🫀 Circulatory Pathways in Humans – How Blood Travels in the Body

Imagine your body is a delivery system, and blood is the delivery truck. It carries oxygen, nutrients, waste materials, and hormones to and from all parts of the body.

Now, to deliver things properly, this blood has to follow certain routes or pathways, just like trucks follow highways or streets to deliver goods.

In humans, there are two main circulatory routes:

1. Pulmonary Circulation – for the lungs
2. Systemic Circulation – for the rest of the body

Let’s explore each one in detail.

🌬️ 1. Pulmonary Circulation – Blood to the Lungs and Back

This pathway is only concerned with sending blood to the lungs to pick up oxygen and get rid of carbon dioxide.

Let’s follow the journey:

1. Deoxygenated blood (blood with less oxygen and more carbon dioxide) comes from the body and enters the right atrium of the heart.
2. From there, it moves into the right ventricle.
3. The right ventricle pumps this blood into the pulmonary trunk.
4. The pulmonary trunk splits into two arteries:
   • Right pulmonary artery → goes to the right lung
   • Left pulmonary artery → goes to the left lung

💡 Note: These are the only arteries in the body that carry deoxygenated blood.

5. Inside the lungs:
   • The pulmonary arteries become smaller vessels (called arterioles), and finally become capillaries.
   • These lung capillaries surround the air sacs (alveoli).
   • Gas exchange takes place here:
     • CO₂ is released from the blood
     • O₂ is absorbed into the blood
6. Now the blood is oxygenated.
7. This oxygen-rich blood is collected by pulmonary venules, which join to form pulmonary veins.
8. These pulmonary veins carry oxygenated blood back to the heart, entering the left atrium.

💡 Note: These are the only veins in the body that carry oxygenated blood.

🟢 Purpose of Pulmonary Circulation:
To bring oxygen-poor blood to the lungs and return oxygen-rich blood to the heart.

🌍 2. Systemic Circulation – Blood to the Body and Back

Now that oxygenated blood has entered the left atrium, it must be pumped to all the body tissues.

Let’s trace the systemic circulation:

1. Oxygenated blood moves from the left atrium to the left ventricle.
2. The left ventricle pumps this blood into the aorta, the largest artery in the body.
3. From the aorta, blood travels through arteries → arterioles → capillaries, reaching every organ, muscle, bone, and tissue.
4. In capillaries, blood:
   • Delivers oxygen and nutrients
   • Picks up carbon dioxide and waste
5. The now deoxygenated blood moves into venules, then into veins, and finally returns to the right atrium of the heart via two large veins:
   • Superior vena cava – from the upper body
   • Inferior vena cava – from the lower body