Values

Arterial End of Capillary

30 mm Hg Capillary blood pressure
25 mm Hg Oncotic pressure
05 mm Hg Pressure gradient
  • Filtration occurs.
  • There is no filtration of plasma proteins.

Venous End of Capillary

15 mm Hg Capillary blood pressure
25 mm Hg Oncotic pressure
10 mm Hg Pressure gradient
  • Reabsorption occurs.
  • Plasma proteins remain inside the blood capillary because the capillary membrane is not permeable to plasma proteins.

7 Applied Physiology: Edema

Definition of Edema

Edema is defined as the swelling caused by excess accumulation of fluid in tissues.

It may be:

Generalized
Local

Generalized Edema

Edema that involves the entire body is called generalized edema.

Local Edema

Edema that occurs in specific areas of the body is called local edema.

Specific areas include:

  • Abdomen
  • Lungs
  • Extremities such as feet, ankles and legs.
Accumulation of fluid may be inside or outside the cell.

8 Types of Edema

Edema is classified into 2 types depending upon the body fluid compartment where accumulation of excess fluid occurs:

1. Intracellular Edema

Accumulation of fluid inside the cell.

2. Extracellular Edema

Accumulation of fluid outside the cell.

1. Intracellular Edema

Intracellular edema is the accumulation of fluid inside the cell.

It occurs because of 3 reasons:

  1. Edema due to malnutrition
  2. Edema due to poor metabolism
  3. Edema due to inflammation of the tissues

2. Extracellular Edema

Extracellular edema is defined as accumulation of fluid outside the cell.

It occurs because of:

  1. Abnormal leakage of fluid from capillaries into interstitial space.
  2. Obstruction of lymphatic vessels that prevents return of fluid from interstitial fluid back into blood.

Common Conditions Resulting in Extracellular Edema

1. Edema due to right-sided heart failure

→ Peripheral edema

2. Edema due to left-sided heart failure

→ Pulmonary edema

3. Edema due to renal disease

→ Generalized edema

4. Edema due to decreased amount of plasma proteins

→ Peripheral edema

5. Edema due to lymphatic obstruction

→ Lymphedema

6. Edema due to increased endothelial permeability

→ Peripheral edema

9 Pitting and Non-pitting Edema

  • Interstitial fluid is present in the form of a gel that is almost like a semisolid substance.
  • This is because interstitial fluid is not present in fluid form, but is bound in a proteoglycan meshwork.
  • It does not allow any free space for the movement of fluid.

Pitting Edema

  • When interstitial fluid volume increases, most of the fluid becomes free fluid that is not bound to proteoglycan meshwork.
  • It flows freely through tissue spaces.
  • This produces a swelling called edema.
  • This type of edema is known as pitting edema.

On Pressing with Finger

  1. Displacement of fluid occurs.
  2. A depression or pit is produced.
  3. When the finger is removed:
    • The pit remains for a few seconds.
    • Sometimes it remains for as long as 1 minute.
  4. This continues till the fluid flows back into that area.

Non-pitting Edema

  • Edema also develops due to:
    • Swelling of the cells
    • Clotting of interstitial fluid in the presence of fibrinogen
  • This is called non-pitting edema.
  • It is hard.
  • A pit is not formed while pressing.
Tissue Fluid and Edema