The human body is made up of interconnected systems that work together to maintain health. This section explains the major body systems and their functions. Understanding how these systems operate helps you see the bigger picture of your health.

Overview:
Body systems are organized networks of organs, tissues, and cells that perform essential functions for survival, growth, and homeostasis. Understanding their structure helps explain how the body works as a coordinated whole.

1. Cardiovascular System

Responsible for transporting blood, nutrients, oxygen, and waste products throughout the body.

Arteries: Blood vessels that carry oxygenated blood away from the heart to tissues.
Capillaries: Tiny vessels where gas, nutrient, and waste exchange occurs between blood and tissues.
Heart: Muscular organ that pumps blood through the circulatory system.
Veins: Blood vessels that return deoxygenated blood back to the heart.
Vena Cava (Superior & Inferior): Major veins that return blood from the body to the heart.
Aorta: The largest artery in the body; distributes oxygen-rich blood from the heart.
Coronary Arteries: Supply blood to the heart muscle itself.
Valves (Heart Valves): Prevent backflow of blood between chambers and major vessels.

2. Digestive System

Breaks down food into nutrients, absorbs them, and eliminates waste.

Esophagus: Tube connecting mouth to stomach; moves food via peristalsis.
Gallbladder: Stores and releases bile to help digest fats.
Large Intestine (Colon): Absorbs water and electrolytes; forms and stores feces.
Liver: Produces bile, detoxifies chemicals, and metabolizes nutrients.
Pancreas: Produces digestive enzymes and hormones like insulin.
Small Intestine (Duodenum, Jejunum, Ileum): Absorbs nutrients and minerals from food.
Stomach: Secretes acid and enzymes to break down food.
Teeth & Tongue: Mechanically break down food and aid in swallowing.

3. Endocrine System

Glands that produce hormones to regulate growth, metabolism, reproduction, and stress response.

Adrenal Glands: Produce cortisol, adrenaline, and aldosterone; regulate stress and metabolism.
Hypothalamus: Brain region that controls the pituitary and links nervous and endocrine systems.
Pancreas: Secretes insulin and glucagon for blood sugar regulation.
Parathyroid Glands: Regulate calcium and phosphate balance.
Pineal Gland: Regulates sleep-wake cycles via melatonin.
Pituitary Gland: “Master gland” controlling growth, reproduction, and other glands.
Thyroid Gland: Regulates metabolism and growth via thyroid hormones.
Gonads (Ovaries/Testes):Produce sex hormones (estrogen, progesterone, testosterone) and support reproduction.

4. Immune System

Defends the body against infections, pathogens, and abnormal cells.

Bone Marrow: Produces white blood cells and red blood cells.
Lymph Nodes: Filter lymph fluid; trap pathogens and foreign particles.
Spleen: Filters blood, removes old cells, and produces immune cells.
Thymus: Trains T-cells for adaptive immunity.
Tonsils & Adenoids: Lymphoid tissue that traps pathogens entering through mouth/nose.
White Blood Cells (Leukocytes): Defend against infections and foreign substances.
Lymphatic Vessels: Transport lymph fluid throughout the body.

5. Nervous System

Controls communication between body parts, senses, thoughts, and reflexes.

Brain (Cerebrum, Cerebellum, Brainstem): Central processing, coordination, and survival functions.
Cranial Nerves: 12 pairs that transmit sensory and motor signals to/from the head and neck.
Spinal Cord: Conducts signals between brain and body; controls reflexes.
Peripheral Nerves: Connect CNS to limbs and organs.
Sensory Organs (Eyes, Ears, Skin, Tongue, Nose): Receive stimuli from environment.
Neurons: Individual nerve cells that transmit electrical and chemical signals.
Synapses: Junctions between neurons for signal transmission.

6. Respiratory System

Supplies oxygen to the blood and removes carbon dioxide.

Alveoli: Tiny air sacs in lungs where gas exchange occurs.
Bronchi / Bronchioles: Airways that branch from trachea into lungs.
Diaphragm: Primary muscle for breathing.
Larynx: Voice box; routes air to lungs and prevents food entry.
Lungs: Organs that oxygenate blood and remove carbon dioxide.
Nasal Cavity / Nose: Filters, warms, and humidifies air.
Pharynx: Throat area connecting nose/mouth to larynx and esophagus.
Trachea: Windpipe; conducts air to lungs.

7. Skeletal System

Supports body structure, protects organs, and enables movement.

Axial Skeleton (Skull, Vertebral Column, Rib Cage): Central support and organ protection.
Appendicular Skeleton (Limbs, Shoulder & Pelvic Girdles): Facilitates movement.
Cartilage: Flexible connective tissue cushioning joints.
Ligaments: Connect bones to other bones; stabilize joints.
Tendons: Connect muscles to bones for movement.
Bone Marrow: Produces blood cells.
Joints (Hinge, Ball-and-Socket, Pivot): Points where bones meet and move.
Skull: Protects brain; forms structure of head and face.

8. Urinary System

Removes waste from the blood, maintains fluid/electrolyte balance.

Kidneys: Filter blood to produce urine, regulate electrolytes, and maintain blood pressure.
Ureters: Tubes transporting urine from kidneys to bladder.
Bladder: Stores urine until excretion.
Urethra: Conducts urine out of the body.
Nephrons: Functional units of kidneys that filter blood.

9. Reproductive System

Responsible for producing gametes (sperm and eggs), supporting fertilization, and regulating sex hormones.

Epididymis: Coiled tube on top of the testes where sperm mature and are stored.
Fallopian Tubes: Transport eggs from ovaries to uterus.
Uterus: Supports fertilized egg and fetal development.
Ovaries: Produce eggs (ova) and female sex hormones, including estrogen and progesterone.
Penis: Male external organ used for sexual reproduction and urination.
Prostate Gland: Produces fluid that nourishes and protects sperm.
Seminal Vesicles: Secrete seminal fluid that provides energy for sperm movement.
Testes: Produce sperm and male sex hormone testosterone.
Vagina: Female muscular canal that serves as the birth canal and receives sperm during intercourse.