The human circulatory system is one of the most fascinating systems in the body. It ensures that oxygen, nutrients, and hormones reach every cell while waste products are carried away. To understand this system, students must first learn the difference between veins and arteries, and then understand how blood flows through the heart step-by-step.
This article simplifies these concepts using clear explanations based on your uploaded image.
Veins vs Arteries: What’s the Difference?
Although both veins and arteries are blood vessels, they perform opposite functions.
1. Veins
Veins carry deoxygenated blood back to the heart.
(Memory Trick: Veins = Visit the heart)
Characteristics:
- Carry blood toward the heart
- Usually contain deoxygenated blood
- Have valves to prevent backflow
- Thin walls and wider lumen
Examples:
- Superior Vena Cava
- Inferior Vena Cava
- Pulmonary Vein (exception)
2. Arteries
Arteries carry oxygenated blood away from the heart.
(Memory Trick: Arteries = Away from the heart)
Characteristics:
- Carry blood away from the heart
- Mostly carry oxygenated blood
- Thick, muscular walls
- High pressure blood flow
Examples:
- Aorta
- Coronary Arteries
- Pulmonary Artery (exception)
3. The Only Exceptions
Two vessels break the general rule:
| Vessel | Type | Blood It Carries |
|---|---|---|
| Pulmonary Artery | Artery | Deoxygenated blood |
| Pulmonary Vein | Vein | Oxygenated blood |
Why?
Because the lungs are the only place where blood gets oxygenated—so vessels leading to and from them behave differently.
Blood Flow Through the Heart (Step-by-Step)
The heart works like a double pump:
- The right side pumps deoxygenated blood to the lungs
- The left side pumps oxygenated blood to the body
Below is the full pathway based on your diagram.
Right Side of the Heart (Deoxygenated Blood)
This side receives blood low in oxygen and sends it to the lungs.
Steps 1–6: Path of Deoxygenated Blood
1️⃣ Superior Vena Cava / Inferior Vena Cava
Bring deoxygenated blood from the body to the heart.
2️⃣ Right Atrium
Blood enters the first chamber on the right side.
3️⃣ Tricuspid Valve
Prevents backflow and allows blood to pass into the ventricle.
4️⃣ Right Ventricle
Pumps blood toward the lungs.
5️⃣ Pulmonary Valve
Opens to let blood flow into the pulmonary artery.
6️⃣ Pulmonary Artery
Carries deoxygenated blood to the lungs for oxygenation.
(Remember: This is the exception artery!)
Outcome:
Carries deoxygenated blood to the lungs.
Left Side of the Heart (Oxygenated Blood)
This side receives freshly oxygenated blood from the lungs and pumps it to the entire body.
Steps 7–12: Path of Oxygenated Blood
7️⃣ Pulmonary Vein
Brings oxygen-rich blood from lungs to heart.
(Exception vein!)
8️⃣ Left Atrium
The receiving chamber on the left side.
9️⃣ Bicuspid/Mitral Valve
Allows blood to pass into the left ventricle and prevents backflow.
🔟 Left Ventricle
The strongest chamber—pumps blood with high pressure.
1️⃣1️⃣ Aortic Valve
Opens to release blood into the aorta.
1️⃣2️⃣ Aorta
The largest artery that distributes oxygenated blood to the whole body.
Outcome:
Carries oxygenated blood to tissues and organs.
Complete Blood Flow Summary
Right Side → Lungs → Left Side → Body
1. Body → Right heart → LungsThis cycle repeats with every heartbeat.
Why Is This Sequence Important?
Understanding the flow of blood helps students:
- Learn how heart valves prevent backflow
- Understand oxygen exchange
- Recognize causes of heart diseases
- Visualize how circulation keeps the body alive
This foundational knowledge is essential for nursing, biology, NEET/medical prep, and physiology studies.
Quick Revision (Super Short)
- Veins = back to heart, usually deoxygenated
- Arteries = away from heart, usually oxygenated
- Exception: Pulmonary artery (deoxygenated), pulmonary vein (oxygenated)
- Right side → sends blood to lungs
- Left side → sends blood to body
- Right = deoxygenated
- Left = oxygenated
FAQs About Veins, Arteries & Heart Blood Flow
1. Why do arteries need thick walls?
Because they carry high-pressure blood pumped directly by the heart.
2. Why do veins have valves?
To prevent blood from flowing backward, especially against gravity.
3. What happens in the lungs?
Blood releases carbon dioxide and absorbs oxygen.
4. Which side of the heart is stronger?
The left ventricle, because it pumps blood to the entire body.
5. Is the pulmonary circuit different from the systemic circuit?
Yes:
- Pulmonary → heart ↔ lungs
- Systemic → heart ↔ rest of the body
6. Why is blood flow taught before ECG or cardiac diseases?
Because understanding normal flow is essential before learning abnormalities.
