Anatomy and Physiology The lymphatic system

You will recall that when we looked at blood vessels we noted that the lymphatic system drained from the capillary bed. The hydrostatic and colloidal pressure operating in the capillary bed forces fluid into the venules. Some fluid remains behind in tissue spaces and this is called the interstitial fluid. It contains some metabolic waste products, such as carbon dioxide, and some proteins that were not pushed through into the veins. We produce about 3 litres of this fluid each day and clearly, it must be returned to the general circulatory system. This is done through the lymphatic system.

Lymphatic capillaries The smallest lymph vessels are lymphatic capillaries. Endothelial cells overlap to form a simple one way valve. Fluid enters through a gap in the valve. Bundles of fine collagen filaments anchor the endothelial cells to surrounding structures. Its like a swinging door in which the door can only open inwards. Fluid builds up on the outside and eventually the force pushes open the door and the fluid enters. This allows proteins to enter also. When the tissues are inflamed the opening becomes larger and cells and cellular debris can enter. This is both good and bad. Good because this is the way that we can trap and filter pathogens and bad because this is a route for cancer cells to spread.

Lymphatic trunks Lymph capillaries merge to form lymphatic vessels. These join to form lymphatic trunks. The lymphatic trunks merge into two lymphatic ducts. As lymphatic vessels become larger, the walls become endothelial plus collagen, elastin and smooth muscle. At six places in our body we have a major accumulation of lymph nodes. These are part of the lymphatic system which is dedicated to trapping and dealing with invading organisms. Special white blood cells are located in the lymph nodes ready to attack any invader.

Lymphatic ducts The lymphatic trunks converge into two lymphatic ducts. The right lymphatic duct drains the upper right quadrant and the thoracic duct drains the rest. These two ducts re-enter our vein on both sides of our body into the subclavian vein. Study the slide carefully to familiarise yourself with these points.

Forces that move fluid between compartments The forces that move fluid along the lymphatic system include capillary hydrostatic pressure (a nick in a hose) and Interstitial colloid osmotic (because of proteins in fluid). Note that if proteins move into interstitial space because of inflammation then more water is drawn from capillaries by osmosis.

Flow Lymph vessels have one way valves and pressure build up assists lymph flow. Lymph fluid is also propelled by skeletal muscle movement and breathing in (inspiration) helps draws fluid upwards. This is why people who are chronically confined to their bed often have swollen feet. The same reason applies if you travel for 24 hours on a jet flight and have little movement.

Lymphostasis This is a condition caused by stagnation of lymph. It is caused either by malformation or obstruction and results in lymphoedema. Retention of fluid can contribute to proliferation of fibroblasts and result in layers of elastic and collagen fibres being laid down.

Oedema from lymphatic obstruction Obstruction can be caused by malignant tumours, excessive Fibrinous inflammation surgery or a parasite that blocks the lymph vessels. Secondary lymphoedema can arise from treatment of breast cancer where removal of breast tissue results in obstruction of lymphatic system.

Activity Describe the structure and function of the lymphatic system.