11.1 Life Science Systems and the Human Body
Key Takeaways
- Life Science is roughly 40% of the GED Reasoning Through Science test, so body-system reasoning shows up across many of the 40-ish questions in the 90-minute section.
- Cells, tissues, organs, and organ systems form a hierarchy that lets you explain health, disease, exercise, nutrition, and injury scenarios with evidence.
- Homeostasis uses negative-feedback loops, such as sweating, shivering, or insulin release, to push body conditions back toward a useful set point.
- Tables and graphs about heart rate, breathing, temperature, or blood glucose almost always ask for a trend, a comparison, or a supported conclusion, never a diagnosis.
- A strong health-science answer names the variable being changed, the body response being measured, and whether the data actually support the claim.
Body Systems as Evidence Systems
The GED Reasoning Through Science test runs 90 minutes with no break, contains about 40 items, and is scored 100-200 with a passing mark of 145. Roughly 40% of those items are Life Science, another 40% Physical Science, and 20% Earth and Space Science. You do not need medical training. You do need to connect a body response to the system doing the work and decide which conclusion the data support.
A question may describe exercise, illness, nutrition, injury, or a simple experiment, then ask which conclusion follows. The wrong answers usually go beyond the evidence (a diagnosis) or contradict it.
Levels of Organization
Life science starts with structure. Small parts combine into larger working systems, and damage at one level ripples upward.
| Level | Example | GED Use |
|---|---|---|
| Cell | Muscle cell | Basic unit that performs a function |
| Tissue | Muscle tissue | Many similar cells working together |
| Organ | Heart | Structure with a specific job |
| Organ system | Circulatory system | Organs cooperating for a major function |
| Organism | Human body | All systems interacting |
A GED item may ask why damage to one organ affects another system. Lungs exchange oxygen and carbon dioxide, but the circulatory system must move those gases through the blood. If lung function drops, oxygen delivery to muscle cells drops too, because the systems are connected.
Major System Connections
The body is not a set of separate machines. Memorize what each system contributes:
- Nervous system sends fast electrical signals for rapid responses.
- Endocrine system sends slower chemical signals called hormones.
- Digestive system breaks food into usable molecules like glucose and amino acids.
- Respiratory system exchanges oxygen and carbon dioxide at the lungs.
- Circulatory system transports oxygen, nutrients, hormones, and wastes.
- Excretory system removes wastes and regulates water and salt balance.
Homeostasis and Negative Feedback
Many GED questions focus on homeostasis, keeping internal conditions within a useful range around a set point. Sweating cools the body when temperature rises above about 37 degrees Celsius (98.6 degrees Fahrenheit). Shivering produces heat when temperature falls. After a meal, the pancreas releases insulin to lower high blood glucose; between meals, glucagon raises it. Each is a negative-feedback loop: a change is detected, a response opposes the change, and the condition returns toward target. A common trap answer calls insulin release "natural selection" or a "chemical change in the stomach" rather than feedback.
Reading Body-Response Data
A student records these values before and after moderate exercise.
| Time | Heart Rate (beats/min) | Breathing Rate (breaths/min) |
|---|---|---|
| Rest | 72 | 14 |
| 5 min exercise | 108 | 24 |
| 10 min exercise | 126 | 30 |
| 5 min recovery | 92 | 18 |
Both rates rise during exercise and fall during recovery. A supported conclusion: exercise raises the body's demand for oxygen and waste removal. An unsupported conclusion: the student has heart disease. The table never compares the student to a medical standard.
When interpreting a graph or table, ask three questions:
- What was changed or compared (the independent variable)?
- What was measured (the dependent variable)?
- Does the conclusion match the pattern without adding new facts?
Health Experiments on the GED
In a health experiment, the independent variable is the factor the researcher changes, such as sleep time, exercise level, or food type. The dependent variable is the measured response, such as reaction time, pulse, or blood glucose. Controlled variables are held the same so the test is fair. If two groups take different vitamins but also eat different diets, diet becomes a confounding variable and the result is hard to interpret. GED questions often reward the answer that improves the design: add a control group, hold conditions constant, increase sample size, or repeat the trial.
Worked Example: A Sleep Study
A researcher tests whether sleep affects reaction time. Twenty volunteers are split into two groups: one sleeps 8 hours, the other 5 hours, the night before a tap test that measures reaction time in milliseconds (lower is faster). Both groups take the test at 9 a.m. in the same room.
| Group | Sleep (hours) | Average Reaction Time (ms) |
|---|---|---|
| Rested | 8 | 240 |
| Sleep-deprived | 5 | 310 |
Here the independent variable is sleep, the dependent variable is reaction time, and the controlled variables are the test time and room. The supported conclusion is that less sleep was associated with slower reaction times in this sample. An overreach would be claiming five hours of sleep "causes brain damage" or that the result applies to every person on Earth, the study measured one task in 20 people on one morning.
Common GED Life-Science Traps
- Treating a correlation as proof of a single cause when a confounding variable exists.
- Diagnosing disease from normal physiological data (rising pulse during exercise is expected, not pathology).
- Confusing the independent and dependent variables when labeling a graph's axes.
- Assuming a small sample (one person, one trial) proves a general rule.
The safest habit is to name the system, describe its function, and tie the data pattern to that function, then check that your conclusion adds nothing the data did not measure.
A table shows that a student's heart rate rises from 74 beats per minute at rest to 124 during exercise, while breathing rate rises from 15 to 29 breaths per minute. Which conclusion is best supported?
After a meal, a person's blood glucose rises, the pancreas releases insulin, and blood glucose later moves back toward its usual range. What process does this describe?