Body parts that start with I include some that you use every moment without noticing. The iris controls how much light enters your eye, the intestines absorb the nutrients from your food, and the incisors at the front of your mouth take the first bite.
Here are more than 20 human body parts that start with I, grouped by body system, each with a short and accurate explanation.
Quick List: Body Parts That Start With I
| Ileum | Index finger | Intercostal muscles |
| Iliac arteries | Inferior vena cava | Intervertebral discs |
| Iliopsoas | Infraspinatus | Intestines |
| Ilium | Inner ear | Iris |
| Incisors | Insula | Ischium |
| Incus | Interatrial septum | Islets of Langerhans |
| Instep | Interventricular septum |
Eyes, Ears and Brain
Iris: The coloured ring of the eye. Its muscles widen or narrow the pupil at its centre to control how much light reaches the retina. Each person’s iris pattern is unique, so it can be used for identification.
Inner ear: The deepest part of the ear, set in the skull. It contains the cochlea, which turns sound into nerve signals, and the vestibular system, which senses head movement and keeps you balanced.
Incus: The middle of the three tiny bones of the middle ear, also called the anvil. It passes vibrations from the malleus (hammer) to the stapes (stirrup).
Insula: A region of the cerebral cortex folded deep inside each side of the brain. It helps you sense what is happening inside your body, such as hunger, heartbeat and pain, and plays a part in emotions like disgust.
Heart and Blood Vessels
Inferior vena cava: The largest vein in the body. It carries oxygen-poor blood from the legs, pelvis and abdomen back to the right atrium of the heart.
Iliac arteries: The arteries formed when the abdominal aorta splits in two, at about the level of the navel. They supply blood to the pelvis and legs.
Interatrial septum: The thin wall separating the left and right atria of the heart. Before birth, an opening in it lets blood bypass the lungs; this opening normally closes soon after birth.
Interventricular septum: The thick, muscular wall separating the left and right ventricles. It keeps oxygen-rich and oxygen-poor blood apart and carries the heart’s electrical signals.
Digestive System and Pancreas
Intestines: The long tube from the stomach to the anus. The small intestine, about 6 to 7 metres long, absorbs most nutrients. The large intestine, about 1.5 metres long, absorbs water and forms stool.
Ileum: The last and longest section of the small intestine. It absorbs vitamin B12 and bile salts, then passes what remains into the large intestine.
Islets of Langerhans: Clusters of hormone-producing cells scattered through the pancreas. They release insulin and glucagon, which control blood sugar levels.
Incisors: The eight flat, sharp-edged teeth at the front of the mouth, four upper and four lower. They cut and bite food.
Bones and Spine
Ilium: The largest of the three bones that fuse to form each hip bone. Its upper curved edge is the bony ridge you feel when you put your hands on your hips.
Ischium: The lower, back part of the hip bone. Its rounded bumps, the ischial tuberosities, are the “sit bones” that take your weight when you sit.
Intervertebral discs: The rubbery pads between the vertebrae of the spine. They absorb shock and allow the spine to bend and twist.
Muscles, Hands and Feet
Intercostal muscles: Thin layers of muscle between the ribs. They lift and lower the rib cage, working with the diaphragm to help you breathe.
Iliopsoas: A powerful pair of muscles (the iliacus and psoas major) running from the lower spine and pelvis to the top of the thigh bone. It is the strongest muscle for lifting the knee toward the chest.
Infraspinatus: One of the four rotator cuff muscles, on the back of the shoulder blade. It turns the arm outward and helps keep the shoulder joint stable.
Index finger: The finger next to the thumb, also called the pointer or forefinger. It is the finger most used for pointing and for fine tasks.
Instep: The arched upper part of the foot between the toes and the ankle.
Common Mix-Ups to Avoid
- Ileum vs. ilium: The ileum (with an E) is part of the small intestine; the ilium (with an I) is a hip bone. A simple memory trick: the ilium is in the hip.
- Iris vs. pupil: The iris is the coloured ring; the pupil is the black opening in its centre.
- Inferior vs. superior vena cava: The inferior vena cava brings blood from the lower body; the superior vena cava brings blood from the upper body.
Frequently Asked Questions
Which organs start with I?
The intestines (small and large) are the main organs that start with I.
What is the longest body part that starts with I?
The intestines. Together, the small and large intestines measure around 7.5 to 8.5 metres in an adult.
What are easy I body parts for kids?
Iris, index finger, incisors and instep are simple words children can learn and point to.
Conclusion
From the iris in your eye to the intestines that fuel your body, the letter I covers parts of the senses, heart, digestion and skeleton. Grouping them by system makes them easier to learn and remember.
Continue the series with body parts that start with H and body parts that start with J.
Where to Learn More
For a clear overview of how the body’s systems fit together, see the Britannica guide to the human body. For trusted health information about body parts and conditions, visit MedlinePlus from the US National Library of Medicine. To check the meaning and pronunciation of tricky words, use the Cambridge Dictionary. You can also jump to the end of the alphabet with body parts that start with Z.
This article is for general education only and is not medical advice. If you have a health concern, please talk to a doctor.
