Tissue Fluid

Tissue fluid is a watery liquid distinct from the blood surrounding body cells. It brings nutrients and oxygen to the cells and removes wastes products.

The Circulatory System

To be able to understand what tissue fluid is, we need to look at the basics of the circulatory system. The most important structures of the circulatory system are:

• Arteries: carry blood away from the heart to the rest of the body.

• Arterioles: arterioles branch off arteries, and they feed blood into the capillaries.

• Capillaries: capillaries are the smallest blood vessels. They are the site of metabolic exchange.

• Venules: larger than capillaries but smaller than veins.

• Veins: carry blood from the body to the heart.

The blood is composed of white blood cells, red blood cells, platelets, and plasma. Plasma is a liquid that contains plasma proteins.

Now, let's look at the definition of a tissue fluid.

Tissue fluid is a fluid found in the spaces around cells, and it comes from substances that leak out of blood capillaries. Tissue fluid facilitates substance exchange between cells and the blood by helping bring oxygen and nutrients to cells, and removing waste products from them as well.

How is tissue fluid formed?

Tissue fluid is formed from blood plasma leaking out of capillaries via tiny holes in the capillary endothelium. Plasma proteins, however, cannot fit through capillaries, so they are absent from the tissue fluid. Besides transporting oxygen (O₂) and nutrients to cells, tissue fluid also removes waste products.

As the body has intrinsic mechanisms to regulate the composition of blood plasma (i.e. homeostasis), tissue fluid has a mostly constant environment. In other words, the composition of tissue fluid is stable without much variation.

Now, let's talk about the basics of the formation of tissue fluid.

In capillaries, tissue fluid is formed by the interaction between hydrostatic and osmotic pressures. Hydrostatic pressure is the pressure exerted by a fluid (ex. blood), whereas oncotic pressure is the osmotic pressure exerted by plasma proteins within a blood vessel.

• Hydrostatic pressure forces movement of fluid from the capillaries to the tissue at the arteriole end.
• Oncotic pressure (osmosis) draws fluid back into the capillaries from the tissue at the venule end.

Differences between tissue fluid and blood

Although the tissue fluid is formed from the blood plasma, both fluids differ in composition.

Tissue fluid and blood plasma have these in common:

• Solutes - glucose, amino acids, ions

• Gases - oxygen

• Metabolic wastes - water, carbon dioxide

Tissue fluid mainly lacks large plasma proteins (e.g., albumin) and red blood cells present in the blood plasma. These are too large to pass through the small fenestrations of the capillaries. The tissue fluid will still contain white blood cells, as their flexibility lets them squeeze through the fenestrations.

The functions of tissue fluid

Tissue fluid carries oxygen and nutrients to the cells, and waste away from the cells. By making new tissue fluid, old fluid drains towards lymph vessels. Upon entering lymph vessels, it is called lymph (or interstitial fluid).

Tissue fluid serves two functions in the body:

1. Bathes the tissues - tissue fluid can seep into tiny gaps between cells (i.e. enter intercellular spaces) where capillaries cannot reach.

2. Facilitates exchange of materials (e.g., glucose) between cells and the blood - tissue fluid allows contact between cells and the blood.

How are materials transferred between capillaries and tissue fluid?

Do you recall that tissue fluid is formed from plasma that leaks out of the capillary fenestrations? The volume of plasma fluid that leaves the capillary to form tissue fluid results from two opposing pressures - osmotic and hydrostatic pressures. Therefore, the transfer of materials between capillaries and tissue fluid undergoes pressure filtration. This process is when pressure differences push tissue fluid in and out of capillaries.

Osmotic versus hydrostatic pressures in pressure filtration

As osmotic and hydrostatic pressures influence the formation of tissue fluid, these are two critical concepts to grasp to understand the process of pressure filtration:

1. Osmotic (or oncotic) pressure - the tendency of pure water to enter a solution; this pressure causes tissue fluid higher in water potential to move into the capillary.

2. Hydrostatic pressure - pressure in the capillary caused by the preceding arteriole; this pressure causes blood plasma to move out of the capillary.

Pressure filtration process

The arteriole carries blood at high pressures, whereas the venule carries blood at low pressures. Refer to the article on blood vessels for more information.

The pressure filtration process is divided into two steps:

1. Plasma is pushed out from the capillaries at the arteriole end.

• The arteriole end has a high hydrostatic pressure due to the high-pressure blood flow.

• The osmotic pressure in the capillaries is high as more proteins reduce its water potential.

• But hydrostatic pressure > osmotic pressure, so overall plasma moves out of the capillary as tissue fluid, losing oxygen and nutrients.

2. Tissue fluid returns to the capillaries at the venule end.

• The venule end has a low hydrostatic pressure due to the low-pressure blood flow.

• As fluid is lost from the capillaries previously, the osmotic pressure in the capillaries is now higher.

• Osmotic pressure > hydrostatic pressure, so overall tissue fluid moves in, gaining carbon dioxide and metabolic wastes in the process.

Tissue fluid in the lymphatic system?

Tissue fluid that is not returned to the capillaries is carried back to the lymphatic system via lymph vessels.

The lymphatic system is separate from the circulatory system. Lymph vessels of the lymphatic system begin in the tissues; the region of the lymph vessels near tissues often have dead ends. Dead ends are ends that do not connect with any vessels or tissues. Multiple lymph vessels across different parts of the body then merge into larger vessels that form a network throughout the body.

When tissue fluid enters the lymphatic system, it becomes lymph. Therefore, lymph is similar in composition to tissue fluid, just a different name given as it is found in the lymphatic system.

Unlike the transport of blood where the heart is involved in pumping it across the body, the transport of lymph only relies on the contraction of the surrounding skeletal muscles. Hence, lymph flow is very slow. Through the thoracic duct, the largest lymph vessel, lymph travels at about 100 ${\mathrm{cm}}^3/\mathrm{h}$ in a resting human. Lymph vessels adapt to the slow lymph flow by having valves to prevent backflow. The action of lymph valves in preventing backflow is similar to that of heart valves, whereby they shut whenever lymph flows in the unintended direction. Lymph contents eventually enter the left and right subclavian veins found in your shoulders, back into the vena cava.

Apart from preventing the backflow of lymph, lymphatic valves are also large enough to allow plasma proteins to enter. The role of lymphatic valves in regulating plasma protein entry, which allows plasma proteins to enter and prevent plasma proteins from leaking out, is crucial for if the rate of protein loss from plasma is not in balance with the lymph, this can also lead to oedema.