NCERT Solutions for Class 9th Science Chapter 3 .3.2 How are various parts connected in our body? — Activity 3.2: Let us understand further

Book page 363 Updated on2026-09-08

Q1.

Recall everyday experiences given in the first column of Table 3.3. Write your observations and questions in your notebook. Compare your observations with the observations given in Table 3.3.

Answer

Here is the completed Table 3.3, with the observations the book gives and the questions each one should make you ask.

Experience	Observation	Question it raises
When you get a small cut on your skin	Red blood oozes out from the cut. A clot is formed after some time.	What causes blood to clot?
When you get a skin infection	The area turns red and perhaps slightly swollen. You may have a fever.	What makes the infected area red and swollen — which blood cells collect there?
When you exercise or run	You breathe faster. Your face may turn red.	Why does the body need more oxygen during exercise, and how does blood deliver it faster?

All three experiences point to the same tissue: blood, a connective tissue in which cells float in a fluid matrix called plasma.

Component	Share of blood	Job
Plasma	55% of total volume	Fluid matrix — carries cells, nutrients, hormones and wastes
Formed elements — RBCs, WBCs, platelets	45% of total volume	Oxygen transport, defence, clotting

For an adult with about 5 L of blood:
Volume of plasma = 55% of 5 L = (55 ÷ 100) × 5 L = 2.75 L
Volume of formed elements = 45% of 5 L = (45 ÷ 100) × 5 L = 2.25 L

Why blood counts as a connective tissue: a connective tissue is defined by its matrix — cells scattered in a non-living ground substance. In bone that matrix is hard and rigid because calcium and phosphorus compounds are deposited in it; in blood it is watery and jelly-like. Same design, completely different consistency, and therefore completely different jobs.

Q2.

What causes blood to clot?

Answer

Platelets — the smallest of the formed elements — collect at the site of the injury and start the clot that seals it.

What happens at a cut, in order:

The vessel wall is torn and blood escapes.
Platelets reach the injured spot and stick to the torn edges and to one another, forming a temporary plug.
They set off the chemical reactions of clotting, which lay down a mesh of fibres across the wound.
Blood cells are trapped in that mesh; the mass hardens and bleeding stops.

Why the other observations follow the same logic:

Redness of blood — haemoglobin, an iron-rich protein inside the RBCs. RBCs live for about 4 months and are replaced regularly.
Redness and swelling of an infected patch, and pus — WBCs collect at the infected area to fight the microbes, causing inflammation and pus formation.
Faster breathing and a flushed face while running — the muscles need more oxygen, so breathing speeds up and blood flow increases; the extra blood near the skin surface makes the face look red.

Did you know? An RBC lasting about 4 months means roughly 120 days of service, after which it is broken down and replaced — which is why blood donation is safe for a healthy adult.

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