Human Body Systems Overview

The human body is organized in levels, from small to large: cells build tissues, tissues build organs, and organs that work together toward a shared job form an organ system. The heart is an organ. The heart, blood vessels, and blood together form the circulatory system, because they share one job: moving materials through the body. No organ system works alone. Every system depends on at least one other system to survive, and most depend on several. Keeping this network running is called homeostasis, the body's steady internal balance, which the final sections of this sheet explain in detail.

This sheet focuses on eight organ systems that show up most often in an introductory course. Each one has one primary job, even though most also help with other tasks.

Systems only make sense together. Example: Every cell in your body needs a steady supply of oxygen and glucose to make usable energy, and needs CO₂ removed before it builds up to toxic levels. Trace that single need through four systems: 1. The respiratory system brings oxygen into the blood and removes CO₂ at the lungs. 2. The circulatory system carries that oxygen (and glucose absorbed from food) to every cell in the body, then carries CO₂ waste back to the lungs. 3. The digestive system supplies the glucose in the first place, by breaking food down into small molecules the blood can absorb. 4. The nervous system monitors CO₂ and oxygen levels and adjusts breathing rate and heart rate to match the body's needs. If any one of those four systems fails, the others cannot make up for it. A person can eat perfectly and still suffocate without working lungs. This is why organ systems are studied as a connected network, not as separate parts.

Homeostasis is the maintenance of a stable internal environment even when outside conditions change. Body temperature, blood glucose, blood pH, and water balance all stay within a narrow healthy range, no matter whether it's hot or cold outside, or whether you just ate or haven't eaten in hours. Homeostasis is not a single organ. It is a constant, active process carried out by the nervous and endocrine systems sending signals that adjust other systems moment to moment.

The body maintains homeostasis using feedback loops, in which the output of a system feeds back in to control that same system. Three parts appear in every feedback loop: - A sensor (also called a receptor) detects a change in the internal environment. - A control center (often the brain) compares the sensed value to the normal range and decides on a response. - An effector carries out the response, an organ or tissue that changes the value back toward normal. There are two types of feedback loops: negative and positive.

Blood glucose regulation is the clearest example of negative feedback and uses two hormones from the endocrine system that work as opposites. Example: After eating a meal, blood glucose rises. 1. Sensor: cells in the pancreas detect that blood glucose has risen above the normal range. 2. Control center and effector combined: the pancreas responds directly by releasing the hormone insulin into the blood. 3. Insulin signals body cells to take up glucose from the blood and signals the liver to store extra glucose as glycogen. 4. Blood glucose falls back toward the normal range. As it does, less insulin is released, since the original rise that triggered it is gone. This shutting off of the response as the corrected value approaches normal is what makes it negative feedback. Example: Between meals, blood glucose falls. 1. Sensor: the pancreas detects that blood glucose has dropped below the normal range. 2. The pancreas releases the hormone glucagon instead. 3. Glucagon signals the liver to break stored glycogen back down into glucose and release it into the blood. 4. Blood glucose rises back toward the normal range, and glucagon release slows. Insulin and glucagon are antagonistic hormones, meaning they have opposite effects and work together to hold blood glucose in its normal range from both directions.

Example: If body temperature rises above the normal range (such as from exercise or hot weather): 1. Sensor: temperature receptors in the skin and a control center in the brain detect that internal temperature is too high. 2. The brain signals effectors: sweat glands increase sweat production, and blood vessels near the skin widen (dilate), bringing more warm blood near the surface to release heat. 3. Heat leaves the body through evaporating sweat and radiation from the skin, and temperature falls back toward normal. Example: If body temperature drops below the normal range (such as from cold weather): 1. Sensor: the same skin receptors and brain control center detect that internal temperature is too low. 2. The brain signals effectors: skeletal muscles contract rapidly and repeatedly, producing heat, this response is called shivering. Blood vessels near the skin also narrow (constrict), keeping warm blood deeper in the body core. 3. Body temperature rises back toward normal. Notice the same three part pattern as blood glucose: a sensor detects the change, a control center decides the response, and an effector carries it out, then the correction shuts the loop off again.

1. Which two organ systems work together to deliver oxygen from the air to a muscle cell, and what does each one do in that process? 2. A person's blood glucose rises after breakfast. Name the sensor, the effector, and the hormone involved in bringing it back down. 3. Is childbirth an example of negative or positive feedback, and why? 4. Name one organ that plays a role in more than one organ system listed in this sheet, and describe both roles. 5. Using exercise-heart-rate-breathing.csv, calculate the average heart rate before exercise and the average heart rate after exercise across all 5 students. 6. For each student, calculate the percent change in heart rate using percent change = (after − before) ÷ before × 100. Which student had the largest percent increase, and which had the smallest? 7. Calculate the same student's percent change in breathing rate from Question 6 (the student with the largest heart rate increase). Compare the two percent changes. Based on how the respiratory and circulatory systems work together during exercise, as described earlier on this sheet, explain why both values rose together. 8. Calculate the average breathing rate before exercise and the average breathing rate after exercise across all 5 students.