Organ Systems — Class 5 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 5 Science, Unit 1. The five organ systems of the human body. Taught in June.
1. From cell to organ system
All living things are made of cells, the basic unit of life.
Cells of a similar kind form tissues. Tissues working together form an organ. A group of organs working together forms an organ system.
No single organ can keep you alive on its own. The stomach cannot digest food unless the mouth chews it first and the intestine absorbs it afterwards. That is why we study systems, not just parts.
2. The digestive system
We get the energy for our daily activities from the food we eat. But food as we swallow it is far too complex for the body to use, so it must be broken down into simpler forms. That process is digestion.
Three important digestive glands help:
| Gland | What it makes | What that does |
|---|---|---|
| Salivary glands | Saliva | Moistens the food; its enzymes break complex molecules into simple ones |
| Pancreas | Pancreatic juice | Contains digestive enzymes for digestion |
| Liver | Bile | Helps in the digestion of fats |
The liver is the largest internal organ of the body. Do not confuse it with the largest organ overall, which is the skin.
Most digestion and absorption happen not in the stomach but in the small intestine. The stomach starts the work with acid and enzymes; the small intestine finishes it.
3. The respiratory system
The respiratory system provides oxygen to the tissues of the body and removes carbon dioxide from them.
Air follows a fixed path in and out:
Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
The lungs are the primary organs of this system. They are paired, cone-shaped organs, located in the thoracic chamber — the rib cage — on both sides of the heart.
The alveoli are tiny air sacs at the very end of the path. This is where oxygen passes into the blood and carbon dioxide passes out of it.
Below the lungs sits the diaphragm, a sheet of muscle that moves down to draw air in and up to push it out.
4. The circulatory system
The heart is a hollow, muscular organ, somewhat conical in shape. It is covered by a double-walled membrane called the pericardium, and the fluid between those two walls lets the beating heart move without friction.
The heart has four chambers:
| Position | Name |
|---|---|
| Two upper chambers | Atria (singular: atrium), also called auricles |
| Two lower chambers | Ventricles |
The upper and lower chambers are separated by valves, which make sure blood flows one way only.
Not all blood is red. Lobsters and crabs have blue blood, and a cockroach has colourless or white blood.
5. The excretory system
The body must get rid of its wastes. That removal is called excretion.
The kidneys filter the blood to remove waste and produce urine. They are a pair of dark red, bean-shaped organs placed behind the abdomen, one on either side of the vertebral column.
Every minute, the kidneys receive about 1.25 litres of blood. Over a day that is the body's entire blood supply, filtered many times over.
The path the waste takes is worth learning in order:
Kidney → Ureter → Urinary bladder → Urethra
6. The nervous system
The nervous system carries messages around the body. It has two divisions.
The Central Nervous System (CNS) consists of the brain, the spinal cord and the nerves.
The brain controls all our activities — thinking, reading, writing. It is covered by three protective membranes called the meninges: the dura mater, the arachnoid membrane and the pia mater.
The spinal cord is a long, tube-like structure extending from the brain, lying within the backbone.
The Peripheral Nervous System (PNS) consists of the nerves extending from the spinal cord to all parts of the body. It has two parts: the somatic nervous system and the autonomic nervous system.
The functional unit of the brain is the neuron, or nerve cell.
7. Worked examples
Example 1. Name the three digestive glands.
Solution: The salivary glands, the pancreas and the liver.
Example 2. What is the function of pericardial fluid?
Solution: It lies between the two walls of the pericardium and lets the beating heart move without friction.
Example 3. Name the four chambers of the human heart.
Solution: Two atria (auricles) above and two ventricles below.
Example 4. Arrange the excretory system in the correct sequence: urinary bladder, ureter, kidney, urethra.
Solution: Kidney → Ureter → Urinary bladder → Urethra.
Example 5. What is the functional unit of the brain?
Solution: The neuron.
Example 6. Name the two parts of the peripheral nervous system.
Solution: The somatic nervous system and the autonomic nervous system.
Example 7. What is respiration?
Solution: The process by which the body takes in oxygen for its tissues and removes carbon dioxide from them.
8. Practice
- A group of organs together makes up a what?
- What is the process by which the body removes waste called?
- How many chambers does the human heart have?
- Which gland secretes saliva, and what does saliva do?
- Name the primary organs of the respiratory system and say where they lie.
- Write the path of air from the nostrils to the alveoli.
- What shape and colour are the kidneys, and how much blood do they receive each minute?
- Name the three membranes covering the brain.
- Which animals have blue blood?
- Blood is pumped by which organ?
9. Answers
- An organ system.
- Excretion.
- Four — two atria and two ventricles.
- The salivary glands. Saliva moistens the food, and its enzymes break complex molecules into simple ones.
- The lungs, a pair of cone-shaped organs in the thoracic chamber on both sides of the heart.
- Nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.
- Dark red and bean shaped. They receive about 1.25 litres of blood every minute.
- The dura mater, the arachnoid membrane and the pia mater, together called the meninges.
- Lobsters and crabs.
- The heart.
10. Summary
- Cells form tissues, tissues form organs, and organs together form an organ system.
- Digestion breaks food into simpler forms; the salivary glands, pancreas and liver assist it.
- The liver is the largest internal organ, and most absorption happens in the small intestine.
- The respiratory system supplies oxygen and removes carbon dioxide, running from nostrils to alveoli.
- The lungs are paired cone-shaped organs in the rib cage on either side of the heart.
- The heart is a hollow muscular organ covered by the pericardium, with two atria and two ventricles separated by valves.
- The kidneys are dark red bean-shaped organs receiving about 1.25 litres of blood a minute.
- Excretion runs kidney, ureter, urinary bladder, urethra.
- The CNS is the brain, spinal cord and nerves; the brain is protected by three meninges.
- The PNS has a somatic and an autonomic part, and the neuron is the functional unit of the brain.
