🏗️ Structure of Arteries
Arteries are made of three layers that give them strength and flexibility:
- Tunica Externa (Outer Layer)
- Made of elastic and connective tissue.
- Provides strength and protection.
- Like the outer cover of a pipe.
- Tunica Media (Middle Layer)
- Thickest layer, full of muscles and elastic fibers.
- This allows the artery to stretch and contract with each heartbeat.
- Handles high pressure when the heart pumps blood.
- Tunica Intima (Inner Layer)
- A smooth, thin lining of endothelial cells.
- Helps blood flow easily without friction.
🧪 What is Lumen?
- The lumen is the central space through which blood flows.
- In arteries, the lumen is narrower to maintain high pressure.
🔁 Arterioles – The Small Branches
- Arteries branch into smaller tubes called arterioles.
- Arterioles are like smaller streets from the main highway.
- They then branch into capillaries, the tiniest vessels.
2. Capillaries – The Exchange Points
📌 What are capillaries?
Capillaries are the smallest blood vessels in your body. They connect arterioles and venules and reach every cell of your body.
💡 Real-life example: Think of capillaries like small delivery men who reach the door of every house (cell) to deliver oxygen and collect waste.
🧱 Structure of Capillaries
- Walls are made of only one thin layer of cells: endothelial cells.
- Diameter: About 8 micrometers—so small that red blood cells pass one at a time!
🔁 Functions: Exchange of Materials
The main job of capillaries is to exchange substances between blood and body cells.
This happens through:
- Diffusion
- Molecules move from high concentration to low concentration.
- E.g., Oxygen moves from blood → cells; Carbon dioxide from cells → blood.
- Active Transport
- When substances move against the gradient, with energy.
- Endocytosis & Exocytosis
- Endocytosis: Cells take in large molecules.
- Exocytosis: Cells release substances outside.
- Helps move large materials like proteins or hormones.
💧 Interstitial Fluid – Tissue Bath
- As blood flows in capillaries, some fluid leaks out.
- This fluid is called interstitial fluid or tissue fluid.
- It contains:
- Water
- Nutrients
- Hormones
- Gases
- Small proteins
- It bathes cells, allowing them to absorb what they need and release waste.
⚠️ Note:
- Big proteins, red blood cells, and platelets do not leave the capillaries.
- But white blood cells can squeeze out to fight infections.
💡 Example: Imagine a sponge soaked in nutrient-rich water—that’s how your body cells live in interstitial fluid.
3. Veins – The Return Roads