10.7 Lymphatic System of Humans
The lymphatic system is an essential transport and defence system that works alongside the blood circulatory system. Every day, approximately 4 litres of fluid is filtered out from blood capillaries into the intercellular spaces and cannot be directly reabsorbed by blood capillaries. The lymphatic system collects this excess fluid, filters it, and returns it to the bloodstream, while also transporting lipids and defending the body against infections.
Learning Objectives
- Explain the formation of tissue fluid and lymph.
- Compare the composition of blood plasma, tissue fluid, and lymph.
- Identify the components of the lymphatic system and how lymph flows through lymphatic vessels.
- Describe the relationship between the lymphatic system and the blood circulatory system.
- Explain the necessity and functions of the lymphatic system in the human body.
Relationship Between Blood Capillaries, Tissue Fluid, and Lymphatic Vessels
Fluid leaves the arterial end of a capillary, bathes body cells as tissue fluid, partially returns at the venule end, and enters blind-ended lymphatic capillaries to become lymph.
Filtration
High hydrostatic pressure at the arterial end forces plasma into intercellular spaces to form tissue fluid.
Reabsorption
About 85% of the fluid returns to the venule capillaries by osmosis and diffusion.
Lymph formation
The remaining 15% enters blind-ended lymphatic capillaries and becomes lymph.
Short Explanation
Formation of tissue fluid and lymph
Blood arriving at the arterial capillary end is under high hydrostatic pressure, forcing plasma minus large proteins and blood cells into intercellular spaces to form tissue fluid. This fluid allows nutrient and gas exchange with cells. At the venule end, lower hydrostatic pressure and higher plasma osmotic pressure pull 85% of this fluid back into the blood. The remaining 15% enters closed-ended lymphatic capillaries as lymph.
Components of the lymphatic system
Lymphatic capillaries, lymphatic vessels with one-way valves, lymph nodes, spleen, thymus gland, bone marrow, tonsils, and appendix.
How lymph flows
Driven by muscle contractions, breathing movements, peristalsis, and arterial pulses—regulated by one-way valves—lymph flows through lymph nodes, where pathogens are filtered.
Return to blood
Lymph drains through the right lymphatic duct and thoracic duct into the right and left subclavian veins, returning fluid and absorbed lipids safely back into the blood circulatory system.
Body defence
Lymph nodes produce and store lymphocytes that produce antibodies to defend against pathogens.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Why does tissue fluid lack red blood cells, platelets, and large plasma proteins?
Which main lymphatic duct drains lymph from the lower limbs and left arm, and into which blood vessel does it empty?
What causes the number of lipid molecules in lymph to increase significantly after eating a meal rich in fats?
Common Mistakes
- Confusing the drainage areas of the two main lymphatic ducts: thinking the thoracic duct drains the whole body or confusing which subclavian vein connects to which duct.
- Stating that the lymphatic system has a pump similar to the heart; it does not have a pump and relies on skeletal muscle contractions, peristalsis, breathing pressure changes, arterial pulsation, and one-way valves.
- Thinking that all tissue fluid is reabsorbed by blood capillaries; forgetting that 15% (about 4 L daily) must be recovered by the lymphatic system.
Concept Misunderstandings
Lymph and blood plasma have identical compositions.
Lymph does not contain erythrocytes, platelets, or large plasma proteins, and it contains a higher concentration of fats and lymphocytes compared to tissue fluid.
Lymphatic capillaries form a continuous circular loop just like blood capillaries.
Lymphatic capillaries are blind-ended (dead-end tubes that are closed at one end) and collect fluid in one direction, draining towards the veins.
Tissue fluid is completely different from plasma.
Tissue fluid is derived directly from blood plasma filtered out under high pressure, containing water, dissolved nutrients, ions, and waste, but excluding cells and large proteins.
Summary
- High hydrostatic pressure at the arterial capillary end filters plasma into intercellular spaces, forming tissue fluid.
- 85% of tissue fluid returns to venule capillaries; 15% enters blind-ended lymphatic capillaries to become lymph.
- Lymph contains lymphocytes, water, solutes, and high levels of lipids, but lacks erythrocytes, platelets, and large plasma proteins.
- Lymph flow is driven by skeletal muscle contraction, respiratory pressure changes, peristalsis, and pulse, maintained in one direction by valves.
- Lymph from the right upper body empties via the right lymphatic duct into the right subclavian vein.
- Lymph from the rest of the body empties via the thoracic duct into the left subclavian vein.
- The lymphatic system functions in fluid balance, body defence, and lipid transport.
Short Activity
Objective: identify the formation of lymph, its components, and the drainage routes of the lymphatic system.
A. Objective Quiz
Which blood vessel receives lymph directly from the thoracic duct?
Which of the following components is found in lymph but is absent from tissue fluid in high amounts?
B. Fill in the Blanks
The specialised lymphatic capillaries found in the villi of the small intestine that absorb fats and fat-soluble substances are called .
Approximately percent of the fluid that leaves blood at the arteriole end diffuses back into the venule capillary, while the remainder enters lymphatic capillaries.
C. Matching / Drag and Drop
Drag each lymphatic system structure or vessel to its correct function or drainage region. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Structure to Its Role or Drainage Region
Write the correct structure next to each role or drainage region.
| Role / Drainage Region | Structure |
|---|---|
| 1. Receives lymph from the right arm, chest, and right side of head and neck | |
| 2. Receives lymph from the left upper body and all parts below the ribs | |
| 3. Produce and store lymphocytes involved in the production of antibodies | |
| 4. Prevent the backflow of lymph and ensure continuous flow towards the heart |
Answer choices: Right lymphatic duct • Thoracic duct • Lymph nodes • One-way valves
