Why does the body need systems?
The human body is made up of trillions of cells, organised into tissues, organs, and organ systems. Each system carries out a specific set of functions, but no system works alone — they are all interdependent, constantly exchanging materials so the whole body can survive.
Big idea: The digestive system supplies nutrients, and the respiratory system supplies oxygen — but neither nutrients nor oxygen are useful unless the circulatory system transports them to every cell in the body. The excretory system then removes the waste these processes produce.
The Digestive System (brief recap)
The digestive system breaks down food into small, soluble nutrient molecules (glucose, amino acids, fatty acids) that can be absorbed into the blood in the small intestine. This topic is covered in full detail in the Nutrients & Digestion lesson — refer back to that lesson for the structures of the alimentary canal, enzymes, and food tests.
- Main organs: mouth, oesophagus, stomach, small intestine, large intestine.
- Its job in the bigger picture: supply digested nutrients to the blood so the circulatory system can deliver them to every cell.
The Respiratory System
The respiratory system is responsible for gas exchange — taking in oxygen from the air and removing carbon dioxide, a waste product of respiration.
- Nose/mouth & trachea (windpipe): air is warmed, moistened, and filtered as it travels down towards the lungs.
- Bronchi & bronchioles: the trachea splits into two bronchi (one per lung), which branch into smaller and smaller bronchioles — like the branches of a tree.
- Alveoli: tiny air sacs at the end of the bronchioles where gas exchange actually happens. There are millions of them, giving the lungs a huge surface area. Each alveolus is surrounded by a network of capillaries.
Gas exchange: Oxygen diffuses from the air in the alveolus into the blood in the surrounding capillaries (where oxygen concentration is lower). Carbon dioxide diffuses the opposite way — from the blood into the alveolus, to be breathed out.
The Breathing Mechanism
Breathing (ventilation) moves air in and out of the lungs using the diaphragm (a dome-shaped muscle below the lungs) and the intercostal muscles (between the ribs).
| Stage | Diaphragm | Rib cage | Chest volume & pressure |
|---|
| Inhalation | Contracts & flattens (moves down) | Intercostal muscles contract, ribs move up and out | Volume increases, pressure decreases → air rushes in |
| Exhalation | Relaxes & domes upward | Intercostal muscles relax, ribs move down and in | Volume decreases, pressure increases → air is pushed out |
The lungs themselves have no muscle — they are inflated and deflated passively by changes in chest volume and pressure.
The Circulatory System
The circulatory (cardiovascular) system transports oxygen, nutrients, hormones, and waste products around the body. It consists of the heart, blood vessels, and blood.
The Heart — a Double Pump
The heart is a muscular organ divided into four chambers (two atria, two ventricles) that pumps blood through two separate circuits — this is called double circulation:
- Pulmonary circulation: the right side of the heart pumps deoxygenated blood to the lungs, where it picks up oxygen and releases carbon dioxide. Oxygenated blood returns to the left side of the heart.
- Systemic circulation: the left side of the heart pumps oxygenated blood at high pressure to the rest of the body. Deoxygenated blood returns to the right side of the heart.
Double circulation means blood passes through the heart twice for each complete circuit of the body — once for the lungs, once for the body. This keeps oxygenated and deoxygenated blood separate and allows blood to be pumped at higher pressure to the body.
Blood Vessels
| Vessel | Carries blood | Wall structure | Direction |
|---|
| Arteries | Away from the heart (usually oxygenated, except the pulmonary artery) | Thick, muscular, elastic walls to withstand high pressure | Heart → body/lungs |
| Veins | Towards the heart (usually deoxygenated, except the pulmonary vein) | Thinner walls, larger lumen, contain valves to prevent backflow | Body/lungs → heart |
| Capillaries | Connect arteries to veins | Walls just one cell thick, allowing diffusion | Site of exchange with body cells |
The Excretory System
Excretion is the removal of metabolic waste products from the body. The main excretory organs are the kidneys, which filter the blood.
- Blood enters each kidney under high pressure and is filtered by thousands of tiny structures called nephrons.
- Useful substances (glucose, most water, salts) are reabsorbed back into the blood.
- Waste products — especially urea (from the breakdown of excess amino acids in the liver) — and excess water and salts remain to form urine.
- Urine travels via the ureters to the bladder, where it is stored before leaving the body through the urethra.
The lungs also excrete a waste product — carbon dioxide — making the respiratory system part of excretion too.
How the Systems Work Together
No organ system functions in isolation. Consider what happens after you eat a meal and go for a run:
- The digestive system breaks food into glucose and amino acids and absorbs them into the blood.
- The respiratory system brings oxygen into the blood via the alveoli.
- The circulatory system transports both the glucose and the oxygen to every muscle cell in the body, where they are used in cellular respiration to release energy.
- Cellular respiration produces carbon dioxide and other waste (like urea from excess protein breakdown) as by-products.
- The circulatory system carries carbon dioxide back to the lungs (respiratory system) to be breathed out, and carries urea to the kidneys (excretory system) to be removed as urine.
Exam tip: When asked to explain how systems are interdependent, always trace the actual substance being moved (glucose, oxygen, carbon dioxide, urea) from the system that produces/absorbs it, through the circulatory system, to the system or cells that use or remove it.