B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

Systems Working Together: Keeping the Body in Balance

B3.5 B3.4 B2.1 A1.1 A1.10 A1.12 A2.2About 70 minutes

Learning goals

I can describe the primary functions of the excretory, nervous, endocrine and musculoskeletal systems.
I can explain how the respiratory and circulatory systems (and others) interact, and why this is necessary for survival.
I can explain homeostasis and describe a negative feedback loop.
I can trace a substance, such as oxygen or glucose, through several body systems.

Words to know

homeostasis
Keeping conditions inside the body (temperature, blood sugar, water, CO₂) steady even when outside conditions change.

negative feedback
A control loop in which a change triggers a response that reverses the change.

excretory system
The system that removes wastes made by cells, mainly through the kidneys (also lungs and skin).

kidney
An organ that filters the blood, removing urea, extra water and salts to make urine.

urea
A nitrogen-containing waste made by the liver when it breaks down extra amino acids.

endocrine system
Glands that release hormones into the blood to control body activities.

hormone
A chemical messenger made by a gland and carried in the blood to other organs.

insulin
A hormone made by the pancreas that lowers blood glucose by helping cells take in glucose.

A hockey game inside your body

It's the third period and you're skating hard. Your leg muscles are burning through glucose and oxygen. Your breathing is fast and deep; your heart is pounding; you're sweating even though the rink is cold. You didn't decide to do any of that except the skating. Your systems are talking to each other and adjusting automatically. No system works alone, because every cell needs the same few things: oxygen, nutrients, water, a steady temperature and a way to get rid of wastes.

More systems to know

You've studied the digestive, respiratory and circulatory systems in detail. Here are the main jobs of the others:

Excretory system: the kidneys filter the blood, removing urea (a waste made when the liver breaks down extra amino acids), extra water and salts. Each kidney contains about a million tiny filtering units called nephrons. Urine travels down the ureters to the bladder and leaves through the urethra. The lungs (CO₂) and skin (sweat) also excrete wastes. The kidneys also control the body's water balance.
Nervous system: the brain, spinal cord and nerves. Sense organs detect changes; the brain and spinal cord process information and send signals to muscles and glands in a fraction of a second.
Endocrine system: glands (such as the pancreas, thyroid and adrenal glands) release hormones into the blood. Hormones act more slowly than nerve signals but their effects can last longer.
Musculoskeletal system: bones support and protect organs (the skull protects the brain; the ribs protect the heart and lungs) and make blood cells in the marrow; skeletal muscles pull on bones to move them.
Immune system: white blood cells, lymph nodes and other organs that defend against infection.

How body systems serve the cells. The circulatory system is the hub that connects them all. Solid arrows show materials moving toward the cells; dashed arrows show wastes moving away; the dotted arrow shows control signals. (Simplified.)

The classic partnership: respiratory + circulatory

The curriculum's own example: the respiratory system brings oxygen into the body, and the circulatory system carries it to the cells. Neither can do the job alone.

Without the respiratory system, blood would have no fresh oxygen to carry and no way to dump carbon dioxide.
Without the circulatory system, oxygen would reach only the cells lining the lungs. Diffusion alone is far too slow to move oxygen more than a fraction of a millimetre, so cells deep in your legs would die.

The digestive system supplies glucose; the circulatory system delivers it; cells use it with oxygen in cellular respiration; the CO₂ produced goes back through the blood to the lungs; urea from breaking down protein goes to the kidneys. The nervous and endocrine systems coordinate all of it.

Homeostasis and negative feedback

Your body keeps internal conditions within narrow limits: core temperature near 37 °C, blood glucose within a set range, water and CO₂ levels steady. This is homeostasis. Most of it works by negative feedback: a change is detected, and the body responds in a way that reverses the change, like a thermostat in your home in winter.

A negative feedback loop during exercise. The rise in CO₂ is detected by the brain, which speeds up breathing and heart rate; this removes CO₂ faster, bringing the level back toward normal. Four systems are involved: muscular, respiratory, circulatory and nervous.

Worked example

Let's trace an oxygen molecule

Trace one oxygen molecule from the air in a room to a mitochondrion in a muscle cell in your thigh, and the carbon dioxide made there back out of the body. Name each system.

Respiratory: nose → trachea → bronchi → bronchioles → alveolus. O₂ diffuses across the alveolus and capillary walls into the blood.
Circulatory: O₂ binds to haemoglobin in a red blood cell → pulmonary vein → left atrium → left ventricle → aorta → arteries to the leg → capillary in the thigh muscle.
Muscle cell: O₂ diffuses out of the capillary into the muscle cell and into a mitochondrion, where it is used in cellular respiration: glucose + O₂ → CO₂ + H₂O + energy.
Back out: CO₂ diffuses into the capillary → veins → vena cava → right atrium → right ventricle → pulmonary artery → lung capillaries → diffuses into an alveolus → breathed out (respiratory).
Control: the nervous system adjusts breathing and heart rate to match how hard the muscle is working.

Respiratory → circulatory → muscle cell (cellular respiration) → circulatory → respiratory, all coordinated by the nervous system.

Worked example

Let's explain blood glucose control

After a bowl of oatmeal, blood glucose rises. Explain how the body brings it back to normal, naming the organs and systems involved. Then say what goes wrong in type 1 diabetes.

Digestive system: starch in the oatmeal is digested to glucose and absorbed into capillaries in the villi.
Circulatory system: blood glucose rises; blood carries the glucose to the pancreas.
Endocrine system: cells in the pancreas detect the rise and release the hormone insulin into the blood.
Insulin signals body cells (especially muscle and liver cells) to take in glucose; the liver stores extra glucose as glycogen. Blood glucose falls back toward normal: negative feedback. (If glucose drops too low, the pancreas releases a different hormone, glucagon, which raises it.)
Type 1 diabetes: the immune system destroys the insulin-making cells of the pancreas, so blood glucose stays too high. People with type 1 diabetes manage it with insulin and by monitoring their glucose.

Digestive (absorbs glucose) → circulatory (carries it) → endocrine (pancreas releases insulin) → cells take in and store glucose → level falls. In type 1 diabetes, insulin is missing.

Canadian contribution: insulin

In 1921 and 1922 at the University of Toronto, Frederick Banting and Charles Best, working in the lab of J. J. R. Macleod and with biochemist James Collip, extracted insulin and used it to treat people with diabetes for the first time. Before insulin, type 1 diabetes was usually fatal. Banting and Macleod received the 1923 Nobel Prize in Physiology or Medicine. Banting shared his prize money with Best, and Macleod shared his with Collip.

Just for fun

Funny but true: your kidneys filter about 180 L of fluid out of your blood every day, roughly a bathtub's worth. Yet you make only about 1.5 L of urine, because the kidneys reabsorb almost all of that water back into the blood. Talk about recycling.

Hands-on activity

Reaction-time test: nervous + muscular systems

You need: a 30 cm or 1 m ruler, a partner, a table, and a calculator.

Sit with your forearm resting on the table and your hand over the edge, thumb and finger open about 3 cm apart.
Your partner holds the ruler vertically with the 0 cm mark between your fingers, then drops it without warning. Catch it as fast as you can. Record the distance (in cm) at the top of your fingers.
Do 5 trials with each hand. Calculate the mean distance.
Convert distance to reaction time with t = √(2d ÷ 9.8), where d is in metres and t in seconds. For example, d = 15 cm = 0.15 m gives t = √(0.30 ÷ 9.8) ≈ 0.17 s.
Ask a scientific question and test it: Is my dominant hand faster? Do I get faster with practice? Am I slower while counting backward by 7s? Write a hypothesis first, change only one variable, and draw a conclusion from your data.

What to look for: typical results are around 10 to 20 cm (about 0.14 to 0.20 s). Your eyes (sense organs) detect the falling ruler, a signal travels along nerves to the brain, the brain sends a signal to the muscles in your hand, and the muscles contract: nervous, muscular and skeletal systems working together.

Your turn

Explain why the respiratory system and circulatory system must work together for an organism to survive.

What do the kidneys remove from the blood?

Compare how the nervous system and endocrine system send messages.

What is homeostasis? Give one example.

Describe a negative feedback loop for body temperature on a hot day.

Name two jobs of the skeletal system other than movement.

More practice: warm-up

Which hormone lowers blood glucose?

Which organ makes urea?

Which tubes carry urine from the kidneys to the bladder?

Which two organs make up the central nervous system?

Who were two of the Canadian scientists who discovered insulin?

More practice: on your own

A ruler falls 20 cm before you catch it. What is your reaction time? (t = √(2d ÷ 9.8), d in metres)

Why does your breathing rate stay high for a few minutes after you stop running?

Explain how the digestive, circulatory and excretory systems work together to deal with protein from a meal.

A person is dehydrated after a long hike. Predict how the kidneys respond and why.

Why is negative feedback called "negative"?

Challenge

Carbon monoxide (CO) binds to haemoglobin much more strongly than oxygen. Explain, using two systems, why CO poisoning is so dangerous even though breathing continues normally.

Design a fair test of the question "Does listening to loud music change reaction time?" State the variables and how you would make it reliable.

Explain why a person with type 1 diabetes may have very high blood glucose but their cells are still "starving" for energy.

Check your understanding

Quiz

Which system carries oxygen from the lungs to the body's cells?

What is the main job of the kidneys?

Which is an example of negative feedback?

During exercise, which change in the blood is the main signal that increases breathing rate?

How do hormones travel to their target organs?

A ruler falls 10 cm before it is caught. What is the reaction time? (t = √(2d ÷ 9.8))

For parents and teachers

Big idea: expectation B3.5 asks students to explain how different systems interact and why that interaction is necessary for survival (for example, the respiratory system supplies oxygen and the circulatory system transports it). B3.4 includes the main functions of the systems introduced here.

How to help: The ruler-drop test is a great family experiment: compare hands, people, or before and after practice. Ask your teen to narrate which systems are working when you go for a walk after dinner. If someone in the family lives with diabetes, it can be a respectful, real-life example of homeostasis.

Common difficulties:

Describing systems one at a time instead of explaining how they depend on each other.
Forgetting to convert cm to m in the reaction-time formula.
Thinking negative feedback means something bad.