Life Processes · Lesson 10 of 13
The Tubes — Blood Vessels
“Arteries deliver, veins return and capillaries handle the handover.”
• Compare arteries, veins and capillaries. • Relate vessel-wall structure to pressure and function. • Explain exchange between blood and tissues. • Describe the role of platelets in clotting. • Explain lymph formation and return. • Describe the transport of absorbed fat by lymph.
A pump is useful only when connected to a suitable network. Human blood vessels must withstand high pressure near the heart, return blood at lower pressure, and become thin enough for exchange at tissues.
Arteries, Veins And Capillaries
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Direction | Away from heart | Toward heart | Connect small arteries and veins within tissues |
| Pressure | Relatively high | Relatively low | Reduced |
| Wall | Thick and elastic | Thinner | One cell thick |
| Valves | Generally absent | Present in many veins | Absent |
| Main role | Distribute blood | Return blood | Exchange materials |
Arteries receive blood driven directly by ventricular contraction, so their thick elastic walls stretch and recoil. Veins return blood after much of the pressure has been lost; valves prevent backward flow. Arteries branch repeatedly into capillaries. Their extremely thin walls bring blood close to cells and permit exchange of oxygen, nutrients, carbon dioxide and wastes.
Arteries are defined by carrying blood away from the heart, not by always carrying oxygenated blood. The pulmonary artery carries deoxygenated blood to the lungs. Veins carry blood toward the heart; pulmonary veins carry oxygenated blood.
Maintenance By Platelets
A break in a blood vessel causes blood loss and reduces pressure. Platelets help initiate clot formation at the damaged site. The clot plugs the leak, limits blood loss and helps preserve the closed transport system while repair occurs.
Platelets are small blood components that help form clots and seal damaged blood vessels.
Lymph
Some plasma, proteins and blood cells leave capillaries and enter spaces between tissue cells, forming tissue fluid or lymph. Lymph is colourless and contains less protein than blood plasma. It enters lymphatic capillaries, passes into larger lymph vessels and ultimately returns to large veins.
Lymph is the colourless tissue fluid collected by lymphatic vessels and returned to the bloodstream.
Lymph returns excess fluid from tissues, helping prevent persistent swelling. It also carries absorbed fats from the intestine. Thus, lymphatic transport complements blood circulation rather than replacing it.
Problem
A vessel has a thick elastic wall and carries blood under high pressure away from the heart. Identify it.
- 1.High pressure requires a strong elastic wall.
- 2.The direction is away from the heart.
- 3.Both features identify an artery.
Problem
Why would a one-cell-thick wall be unsuitable for a major artery but ideal for a capillary?
- 1.A major artery must withstand strong pulsatile pressure, so a one-cell wall could rupture.
- 2.A capillary experiences lower pressure.
- 3.Its main function is exchange rather than bulk transport.
- 4.A one-cell wall shortens diffusion distance and improves exchange.
Problem
Predict the effect if lymphatic vessels in one limb are blocked.
- 1.Tissue fluid continues to leave blood capillaries.
- 2.Blocked lymphatic vessels cannot return all excess fluid.
- 3.Fluid accumulates in intercellular spaces.
- 4.The limb may become swollen, and normal tissue-fluid balance is disturbed.
Quiz
Which vessels usually contain valves?
Why are capillary walls thin?
What is a main function of platelets?
Where does lymph ultimately return?
Which statement about arteries is correct?
Practice Problems
- Compare arteries, veins and capillaries in structure, pressure and function.
- Explain why the pulmonary artery is still classified as an artery.
- Trace oxygen from blood in a capillary into a tissue cell and carbon dioxide in the opposite direction.
- Explain how clotting preserves both blood volume and transport pressure.
- Describe how lymph forms, what it transports and how it returns to blood.
Key Takeaways
• Arteries carry blood away from the heart under relatively high pressure. • Veins return blood and often contain valves. • Capillaries have one-cell-thick walls for exchange. • Vessel structure matches pressure and function. • Platelets help seal leaks by clotting. • Lymph forms from tissue fluid and returns excess fluid to blood. • Lymph carries absorbed fat from the intestine.