Organ Systems

Essential Question

How do organ systems work together to keep an organism in balance?

No organ system keeps you alive on its own. In this lesson, you’ll see how the circulatory, respiratory, digestive, and excretory systems cooperate to deliver what cells need and remove what they do not, and how feedback loops keep conditions in the body steady.

Systems Work Together

• Cells form tissues, tissues form organs, and organs form organ systems
• An organ system is a group of organs with a shared job
• The respiratory system brings in oxygen
• The digestive system brings in nutrients
• The circulatory system delivers to all cells

An organ system is a group of organs that work together to do a job. Cells need oxygen and nutrients and must get rid of carbon dioxide and other wastes. The respiratory system brings in oxygen and removes carbon dioxide, and the digestive system breaks down food and absorbs nutrients. The circulatory system connects them: its blood carries oxygen and nutrients to every cell and carries wastes away. The excretory system then removes wastes and extra water from the blood. If one system fails, cells in the whole body can be affected.

Gas Exchange

• Air reaches tiny air sacs called alveoli
• Each alveolus is next to a blood capillary
• O2 diffuses into the blood
• CO2 diffuses out of the blood

Air that you breathe in reaches millions of tiny air sacs in the lungs called alveoli. Each alveolus is surrounded by capillaries. Oxygen is more concentrated in the air than in the blood, so it diffuses into the blood, and carbon dioxide is more concentrated in the blood, so it diffuses into the air to be breathed out. The thin wall and the large total surface area of the many alveoli make this exchange fast, which is the surface area idea from lesson 6.8.

Nutrients In, Wastes Out

• The small intestine has villi, finger-like folds
• Nutrients pass through the thin wall into capillaries
• The kidneys filter wastes and extra water from the blood
• Useful substances stay in the blood

The inner wall of the small intestine is covered in finger-like folds called villi, and each has a capillary inside. Digested nutrients pass through the thin wall into the capillary and are carried away by the blood. The kidneys, part of the excretory system, filter the blood. Wastes and extra water leave the body in urine, and the useful substances stay in the blood. Villi are another example of a structure that increases surface area.

Transport in Plants

• Xylem carries water and dissolved minerals from the roots to the leaves
• Phloem carries sugars from where they are made to where they are used or stored
• Stomata let carbon dioxide in and let oxygen and water vapor out

Structure	What it moves	Direction
Xylem	Water and dissolved minerals	From the roots up to the leaves
Phloem	Sugars made by photosynthesis	From where they are made to where they are used or stored
Stomata	Carbon dioxide, oxygen, and water vapor	Carbon dioxide in; oxygen and water vapor out

Plants have no heart or lungs, but they still need to move materials around. Xylem is a set of tubes that carries water and dissolved minerals from the roots up to the leaves. Phloem is a set of tubes that carries sugars from the leaves, where they are made, to parts of the plant that use or store them, such as roots, fruits, and growing tips. Tiny openings in the leaf surface called stomata let carbon dioxide enter for photosynthesis and let oxygen and water vapor leave. The next lesson, 6.10, looks at how guard cells open and close stomata.

Feedback Loops

• Feedback loops keep conditions in a range
• Negative feedback reverses a change
• Positive feedback adds to a change
• Most homeostasis relies on negative feedback

Homeostasis is the maintenance of steady conditions inside the body. It usually works by negative feedback: when a condition changes, the body responds in a way that reverses the change. When blood carbon dioxide rises, breathing rate increases, which removes carbon dioxide and brings the level back down. In positive feedback, the response adds to the change. When a vessel is cut, platelets stick and release chemicals that attract more platelets, and the process speeds up until a clot seals the cut.

Systems Respond to Exercise

• Muscles need more oxygen during exercise
• Breathing rate and heart rate both rise
• Cardiac output = heart rate x stroke volume
• Both rates return toward resting values afterward

When you exercise, your muscles use more oxygen and make more carbon dioxide. The respiratory system speeds up to bring in more oxygen and remove carbon dioxide, and the circulatory system speeds up to deliver it. The volume of blood the heart pumps each minute is the cardiac output, which equals heart rate x stroke volume (the volume pumped per beat). After exercise, the demand drops and both systems return toward their resting values. This is homeostasis in action.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Trace the Oxygen

Use the model to trace an oxygen molecule from the air you breathe in to a muscle cell, and name the organ system responsible for each step.

Show the solution

1. Air enters through the nose or mouth and travels to the alveoli in the lungs (respiratory system).
2. Gas exchange: oxygen diffuses from the alveolus into a capillary (respiratory and circulatory systems).
3. Transport: the heart pumps the blood to the muscles (circulatory system).
4. Delivery: in the muscle, oxygen diffuses from the capillary into the muscle cells.
5. Answer: Respiratory system, then circulatory system, then muscle cells.

Example 2: Exercise Data

A student recorded heart rate and breathing rate before, during, and after exercise (sample data). Write a claim, evidence, and reasoning about how the two rates change.

Time (min)	Heart rate (beats/min)	Breathing rate (breaths/min)
0	70	14
2	70	14
3	120	24
4	145	32
5	155	36
6	160	38
7	130	28
8	100	20
10	80	16

Show the solution

1. Claim: heart rate and breathing rate both rise during exercise and then return toward resting values.
2. Evidence: heart rate rose from 70 to 160 beats per minute and breathing rate from 14 to 38 breaths per minute by minute 6. By minute 10 they were 80 and 16.
3. Reasoning: exercising muscles need more oxygen and make more carbon dioxide, so the respiratory and circulatory systems work faster. After exercise the demand drops and the rates fall.
4. Answer: Both rates rise during exercise and fall afterward.

Example 3: Cardiac Output

At rest, a person’s heart beats 70 times per minute and pumps 70 mL of blood per beat. During exercise the heart rate is 150 beats per minute and the stroke volume is 100 mL. Find the cardiac output at rest and during exercise, in liters per minute, and how many times greater it is during exercise.

Condition	Heart rate (beats/min)	Stroke volume (mL/beat)
Rest	70	70
Exercise	150	100

Show the solution

1. Formula: cardiac output = heart rate x stroke volume.
2. Rest: 70 beats/min x 70 mL/beat = 4,900 mL/min = 4.9 L/min.
3. Exercise: 150 beats/min x 100 mL/beat = 15,000 mL/min = 15.0 L/min.
4. Compare: 15.0 L/min / 4.9 L/min = about 3.1, so the cardiac output is about 3 times greater during exercise.
5. Answer: 4.9 L/min at rest and 15.0 L/min during exercise, about 3.1 times greater.

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Practice

Describe the jobs of organ systems, calculate cardiac output, read data on heart and breathing rates during exercise, and classify a feedback loop. Try it on your own first, then check your work with the answer key.

• Organ Systems Practice Worksheet (PDF)
• Organ Systems Practice Answer Key (PDF)

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Lesson Slides

• 6.9 Organ Systems Slides (PDF)

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Big Ideas

• Organ systems work together to supply cells and remove wastes
• Alveoli and villi have large surface areas for exchange
• Negative feedback reverses a change and maintains homeostasis
• Positive feedback adds to a change
• Cardiac output = heart rate x stroke volume
• In plants, xylem carries water up, phloem carries sugars, and stomata exchange gases

Back to the essential question: the circulatory system connects the respiratory, digestive, and excretory systems to the cells of the body, so oxygen and nutrients arrive and wastes leave. Feedback loops, mostly negative feedback, keep conditions steady even when demands change, as in exercise. Next, you’ll look at how plants and animals respond to changes in their surroundings.

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