Levels of Organisation » Organ Systems Overview

What you'll learn this session

Study time: 30 minutes

The concept of levels of organisation in living organisms
The structure and function of major organ systems in humans
How different organ systems work together
Examples of organs and their roles within each system
How organ systems maintain homeostasis

Introduction to Organ Systems

Your body is like an incredibly complex machine with many different parts working together. These parts are organised into levels of increasing complexity, from cells to tissues to organs and finally to organ systems. This organisation allows your body to function efficiently and respond to changes in both internal and external environments.

Key Definitions:

Cell: The basic structural and functional unit of all living organisms.
Tissue: A group of similar cells that work together to perform a specific function.
Organ: A structure made up of different tissues that work together to perform specific functions.
Organ System: A group of organs that work together to perform a particular function.
Organism: A complete living entity made up of all the organ systems working together.

Levels of Organisation

Living organisms are organised in a hierarchy of increasing complexity. Understanding this organisation helps us make sense of how our bodies function.

🗅 From Cells to Organisms

The journey from cells to a complete organism follows these steps:

Cells → The basic building blocks (e.g., muscle cells, nerve cells)
Tissues → Groups of similar cells (e.g., muscle tissue, nervous tissue)
Organs → Different tissues working together (e.g., heart, brain)
Organ Systems → Groups of organs with related functions (e.g., circulatory system)
Organism → All systems working together (e.g., human being)

🌐 Why This Organisation Matters

This hierarchical organisation allows for:

Specialisation of functions
Efficient coordination between body parts
Better response to environmental changes
Effective maintenance of internal conditions (homeostasis)
Repair and replacement of damaged parts

Major Organ Systems in Humans

The human body contains several organ systems, each with specific functions. Let's explore the main ones you need to know for your iGCSE Biology course:

The Circulatory System

This system transports substances around the body and helps maintain homeostasis.

❤️ Main Organs

Heart
Blood vessels (arteries, veins, capillaries)
Blood

📝 Key Functions

Transports oxygen, nutrients, hormones and waste products
Helps regulate body temperature
Protects against disease through white blood cells

📈 Fun Facts

Your heart beats about 100,000 times a day
Blood vessels would stretch 100,000 km if laid end to end
Blood makes a complete circuit through your body in about 60 seconds

The Respiratory System

This system is responsible for gas exchange, bringing oxygen into the body and removing carbon dioxide.

👶 Main Organs

Nose and mouth
Trachea (windpipe)
Bronchi and bronchioles
Lungs and alveoli
Diaphragm

📝 Key Functions

Takes in oxygen from the air
Removes carbon dioxide from the body
Filters, warms and moistens incoming air
Enables speech through air movement

📈 Fun Facts

You breathe about 20,000 times a day
The surface area of your lungs is roughly the size of a tennis court
Your left lung is smaller than your right to make room for your heart

The Digestive System

This system breaks down food into nutrients that can be absorbed and used by the body.

🍽 Main Organs

Mouth and salivary glands
Oesophagus
Stomach
Small intestine
Large intestine
Liver
Pancreas
Gallbladder

📝 Key Functions

Mechanical and chemical breakdown of food
Absorption of nutrients into the bloodstream
Elimination of undigested waste
Storage of energy as glycogen in the liver
Production of bile for fat digestion

The Nervous System

This system coordinates the body's actions by transmitting signals between different parts of the body.

🧠 Main Organs

Brain
Spinal cord
Nerves
Sensory organs (eyes, ears, etc.)

📝 Key Functions

Processes sensory information
Controls voluntary movements
Regulates unconscious functions
Enables learning and memory
Coordinates responses to stimuli

How Organ Systems Work Together

No organ system works in isolation. They all depend on each other to maintain the body's overall health and function. Here are some examples of how different systems interact:

💦 Circulatory & Respiratory Systems

The respiratory system takes in oxygen, which the circulatory system then transports to all body cells. The circulatory system also carries carbon dioxide from the cells back to the lungs to be exhaled.

🍜 Digestive & Circulatory Systems

The digestive system breaks down food into nutrients, which are absorbed into the bloodstream. The circulatory system then distributes these nutrients to all body cells.

💪 Muscular & Skeletal Systems

Muscles are attached to bones by tendons. When muscles contract, they pull on bones, allowing movement. The skeletal system provides the framework that the muscular system moves.

🧠 Nervous & Endocrine Systems

Both systems control and coordinate body functions. The nervous system works quickly using electrical impulses, while the endocrine system works more slowly using hormones transported in the blood.

Case Study Focus: Diabetes and Multiple Organ Systems

Diabetes is a condition where the body cannot regulate blood glucose levels properly. It shows how multiple organ systems are interconnected:

Endocrine system: The pancreas doesn't produce enough insulin or cells don't respond to it properly
Digestive system: Affects how the body processes carbohydrates
Circulatory system: Can lead to blood vessel damage and poor circulation
Nervous system: Can cause nerve damage (neuropathy)
Excretory system: Can lead to kidney damage

This example shows how a problem in one system can affect many others, highlighting the importance of organ systems working together to maintain homeostasis.

Homeostasis and Organ Systems

Homeostasis is the maintenance of a constant internal environment despite changes in the external environment. Multiple organ systems work together to maintain homeostasis:

🌡 Temperature Regulation

When body temperature rises:

Blood vessels near the skin dilate (widen)
Sweat glands produce more sweat
These responses help cool the body

Systems involved: Circulatory, Integumentary (skin), Nervous

💧 Water Balance

The body maintains water balance through:

Controlling water intake (thirst)
Regulating water output (urine production)
Reabsorption of water in the kidneys

Systems involved: Urinary, Endocrine, Nervous

🥑 Blood Glucose Regulation

Blood glucose levels are maintained by:

Insulin (lowers blood glucose)
Glucagon (raises blood glucose)
Liver storing and releasing glucose

Systems involved: Endocrine, Digestive, Circulatory

Summary

The human body is an amazing example of organisation and cooperation. From cells to tissues to organs to organ systems, each level builds upon the previous one to create a functioning organism. Understanding how these systems work together helps us appreciate the complexity of life and how disruptions in one system can affect the entire body.

Remember that for your iGCSE exam, you should be able to identify the main organs in each system, explain their functions and describe how different systems interact to maintain homeostasis.

🧠 Test Your Knowledge!