IB Biology Sub-topic B3.2 Notes

Tissue fluid

In the SL syllabus, you learned the basics of blood vessels and cardiovascular disease. However, in the HL syllabus you are expected to know much more detail about blood flow in capillaries, the circulatory system, and the heart. Let's start with capillary networks, which provide an efficient distribution route for delivery of blood. Each capillary exchanges fluid with adjacent tissue to supply cells with fresh oxygen and nutrients and drive out carbon dioxide and waste. However, it is the tissue fluid that circulates between capillaries and cells that delivers these substances directly to and from the cell membranes.

To perform this, tissue fluid is subjected to a number of pressure and concentration gradients. At the arteriolar end of a capillary bed:

1. Arriving plasma is high in oxygen, glucose, and amino acids and low in carbon dioxide.
2. High hydrostatic pressure forces fluid out of the capillary and into the tissue fluid. 
3. The solute concentration of this tissue fluid is high since it is rich in water, mineral ions, glucose, amino acids and oxygen. It has a similar composition to plasma, without blood cells.
4. This fluid bathes the cells of the tissues to supply them with the necessary nutrients.

At the venous end:

1. The blood vessel has a reduced hydrostatic pressure and solute concentration due to movement into the tissue at the arteriolar end.
2. Here, water, CO₂ and waste products that have been exported out of cells are reabsorbed into the bloodstream. The returning is thus low in oxygen, glucose, and amino acids, and high in carbon dioxide and waste.

The returning plasma fluid accounts for 90% of the original leaked fluid. The remaining 10% is collected by the lymph vessels and returned to the circulation near the heart. Let's cover how this occurs.

Lymph fluid

The primary role of the lymphatic system is to transport lymphocytes around the body. They just have a network of lymph vessels equally extensive as the circulatory system. However, they require a fluid circulation for transport to occur.

Thus, lymph capillaries are found between cells where they are held in place by collagen filaments attached to connective tissue. Tissue fluid that leaks from the blood vessels moves into lymph capillaries through gaps between thin cells, forming lymph fluid. As a result, lymph fluid has a similar composition to tissue fluid but contains lymphocytes due to its role in the immune system.

The lymph capillaries merge first into pre-collector and then collector lymphatic vessels. These vessels possess valves to ensure unidirectional flow to the lymph nodes, where the lymph fluid collects from all over the body. This forms a network of tissues, organs and vessels that accumulates tissue fluid that has leaked out of blood vessels. This is finally drained into the subclavian vein to return to the heart.