CORE Biology (9221) | Circulation

Welcome to the Circulation Chapter!

Hello future biologists! This chapter is all about how your body runs its incredible transport system. Think of your circulatory system as the ultimate delivery service, making sure every single cell gets what it needs to survive and, most importantly, create energy.

The study of energy production is called Bioenergetics, and it needs two crucial ingredients: Oxygen and Glucose. The circulatory system is the network that rushes these ingredients to your cells and carries the waste away. Let’s dive into the fascinating world of the heart, blood, and blood vessels!

A Quick Analogy: The City's Infrastructure

• The Heart is the central water pump.
• Blood is the water carrying supplies.
• Blood Vessels are the pipes and roads.

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Section 1: The Components of Blood

Blood isn't just red liquid! It is a complex tissue made up of four main components. If you spin blood down in a centrifuge, you can see these parts separate clearly.

The Four Key Parts of Blood

Imagine blood is a vehicle carrying cargo (oxygen/glucose) and passengers (defence cells).

1. Plasma (The Liquid Driver/Medium)

Plasma is the straw-coloured liquid that makes up over half of your blood (about 55%).

• Function: It is the medium that transports everything!
• It carries water, digested food molecules (like glucose), carbon dioxide (waste), hormones, and heat energy.

2. Red Blood Cells (RBCs) (The Oxygen Delivery Truck)

These cells are the most numerous and give blood its red colour. They are highly specialised for their job.

• Function: Transporting oxygen from the lungs to all the cells in the body.
• They contain a red pigment called haemoglobin, which binds reversibly with oxygen.
• They have a special biconcave disc shape to increase the surface area for oxygen uptake.
• They have no nucleus, allowing more space for haemoglobin. (Don't worry if this seems tricky; just remember: No nucleus = more oxygen!)

3. White Blood Cells (WBCs) (The Defence System)

These are the body's immune system fighters, much larger than RBCs.

• Function: Protecting the body from disease caused by pathogens (like bacteria and viruses).
• They can change shape, allowing them to squeeze out of blood vessels to reach areas of infection.

4. Platelets (The Repair Crew)

These are tiny fragments of cells, not full cells themselves.

• Function: Involved in blood clotting. When a blood vessel is cut, platelets clump together to form a barrier, preventing excessive blood loss and stopping pathogens from entering.

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Section 2: Blood Vessels – The Road Network

There are three main types of blood vessels. Each type has a structure perfectly suited to its specific job in the system.

1. Arteries (The High-Pressure Motorways)

Arteries carry blood AWAY from the heart.

• Direction: Always Away from the heart.
• Walls: Very thick and muscular, and also elastic. This is essential to withstand the very high pressure created when the heart pumps.
• Lumen (Hole): Narrow lumen.
• Analogy: They are the strong, thick main motorways carrying fast-moving traffic.

Did you know? You can feel your pulse because the thick, elastic walls of the artery stretch and recoil as the blood surges through after each heart beat.

2. Veins (The Low-Pressure Return Roads)

Veins carry blood TO the heart.

• Direction: Always To the heart.
• Walls: Much thinner walls, with less muscle, because the blood pressure is very low here.
• Lumen: Wide lumen.
• Valves: Veins contain valves. Because the pressure is low, gravity or muscle movement can cause blood to flow backwards. The valves prevent this backflow, ensuring blood only moves towards the heart.

Memory Aid: Artery = Away. Vein = ReVerse (backwards).

3. Capillaries (The Exchange Side Streets)

These are the smallest and most numerous vessels. They form a vast network that reaches every single cell in the body.

• Function: This is where the actual exchange happens—oxygen and glucose diffuse out, and carbon dioxide and urea (waste) diffuse in.
• Walls: Their walls are only one cell thick. This thin wall creates a very short diffusion distance, making the exchange of substances fast and efficient.
• Size: So narrow that red blood cells must pass through in single file.

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Section 3: The Heart – The Pump

The heart is a powerful, muscular organ, constantly pumping blood around the body. It works non-stop to ensure oxygen and glucose reach the cells for aerobic respiration (energy release).

The Basic Structure of the Human Heart

The heart is divided into four chambers: two Atria (receiving chambers) and two Ventricles (pumping chambers).

• Atria (Singular: Atrium): The smaller, upper chambers that receive blood entering the heart.
• Ventricles: The larger, muscular lower chambers that pump blood out of the heart.

Crucial Fact: The heart is split into a Right Side and a Left Side, which are completely separate.

1. The Right Side (Handling Deoxygenated Blood)

This side receives deoxygenated (oxygen-poor) blood returning from the body.

Path: Body $\rightarrow$ Right Atrium $\rightarrow$ Right Ventricle $\rightarrow$ Lungs (to pick up oxygen).

2. The Left Side (Handling Oxygenated Blood)

This side receives oxygenated (oxygen-rich) blood returning from the lungs.

Path: Lungs $\rightarrow$ Left Atrium $\rightarrow$ Left Ventricle $\rightarrow$ Body (to deliver oxygen).

Why is the Left Ventricle thicker?

You will notice the muscular wall of the Left Ventricle is much thicker than the Right Ventricle.

• The Right Ventricle only needs to pump blood a short distance (to the nearby lungs).
• The Left Ventricle must generate enough force to pump blood all the way around the entire body—this requires a much more powerful muscle!

The Importance of Valves

Just like in veins, the heart contains valves between the atria and ventricles, and where the major arteries leave the heart.

Function of Valves: To ensure blood flows in the correct direction and prevent it from flowing backwards when the heart muscles contract.

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Section 4: The Double Circulatory System

Humans (and all mammals) have a double circulatory system. This means blood passes through the heart twice for every complete circuit of the body.

What is the Double System?

It is two separate loops running side-by-side:

1. The Pulmonary Circuit (The Lung Loop)

This circuit moves blood between the heart and the lungs.

• Deoxygenated blood leaves the Right Ventricle and goes to the lungs.
• In the lungs, it drops off CO\(_2\) and picks up O\(_2\).
• Oxygenated blood returns to the Left Atrium.

2. The Systemic Circuit (The Body Loop)

This circuit moves blood between the heart and the rest of the body (all organs, muscles, and tissues).

• Oxygenated blood leaves the Left Ventricle and goes to the body cells (delivering O\(_2\) and glucose).
• Deoxygenated blood returns to the Right Atrium.

Why is the Double System Better?

The double system is highly efficient, which is crucial for mammals that need a lot of energy (high metabolic rate).

• Higher Pressure: When blood flows through the lungs (in capillaries), the pressure drops a lot. The heart immediately re-pumps the blood before it goes to the rest of the body, restoring high pressure. This means oxygenated blood reaches the organs much faster.
• Separation: It keeps oxygenated blood completely separate from deoxygenated blood, ensuring that body tissues only receive the richest supply of oxygen possible.

Great job! Understanding the double system means you understand how your body efficiently feeds the energy demands of every cell, linking circulation right back to bioenergetics!