grade 8 Blood Circulation

Blood Circulation

The heart beats about 100,000 times a day and pumps blood through the body. Pulmonary circulation goes from the right ventricle to the lungs for oxygen, and systemic circulation from the left ventricle delivers oxygen and nutrients everywhere. Arteries have thick walls, capillaries exchange materials, and veins have valves that stop backflow. Follow the four chambers and both circulation routes and the kinds of vessels line up.

The Heart Is a Pump for Blood

❤️ A Pump That Beats 100,000 Times a Day

①The heart has four chambers (LA, LV, RA, RV)

②The left ventricle has the thickest muscle — it pumps blood to the entire body

③Blood delivers O₂ and nutrients to cells and removes CO₂ and waste

Heart and Circulation Routes

Pulmonary Circulation

Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium

Venous blood (CO₂↑) gains O₂ in the lungs and becomes arterial

Systemic Circulation

Left ventricle → aorta → body → vena cava → right atrium

Arterial blood (O₂↑) delivers O₂ to cells, becoming venous (CO₂↑)

Types of Blood Vessels

💡 Vessel Comparison

①Artery: thick, elastic wall to withstand high pressure

②Capillary: extremely thin wall (one cell thick) — material exchange

③Vein: thin wall with valves to prevent backflow

Composition of Blood

Blood = Plasma + Cells

Plasma (55%) + RBCs + WBCs + platelets

Plasma: nutrient/waste transport / RBC: O₂ transport / WBC: immunity / Platelets: clotting

Summary

Blood-Circulation Core

Pulmonary: RV→lungs→LA / Systemic: LV→body→RA

All four chambers contract & relax in concert, never resting

🎯 Exam Key Points

①Pulmonary: RV→lungs→LA (gas exchange)

②Systemic: LV→body→RA (material delivery)

③Arterial blood ≠ artery! (the pulmonary artery carries venous blood)

④Left ventricle is thickest because it must pump to the whole body

⑤Material exchange actually happens at capillaries

Examples and Unit Test

Example 1

Which route does blood from the right ventricle take to reach the left atrium? (pulmonary circulation)

Blood leaving the right ventricle passes through the pulmonary artery into the lungs.

In the lungs it releases CO₂ and takes in O₂, then returns to the left atrium through the pulmonary vein.

▸ Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium (venous blood becomes arterial)

In pulmonary circulation, venous blood takes in oxygen and becomes arterial blood.

Example 2

Is the blood in the pulmonary artery arterial or venous, and why?

The names artery and vein depend on whether blood flows out of the heart, so the pulmonary artery carries blood out of the right ventricle.

But the blood inside it has just returned from the body and is rich in CO₂, so it is venous blood.

▸ The pulmonary artery is an artery, yet the blood inside it is venous (high CO₂)

The vessel name (artery or vein) and the blood type (arterial or venous) are separate ideas.

Unit Test

Which statement about blood circulation is correct?

①Pulmonary circulation starts at the left ventricle

②Arterial blood flows in the pulmonary artery

③Systemic circulation starts at the left ventricle and returns to the right atrium

④Capillaries have thick walls that aid material exchange

⑤Red blood cells are in charge of immunity

▸ ③ Systemic circulation starts at the left ventricle and returns to the right atrium

Pulmonary circulation starts at the right ventricle (① wrong), and venous blood flows in the pulmonary artery (② wrong).

Capillaries have very thin walls that ease exchange (④ wrong), and white blood cells handle immunity while red cells carry O₂ (⑤ wrong); systemic circulation runs left ventricle to body to right atrium (③ correct).

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