Human Body — Circulatory System
1. Introduction
The circulatory system transports blood, oxygen, nutrients, and waste products throughout the body.
It is also called the cardiovascular system (cardio = heart, vascular = vessels).
Main components:
- Heart — the pumping organ.
- Blood — the transport fluid.
- Blood vessels — the network of tubes (arteries, veins, capillaries).
2. The Heart
The heart is a muscular organ about the size of a fist. It is located in the chest, slightly to the left.
Structure of the Heart
The heart has four chambers:
| Chamber | Location | Function |
|---|---|---|
| Right atrium | Upper right | Receives deoxygenated blood from the body |
| Right ventricle | Lower right | Pumps deoxygenated blood to the lungs |
| Left atrium | Upper left | Receives oxygenated blood from the lungs |
| Left ventricle | Lower left | Pumps oxygenated blood to the body |
Important: The left ventricle has the thickest wall because it needs to pump blood to the entire body.
Valves in the heart prevent blood from flowing backward.
Worked Example: Why does the left ventricle have a thicker wall than the right ventricle?
The left ventricle pumps blood to the entire body (long distance, high pressure). The right ventricle only pumps blood to the nearby lungs (short distance, low pressure).
3. Blood
Blood is a red fluid that circulates through the body. It makes up about 7-8% of body weight.
Components of Blood
| Component | Percentage | Function |
|---|---|---|
| Plasma | 55% | Liquid part, carries nutrients, hormones, waste |
| Red blood cells (RBCs) | 44% | Carry oxygen (haemoglobin), disc-shaped |
| White blood cells (WBCs) | <1% | Fight infections (part of immune system) |
| Platelets | <1% | Help in blood clotting |
Common Mistake: Thinking blood is a simple red liquid. Blood is a complex tissue made of different components, each with specific functions.
Functions of Blood
- Transport of oxygen: RBCs carry oxygen from lungs to all body cells.
- Transport of nutrients: Carries digested food from intestines to cells.
- Removal of waste: Carries CO2 to lungs and other waste to kidneys.
- Fighting infections: WBCs protect against diseases.
- Clotting: Platelets prevent blood loss by forming clots.
- Temperature regulation: Distributes heat throughout the body.
Exam Focus (3 marks): 'Name the four components of blood and state one function of each.'
Plasma: carries nutrients and waste. RBCs: carry oxygen. WBCs: fight infections. Platelets: help in blood clotting.
4. Blood Vessels
Blood flows through a network of tubes called blood vessels.
| Vessel | Direction | Structure | Function |
|---|---|---|---|
| Arteries | Away from heart | Thick, elastic walls, no valves | Carry oxygenated blood (except pulmonary artery) |
| Veins | Toward the heart | Thin walls, have valves | Carry deoxygenated blood (except pulmonary vein) |
| Capillaries | Connect arteries and veins | Very thin walls (one cell thick) | Exchange of gases and nutrients |
Worked Example: Why do veins have valves but arteries do not?
Blood in veins flows at low pressure and must travel upward often (against gravity). Valves prevent backflow. Arteries have high-pressure blood flow from the heart, so no valves are needed.
Common Mistake: Thinking all arteries carry oxygenated blood and all veins carry deoxygenated blood. The pulmonary artery carries deoxygenated blood from heart to lungs. The pulmonary vein carries oxygenated blood from lungs to heart.
5. Pulse
The pulse is the rhythmic expansion and contraction of an artery caused by the pumping of the heart.
- The pulse rate corresponds to the heart rate.
- Normal pulse rate (at rest): 70-80 beats per minute for adults, higher for children.
- Pulse can be felt at the wrist (radial artery) or neck (carotid artery).
Worked Example: A student counts 18 pulses in 15 seconds. What is the heart rate per minute?
Heart rate = 18 x (60/15) = 18 x 4 = 72 beats per minute.
6. Comparison Table: Arteries vs Veins
| Feature | Arteries | Veins |
|---|---|---|
| Direction | Away from heart | Toward the heart |
| Wall thickness | Thick and elastic | Thin and less elastic |
| Valves | Absent | Present |
| Blood pressure | High | Low |
| Blood type | Oxygenated (mostly) | Deoxygenated (mostly) |
7. Self-Test
- Name the four chambers of the heart.
- What is the function of the heart valves?
- List the three types of blood vessels and their functions.
- What are the components of blood? State the function of RBCs.
- Why do veins have valves?
- What is pulse? How can it be measured?
- Which chamber of the heart has the thickest wall? Why?
8. Answers to Self-Test
- Right atrium, right ventricle, left atrium, left ventricle.
- Valves prevent blood from flowing backward in the heart.
- Arteries: carry blood away from heart. Veins: carry blood toward the heart. Capillaries: exchange of gases and nutrients.
- Plasma (carries nutrients), RBCs (carry oxygen), WBCs (fight infection), Platelets (clotting).
- Veins carry blood at low pressure, often upward against gravity. Valves prevent backflow.
- Pulse is the rhythmic expansion of arteries felt with each heartbeat. It can be measured at the wrist or neck.
- Left ventricle. It pumps blood to the entire body, requiring the strongest contraction.
