Human Body Systems: Functions, Organs, and Homeostasis

Posted on Jun 25, 2025 in Biology

Human Body Systems and Homeostasis

This document provides a comprehensive look at the major human body systems and their vital role in maintaining homeostasis.

Muscular System

  • Function: Movement, stability, posture, and heat production.
  • Main Organs: Muscles, including skeletal, smooth, and cardiac muscles.

Skeletal muscles are attached to bones and facilitate movement. Smooth muscles are found in organs like the stomach and intestines, aiding internal movement. Cardiac muscle is unique to the heart, responsible for pumping blood. Regarding skeletal muscles, insulin helps muscles take in glucose for energy.

Skeletal System

  • Function: Support, protection, movement, mineral storage, and blood cell production.
  • Main Organs: Bones, joints, and cartilage.

The skeletal and endocrine systems work together as the endocrine system controls bone growth and calcium levels through hormones like calcitonin and parathyroid hormone. The muscular and skeletal systems collaborate as muscles pull on bones to create movement.

Circulatory System

  • Function: Transport of nutrients, gases, hormones, and waste.
  • Main Organs: Heart, blood vessels (arteries and veins), and blood.

Arteries carry blood away from the heart (usually oxygen-rich), while veins carry blood to the heart (usually oxygen-poor). Blood vessel dilation or constriction plays a crucial role in maintaining body temperature: dilation releases heat to cool the body, and constriction conserves heat. The circulatory and skeletal systems maintain homeostasis by delivering oxygen and nutrients to bones, while bones protect blood vessels.

Respiratory System

  • Function: Gas exchange (oxygen and carbon dioxide).
  • Main Organs: Nose, trachea, bronchi, lungs, and alveoli.

The path an oxygen molecule takes through the respiratory system is:

  1. Nose
  2. Trachea
  3. Bronchi
  4. Lungs
  5. Alveoli (where gas exchange occurs)

The circulatory and respiratory systems work together to maintain homeostasis by bringing oxygen into the body and transporting it throughout.

Nervous System

  • Function: Control and coordination of body activities via electrical signals.
  • Main Organs: Brain, spinal cord, and nerves.

The nervous and muscular systems collaborate to produce movement by sending signals to muscles to contract. The nervous and excretory systems work together by regulating when and how the excretory system removes waste.

Endocrine System

  • Function: Regulation of metabolism, growth, mood, sexual function, and homeostasis through hormones.
  • Main Organs: Pituitary gland, thyroid, pancreas, adrenal glands, and parathyroid glands.

Key hormones and their functions:

  • Antidiuretic Hormone (ADH): Produced by the pituitary gland, targets kidneys to regulate water balance.
  • Calcitonin: Produced by the thyroid gland, targets bones to lower blood calcium levels.
  • Glucagon: Produced by the pancreas, targets the liver to raise blood sugar levels.
  • Insulin: Produced by the pancreas, targets the liver and muscles to lower blood sugar levels.
  • Parathyroid Hormone (PTH): Produced by the parathyroid glands, targets bones to increase blood calcium levels.

Digestive System

  • Function: Breaking down food, absorbing nutrients, and eliminating waste.
  • Main Organs: Mouth, esophagus, stomach, small intestine, large intestine, and anus.

The path food takes through the digestive system is:

  1. Mouth
  2. Esophagus
  3. Stomach
  4. Small Intestine
  5. Large Intestine
  6. Anus

The digestive and circulatory systems work together by breaking down food and transporting nutrients. The endocrine and digestive systems maintain homeostasis, with hormones like insulin and glucagon regulating digestion and nutrient storage.

Excretory System

  • Function: Removing waste and regulating water balance.
  • Main Organs: Kidneys, ureters, bladder, and urethra.

The circulatory and excretory systems work together to filter waste through the kidneys. The endocrine system assists the excretory system by regulating salt and water balance through hormones like aldosterone.

Levels of Structural Organization

The levels of structural organization in the human body, from least to most complex, are:

  1. Cell: The basic unit of life (e.g., red blood cell).
  2. Tissue: A group of similar cells working together (e.g., muscle tissue).
  3. Organ: A body part made of different tissues working together (e.g., stomach).
  4. Organ System: A group of organs working together to perform a function (e.g., digestive system).
  5. Organism: A whole living being (e.g., human).

Feedback Loops

Positive feedback drives a system toward an extreme, while negative feedback stabilizes a system by counteracting changes.

  • Positive Feedback Examples: Childbirth, lactation, and blood clotting.
  • Negative Feedback Examples: Blood sugar regulation, thermoregulation, and water balance.

Key Feedback Loop Vocabulary

Control Center:
Usually the brain, which compares information and initiates a response.
Effector:
An organ, tissue, or cell that changes its activity in response to a signal.
Receptor:
A cell or organ that detects changes in the internal or external environment.

Temperature Regulation (Negative Feedback)

  • When body temperature increases, sweat glands are activated and blood vessels dilate to release heat.
  • When body temperature decreases, muscles contract (shivering), and blood vessels constrict to conserve heat.

Blood Sugar Regulation (Negative Feedback)

  • When blood sugar levels are high, insulin is released from the pancreas, allowing body cells to take in glucose and the liver to store it.
  • When blood sugar levels are low, glucagon is released from the pancreas, and the liver releases glucose.

Water Balance Regulation (Negative Feedback)

  • When water levels increase, antidiuretic hormone (ADH) is released to help the kidneys reabsorb water.
  • When water levels decrease, aldosterone helps the kidneys conserve water and balance salt levels.

Breathing Regulation (Negative Feedback)

  • When carbon dioxide levels rise, the breathing rate increases to expel CO2.
  • When carbon dioxide levels fall, the breathing rate decreases.