P5 Circulatory System Notes

A. Parts in the Human Circulatory System and Their Functions

The circulatory system consists of three basic components:

Heart — a muscular pump made of heart muscles
Blood vessels — tubes through which blood is transported
Blood — circulatory fluid that carries substances around the body

The heart transports substances to all parts of the body. It also removes waste materials.

Blood is made up of plasma, red blood cells, white blood cells and platelets. The red blood cells carry oxygen to all parts of our body.

B. Functions of the Circulatory System

Transports digested food, oxygen and water to all parts of the body
Transports waste materials like carbon dioxide from the body
Protects the body from infection

C. The Heart

The heart is made of heart muscles that pump blood to all parts of the body by continuously contracting and relaxing.
The heart is located between the lungs, and is protected by the ribcage.

💡 Key Point: Blood carries digested food, oxygen, water and waste substances around our body. Blood is made up of plasma, red blood cells, white blood cells and platelets.

D. The Blood Vessels

Blood vessels are tubes that carry blood to all parts of the body.
The three main types of blood vessels are:

Type	Function
Arteries	Carry oxygen-rich blood away from the heart to the body
Veins	Carry oxygen-poor blood back to the heart
Capillaries	Very thin vessels where exchange of substances takes place

E. How Blood Flows in the Circulatory System

The human circulatory system is described as having double circulation, while fish have single circulation.

In humans, blood passes through the heart two times in one complete circuit. In fish, it passes only one time.

1

Blood rich in oxygen flows from the lungs to the heart.

2

The heart pumps the oxygen-rich blood to other parts of the body.

3

Body cells take in oxygen and digested food from the blood to carry out respiration.

4

Carbon dioxide and other waste substances produced by body cells enter the blood.

5

The blood becomes poor in oxygen and flows back to the heart.

6

The heart pumps the oxygen-poor blood back to the lungs, where carbon dioxide is exchanged for oxygen.

In fish: Heart → gills → Body → Heart (single loop).

💡 Two loops in humans: First loop — heart → lungs → heart. Second loop — heart → body → heart.

F. Fish vs Human Circulatory System

Differences:

Feature	Human	Fish
Circulation type	Double circulation — blood passes through heart twice per circuit	Single circulation — blood passes through heart once per circuit
Blood flow loops	Two loops: 1. Heart → Lungs → Heart 2. Heart → Body → Heart	One loop: Heart → Gills → Body → Heart

Similarities:

Both circulatory systems consist of a heart, blood and blood vessels.
In both systems, the heart is present to pump blood to all parts of the organism.
In both systems, blood is transported in blood vessels.

G. Organ Systems That Work Closely Together with the Circulatory System

The circulatory system works closely together with other body systems:

Digestive System ↔ Circulatory System:

Digested food and water from the digestive system enter the circulatory system to be circulated around the body.

Respiratory System ↔ Circulatory System:

Oxygen from the respiratory system enters the circulatory system to be circulated.
Carbon dioxide from oxygen-poor blood leaves the circulatory system and enters the respiratory system to be removed out of the body.

💡 Key Point: The three systems are interdependent — none can function alone. Digestive provides food, respiratory provides oxygen, circulatory distributes both.

H. Differences Between How Substances Are Transported in Humans and Plants

Differences:

Feature	Humans	Plants
Pump	Have a heart to act as a pump for substances	Do NOT have a heart
Tubes	Blood vessels transport substances	Water-carrying tubes and food-carrying tubes
Transport medium	All substances carried by blood in blood vessels (humans)	Different substances carried in different tubes (plants)

In plants:

Food-carrying tubes transport food from leaves to all parts of the plant.
Water-carrying tubes transport water and dissolved mineral salts from roots to all parts of the plant.

Similarities:

Both systems consist of tubes for the transportation of substances.
In both systems, water and nutrients are transported through the tubes to all parts of the organism.

I. Common Exam Question

Q: How does our body remove carbon dioxide produced during respiration?

1

Carbon dioxide enters the blood, which flows to the heart.

2

The heart pumps the blood rich in carbon dioxide to the lungs.

3

Carbon dioxide leaves the blood and enters the lungs.

4

Carbon dioxide is removed from the body through the nose when a person exhales.

⚠️ Exam Tip: Always mention all 4 steps — blood → heart → lungs → nose (exhale). Missing any step loses marks!

J. Heart Beat

The heart contracts and relaxes in a rhythmic cycle.
One complete cycle of contraction and relaxation is known as a heartbeat.
The heart rate is the number of heartbeats in a minute, expressed as "beats per minute" (bpm).
The heart rate changes according to a person's activity.

Effects of exercising on heart rate:

When we exercise, our body needs more energy.
Thus, our heart pumps faster to transport more oxygen and digested food in the blood to other parts of our body for a greater rate of respiration to release more energy.
This also allows carbon dioxide given out during respiration to be removed faster.

Effects of narrowed blood vessels:

Fatty deposits result in a smaller opening for blood to flow through, reducing the amount of blood flowing through the blood vessels.
Hence, the heart has to pump faster to transport the same amount of oxygen and digested food to other parts of the body for respiration.

💡 Key Point: Anything that reduces blood flow (exercise → more demand, narrowed vessels → less supply) causes the heart to beat faster.

❤️ Human Circulatory System