The Human Body

The human body is a complex biological system composed of approximately 37.2 trillion 
cells. The average adult human body contains about 206 bones and over 600 muscles.

Major Organ Systems:

Circulatory System:
The heart pumps approximately 2,000 gallons of blood daily. Blood travels through 
about 60,000 miles of blood vessels. Red blood cells live for about 120 days and 
carry oxygen using hemoglobin. The average adult has about 5 liters of blood.

Respiratory System:
The lungs contain about 300 million alveoli for gas exchange. An average person 
takes about 20,000 breaths per day. The right lung has three lobes while the left 
lung has two lobes to accommodate the heart.

Nervous System:
The brain contains approximately 86 billion neurons. The spinal cord is about 
45 cm long and transmits signals between the brain and body. Nerve impulses can 
travel at speeds up to 120 meters per second.

Digestive System:
The small intestine is about 6 meters long. The large intestine is approximately 
1.5 meters long. Food takes 24 to 72 hours to pass through the digestive system.
The liver is the largest internal organ, weighing about 1.4 kg.

Skeletal System:
Bones are made of collagen and calcium phosphate. The femur (thigh bone) is the 
longest bone. The stapes in the ear is the smallest bone. Babies are born with 
about 270 bones, which fuse to 206 by adulthood.

The skin is the largest organ, covering about 2 square meters in adults. It weighs 
approximately 3.6 kg and has three layers: epidermis, dermis, and hypodermis.

The human body produces about 1 liter of saliva daily. The stomach can hold about 
1.5 liters of food. The kidneys filter about 180 liters of blood daily.