Structure and Function
Section: Biology | Syllabus: Cambridge Primary Science (0846)
Human Circulatory System
- The circulatory system carries food and oxygen to all parts of the body, and carries waste substances (like carbon dioxide) to organs that remove them, such as the lungs and kidneys
- It is composed of the heart and blood vessels
- Blood is composed of red blood cells, white blood cells, platelets, and plasma
- Blood vessels are composed of arteries, veins, and capillaries
- The heart is a muscular organ that pumps blood around the body - this process is called circulation
The circulatory system carries blood, food, and oxygen to all parts of the body
The Heart: Your Body's Pump
- Location: found in the chest, slightly to the left, and protected by the ribs
- Structure: has two sides separated by a wall called the septum
- the left side pumps oxygen-rich blood to the body
- the right side pumps oxygen-poor blood to the lungs
- Function: a strong muscle that beats continuously, pumping blood around the body with every contraction
- it takes less than one minute for blood to circulate to all parts of the body
The heart's septum divides it into a left side and a right side
Blood Vessels and Blood
Blood is a red liquid that moves through the body in three kinds of blood vessels:
- Arteries: carry blood away from the heart to the lungs (to pick up oxygen) and the rest of the body
- have thick, elastic muscular walls to handle high pressure
- Veins: carry blood back to the heart
- almost all veins carry blood low in oxygen, except the ones coming from the lungs
- have valves to keep blood flowing in one direction
- Capillaries: tiny, narrow vessels with very thin walls that join arteries and veins
- allow substances like oxygen and food to move into cells, and waste to move out
Arteries have thick muscular walls, veins have valves, and capillary walls are just one cell thick
Substances cross the thin capillary wall to reach body cells
Heartbeat and Pulse
- A child's heart beats about 90 times a minute, while an adult's beats about 70
- Your pulse is caused by the pressure of blood as the heart pumps it - you can feel it on your wrist or neck
- When you are active, your heart beats faster to supply more food and oxygen to your muscles
- Other vertebrates (animals with backbones, like dogs, birds and fish) also have a heart and blood vessels that work in a similar way to humans
- blood always moves in a cycle: Heart → Lungs → Heart → Body → Heart
Blood flows in a continuous cycle: Heart → Lungs → Heart → Body → Heart
Worked Example: The Circulatory System
- Question: why does the heart pump blood to the lungs before it pumps blood to the rest of the body?
- Answer: the heart pumps blood to the lungs first so it can pick up oxygen and get rid of carbon dioxide waste, before delivering the oxygen-rich blood to the rest of the body's cells
Common Mistakes
- ❌ Thinking arteries always carry oxygenated blood
- ✓ the best definition is that arteries carry blood away from the heart - the artery to the lungs actually carries blood without oxygen
Human Respiratory System
- The respiratory system allows us to get oxygen into our bodies and remove carbon dioxide (a waste gas) - we need to breathe to stay alive
The main parts of the respiratory system
Key Parts of the Respiratory System
- Lungs: located in the chest and protected by the ribs
- like stretchy sponges that fill up with air
- Air tubes: lead from the mouth and nose, down the windpipe (trachea), and into the lungs
- Chest muscles: allow air to move in and out of the lungs
Breathing Mechanics
- Breathing in (inhaling): the diaphragm muscle contracts and moves downwards, while the ribs move upwards and outwards
- the chest gets bigger, and the lungs fill with air
- Breathing out (exhaling): the diaphragm relaxes and moves upwards, while the ribs move downwards and inwards
- the chest gets smaller, pushing air out of the lungs
- Gas exchange: in the lungs, oxygen moves into the blood, which carries it to the heart and the rest of the body, while carbon dioxide moves from the blood into the lungs to be breathed out
Breathing in fills the lungs with air; breathing out pushes air back out
- Many vertebrates (mammals, birds, reptiles, amphibians) have similar respiratory systems, using lungs to exchange gases between air and blood
- this is because vertebrates share a similar skeleton, including a ribcage that protects the lungs in the same way
Worked Example: Path of Air Through the Body
- Question: describe the path air takes from the nose to the alveoli. [3 marks]
- Step 1: air enters through the nose (or mouth) [1]
- Step 2: travels down the windpipe [1]
- Step 3: enters the lungs, where gas exchange occurs [1]
Human Reproductive System
- The reproductive system makes it possible for humans to produce offspring - its main job is to make special cells called sex cells
- in males, sex cells are called sperm
- in females, sex cells are called eggs or ova (singular: ovum)
- Fertilisation is the process where a sperm and an egg join together to form a new living being
Reproductive Organs
| Male Parts | Function |
|---|---|
| Testes | Produce sperm cells |
| Sperm duct | Carries sperm to the penis |
| Penis | Transfers sperm into the female's body |
| Female Parts | Function |
|---|---|
| Ovaries | Produce egg cells (ova) |
| Oviducts | Where fertilisation takes place |
| Uterus (womb) | Where the baby develops |
| Birth canal | Receives sperm and is the path for birth |
Labelled male and female reproductive organs
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