- Tissue fluid is a substance used to transport nutrients and oxygen from the blood to the cells and carry waste (such as carbon dioxide) from the cells to the blood.
- The tissue fluid is formed when the blood at the arterial end of the capillary is under high hydrostatic pressure (due to the heart contracting) which pushes fluid through the tiny gaps in the capillary wall.
- The fluid that is pushed out of the gaps from the blood consists of plasma and dissolved nutrients and oxygen.
- All of the bigger cells (including red and white blood cells and platelets) and the plasma proteins remain in the blood because they are too big to fit through the small capillary holes.
- This fluid is known as tissue fluid and it surrounds the cells in order to diffuse and exchange oxygen, nutrients and carbon dioxide into or out of the cell.
- The water potential of the tissue fluid tends to be higher (less negative) than that of the blood, therefore water in the tissue fluid will flow back into the blood via osmosis.
- Also the tissue fluid is under some hydrostatic pressure as well, which will force the tissue fluid back into the capillaries.
The Lymphatic System
- Some tissue fluid is drained away into the lymphatic system instead of flowing back into the capillaries.
- The lymphatic system is made up of vessels that are similar to capillaries which start in the tissues and then drain the excess fluid into larger vessels that will eventually join the blood systems in the chest cavity.
- The lymphatic system is part of the immune system that protects the body from infection.
- Lymph is very similar to tissue fluid but with a few differences, one of which being that there is more lymphocytes in lymph.
- Lymphocytes are made in swellings found along the lymphatic system called lymph nodes.
- The lymph node's role is to filter bacteria and foreign material from the lymph fluid and the lymphocytes job is to then destroy and engulf the bacteria and particles.
Open Circulatory Systems
An open circulatory system is where the blood is not held in the blood vessels but instead it bathes the cells and tissues directly as it circulates through the body cavity.
Depending on the animal there are a couple of ways in which the blood is circulated through the cavity.
One way it's circulated is by the movement of the body muscles and the other is a pumping organ much like the heart.
Blood enters the 'heart' through pores and is pumped up the heart and then pours out into the body cavity at the other end nearest the head of the animal (usually insects).
Open circulatory systems works in small animals because the blood does not have far to travel around the body and also the carriage of gases occurs in a different system all together, they do not rely on blood for transport.
An open circulatory system would not be efficient in a large animal because the blood in an open system is under low pressure and is slow moving which would mean that lots of body parts would not receive sufficient amounts of oxygen and nutrients.
Closed Circulatory Systems
A closed circulatory system is where the blood stays entirely in blood vessels and tissue fluid bathes the cells and tissues.
The heart can then pump the blood at a higher pressure and thus allows the blood to flow more quickly.
This is efficient because it allows gases to be exchanged quicker and and quickly transport nutrients to where they are needed.
Some animals have a single circulatory system in which the blood only flows through the heart once during each circulation of the body.
Other animals (such as humans) have a double circulatory system in which the blood travels twice through the heart during each circulation of the body.