Human Body: Cells, Tissues, and Organ Systems

• All living things consist of cells.
• Cells form tissues, tissues combine into organs, and organs collaborate within organ systems.
• The main organ systems include digestive, respiratory, circulatory, and excretory.

Digestive System

• Breaks down food into nutrients, which provide energy for growth and repair.
• Major organs include the mouth, esophagus, stomach, small intestine, large intestine, liver, and pancreas.
• Digestion starts in the mouth, where saliva contains enzymes.
• The esophagus moves food to the stomach through peristalsis.
• The stomach uses acid and enzymes to break down food, especially proteins.
• The small intestine absorbs nutrients, using villi and microvilli to increase surface area.
• The large intestine absorbs water and forms solid waste (feces).
• The liver produces bile to digest fats.
• The pancreas secretes digestive enzymes and bicarbonate.
• Villi in the small intestine are specialized for nutrient absorption.
• Enzymes break down macromolecules (carbs, proteins, fats) into simpler forms (sugars, amino acids, fatty acids).

Respiratory System

• Facilitates oxygen intake and carbon dioxide expulsion.
• Major organs include the nose/mouth, trachea, lungs, bronchi/bronchioles, and alveoli.
• Air, filtered warmed, and moistened, enters through the nose/mouth
• The trachea (windpipe) serves as the path to the lungs.
• Lungs facilitate gas exchange, taking in oxygen and expelling carbon dioxide.
• Bronchi/bronchioles are airways that lead to smaller regions in the lungs.
• Alveoli are tiny sacs where gas exchange occurs.
• Alveoli walls are thin for efficient gas exchange.
• Cilia and mucus in the airways protect against pathogens and particles.

Circulatory System

• Transports oxygen, nutrients, hormones, and waste throughout the body.
• Major organs include the heart, blood vessels (arteries, veins, and capillaries), and blood.
• The heart pumps blood.
• Arteries carry oxygen-rich blood away from the heart.
• Veins carry oxygen-poor blood back to the heart.
• Capillaries are tiny vessels that enable nutrient and waste exchange at the cellular level.
• Blood contains red blood cells (oxygen), white blood cells (immune response), platelets (clotting), and plasma (carries nutrients/waste).
• Red blood cells contain hemoglobin for oxygen transport.
• Heart valves ensure one-way blood flow.

Excretory System

• Removes waste products and maintains water balance.
• Major organs include the kidneys, ureters, bladder, and urethra.
• Kidneys filter blood to remove waste and produce urine.
• Ureters transport urine from the kidneys to the bladder.
• The bladder stores urine.
• The urethra excretes urine.
• Nephrons in the kidneys filter blood, reabsorb necessary substances, and secrete waste.
• The glomerulus filters blood in the kidneys.
• The Loop of Henle concentrates urine, reabsorbing water and salts.

Key Concepts

• Cells assemble to form tissues, tissues combine to create organs, and organs work together in systems.
• All systems (digestive, respiratory, circulatory, excretory) work together to maintain homeostasis.
• Each system has specialized structures to perform its job efficiently.

How Systems Work Together

• The digestive system breaks down food, and the circulatory system carries the resulting nutrients.
• The respiratory system brings oxygen, and the circulatory system transports it to cells for energy.
• The excretory system removes waste and maintains water and salt balance.
• All organ systems support and rely on each other.

Explore cell structure and organization in the human body. Learn how cells form tissues, tissues create organs, and organs work in systems. The main systems covered are digestive, respiratory, circulatory and excretory.