Functions of the skeleton and a healthy skeleton

25 min

More than a frame

If your skeleton were only a frame, it could be made of wood. But at this moment your bones are making millions of new blood cells, storing calcium for your heart and nerves, and quietly rebuilding themselves. The teenage years are when bones grow fastest and lay down most of the mineral they will have for life, so the habits you form now matter for the rest of your life.

Definition

Red bone marrow: soft tissue inside the spongy bone of the ribs, sternum, vertebrae, pelvis and skull and at the ends of long bones, which makes red blood cells, white blood cells and platelets.

Mineral (in bone): an inorganic substance, mainly calcium phosphate, that makes bone hard. Bone acts as the body's store of calcium and phosphate.

Posture: the position in which you hold your body when sitting, standing, lifting or carrying.

The five functions of the skeleton

Support. The skeleton holds the body up and gives it its shape; the vertebral column supports the head and trunk, and the legs carry the whole weight.
Protection. The cranium encloses the brain, the vertebrae surround the spinal cord, and the ribcage shields the heart and lungs.
Movement. Muscles are attached to bones by tendons. When a muscle contracts it pulls a bone, which moves at a joint.
Making blood cells. Red bone marrow makes red blood cells, white blood cells and platelets.
Storing minerals. Bone stores calcium and phosphate. When the blood needs calcium for muscles and nerves, it is released from the bones; when there is extra, it is laid down in them.

Figure 1: Inside a long bone. Hard compact bone stores minerals; red marrow in the spongy ends makes blood cells.

Bone or group of bones	Main functions
Skull (cranium)	protects the brain, eyes and inner ears; holds the teeth
Vertebral column	supports the head and trunk; protects the spinal cord; anchors the ribs and back muscles
Ribs and sternum	protect the heart and lungs; move during breathing; red marrow makes blood cells
Pelvic girdle	passes the weight of the upper body to the legs; protects the bladder and other organs of the lower abdomen
Femur, tibia and foot bones	support the body's weight; act as levers for walking, running and jumping
Humerus, radius and ulna	levers for lifting, pushing and throwing
Carpals, metacarpals and phalanges	fine movements: gripping a hoe, writing, typing

Worked example 1: which function is it?

Name the function of the skeleton shown in each case: (a) a footballer's rib is bruised but her lungs are unharmed; (b) a doctor takes a sample from the hip bone to study how a patient's blood cells are made; (c) during a long race, calcium moves from the bones into the blood.

(a) The ribs took the blow instead of the lungs: protection.
(b) Red marrow in the pelvis makes blood cells: blood cell production.
(c) Calcium stored in bone is released for the working muscles: mineral store.

Practices for a healthy skeleton

Eat enough calcium. Bones are built from calcium phosphate. FAO and WHO suggest about 1,000 to 1,300 mg of calcium a day for young people aged 10 to 18, the time of fastest bone growth. Good Rwandan sources are milk and fermented milk (ikivuguto), small dried fish eaten whole with their bones, such as indagara and isambaza, beans, and dark green leaves such as cassava leaves (isombe) and amaranth (dodo). Unit 7 describes what happens when the diet lacks calcium.

Get vitamin D and sunlight. Vitamin D lets the intestine absorb calcium from food. The skin makes vitamin D when sunlight falls on it, so time outdoors every day matters; egg yolk and oily fish also contain some.

Exercise. Bone responds to the loads put on it. Running, football, basketball, dancing (such as intore) and skipping make bones thicker and stronger; long periods of sitting do the opposite.

Sit and stand well. Sit with your back against the chair and your feet flat on the floor; hold a phone at eye level instead of bending your neck down for hours.

Lift and carry safely. Bend your knees, keep your back straight and hold the load close. Carry a school bag on both shoulders, with heavy books packed low and near the back. Carrying water or crops on the head is common in Rwanda: use an ingata (head ring) to spread the pressure, keep the load centred and your neck straight, and share heavy loads.

Avoid smoking and alcohol, which weaken bones, and use protection (helmets, shin guards) in sport and on motos.

Figure 2: Lifting and carrying. Bending the knees lets the strong leg muscles do the work; a centred head load keeps the vertebral column straight.

Cyclists on a Car Free Day in Kigali, when main roads are closed to cars for a few hours so that people can walk, run and cycle. Regular weight-bearing exercise makes bones thicker and stronger. Photo: Fabos ishimwe, CC BY-SA 4.0, via Wikimedia Commons

Worked example 2: comparing loads and calcium

Ishimwe has a mass of 40 kg. (a) He carries a full 20-litre jerrycan of water (mass about 20 kg). What percentage of his body mass is this? Compare with his 4 kg school bag. (b) A 250 ml cup of milk contains about 300 mg of calcium. What percentage of a 1,000 mg daily need is this?

(a) Jerrycan: $\frac{20}{40} \times 100\% = 50\%$ of his body mass. School bag: $\frac{4}{40} \times 100\% = 10\%$ .
The jerrycan is five times as heavy a load for his body. Two half-full containers, a wheelbarrow, or sharing the load with a friend would protect his back and neck.
(b) $\frac{300}{1000} \times 100\% = 30\%$ , so one cup provides almost a third of his daily calcium.

Warning

Common mistakes and safety. (1) "Bones are dead, so food cannot change them." Bone is living tissue that is rebuilt all the time. (2) "Heavy school bags cause a curved spine (scoliosis)." They do not, although they can cause back, neck and shoulder pain. (3) If someone may have a broken bone, do not try to straighten or move the limb: support it in the position you found it, and get medical help, after the primary survey you learned in Unit 1.

KS3 Biology: Skeletal System, Structure of Bones and Functions of the Skeleton (Cognito). Watch for the functions of the skeleton and where red and yellow marrow are found.

Tip

Memory hook: "SPMBM", Support, Protection, Movement, Blood cells, Minerals. For a healthy skeleton remember "Calcium, Sunlight, Exercise, Posture". In an exam, give a reason with each practice: "Eat small fish eaten whole, because they are rich in calcium, which hardens bone."

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Practice: functions and a healthy skeleton

List the five functions of the skeleton.
Name the bones that protect (a) the brain, (b) the heart and lungs, (c) the spinal cord.
Where is red bone marrow found, and what does it make?
Explain why vitamin D is needed for healthy bones, and how the body obtains it.
Give two Rwandan foods rich in calcium.
Mutesi (45 kg) carries a 9 kg basket of avocados on her head. What percentage of her body mass is this? Give two ways she can carry it more safely.
Describe how to lift a heavy sack from the floor safely, and explain why.
A student spends most weekends indoors watching videos. Evaluate the effect on his skeleton and suggest changes.

Show answersHide answers

Support, protection, movement, making blood cells, storing minerals.
(a) The cranium (skull); (b) the ribs and sternum (ribcage); (c) the vertebrae (vertebral column).
In the spongy bone of flat bones such as the ribs, sternum, vertebrae, pelvis and skull, and in the ends of long bones. It makes red blood cells, white blood cells and platelets.
Vitamin D lets the intestine absorb calcium, which hardens bone. The skin makes it in sunlight; egg yolk and oily fish also supply some.
Any two: milk or ikivuguto; small dried fish eaten whole (indagara, isambaza); beans; cassava leaves (isombe); amaranth.
$\frac{9}{45} \times 100\% = 20\%$ . Any two: use an ingata; keep the load centred and her neck straight; carry a smaller load and make two trips; share it with someone.
Stand close to the sack, bend the knees with the back straight, hold the sack close to the body and straighten the legs to lift. The strong leg muscles do the work, and the vertebral column stays straight, so the back is not strained.
Little weight-bearing exercise means his bones are not stimulated to grow thicker and stronger, and staying indoors means less sunlight, so less vitamin D for absorbing calcium. He should play sport, walk or cycle outdoors, and eat calcium-rich food.

Key points

The skeleton supports, protects, allows movement, makes blood cells (red marrow) and stores calcium and phosphate.
Healthy bones need calcium (milk, small fish eaten whole, beans, green leaves), vitamin D (sunlight) and weight-bearing exercise.
Good posture and safe lifting and carrying protect the vertebral column.

1More than a frame

2The five functions of the skeleton

Practices for a healthy skeleton

More than a frame

The five functions of the skeleton