Organ System
An organ system is a group of organs that work together to carry out a major body function, such as digestion, circulation, or breathing.
Definition
An organ system is a team of organs that work together on one big job; the digestive system, for example, is the mouth, stomach, and intestines cooperating to break down food. The human body has eleven such systems: integumentary, skeletal, muscular, nervous, endocrine, circulatory (cardiovascular), lymphatic (immune), respiratory, digestive, urinary (excretory), and reproductive, and they interact to maintain homeostasis. Each is a functionally integrated set of organs whose combined activity accomplishes a physiological role such as gas exchange, nutrient processing, waste elimination, defense, or coordination. The systems are coupled through the circulatory system, which links all the others by transport, the nervous system, which provides fast electrical control, and the endocrine system, which provides slower hormonal control, and they participate in negative feedback loops. Systems physiology models these interactions quantitatively, treating the body as a set of coupled compartments with flows of matter, energy, and information.
Example
When you run, your respiratory system pulls in air, your circulatory system carries the oxygen, and your muscular system moves your legs, so several systems are working at once. During exercise the nervous system speeds up the heart, the circulatory system pushes more blood to the muscles, the respiratory system raises breathing to about $40$ to $60$ breaths per minute, and the integumentary system sweats to shed heat. Regulating blood glucose involves the digestive system (absorption), the endocrine system (insulin and glucagon from the pancreas), the circulatory system (delivery), and the liver (storage as glycogen), and together they hold fasting blood glucose near $70$ to $100\ \text{mg/dL}$ despite meals that deliver tens of grams of sugar at once.
Key Insight
Organ systems do not work alone; food from the digestive system is useless until the circulatory system delivers it to the cells. The levels of organization run cell, tissue, organ, organ system, organism, and each level does something the level below could not do alone. Disease is usually a systems problem: kidney failure raises blood pressure, which strains the heart, which reduces kidney perfusion, so understanding organ systems means understanding those loops, not just the organs.