Human Immune System

Lymphatic System

• The thymus is an organ located in the upper chest beneath the breastbone that matures a specific type of white blood cell that fights off foreign organisms.
• Tonsils and adenoids are lymphoid organs that trap pathogens from food and air, serving as the body's first line of defense against foreign invaders.
• Bone marrow is the soft, spongy tissue in the center of certain bones, such as the hip bone and breastbone, where white blood cells, red blood cells, and platelets are produced.
• Peyer's patches are small masses of lymphatic tissue in the mucous membrane that lines the small intestine, monitoring and destroying bacteria in the intestines.
• The appendix contains lymphoid tissue that can destroy bacteria before it breaches the intestine wall during absorption, and may also house "good" bacteria and repopulate the gut with good bacteria after an infection.

Digestive System

• The digestive system consists of the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the body.
• The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, is a long continuous tube that extends from the mouth to the anus, including the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• The simple molecules resulting from chemical digestion pass through cell membranes of the lining in the small intestine into the blood or lymph capillaries, a process called absorption.
• Elimination involves the removal of indigestible wastes through the anus in the form of feces, known as defecation or elimination.
• The digestive process involves the movement of food and liquid through the GI tract, breaking food and liquid into smaller parts, and absorbing and moving nutrients to where they are needed.
• The large intestine absorbs water, and the waste products of digestion become stool.
• Nerves and hormones help control the digestive process.

Digestive Process

• Food moves through the GI tract by a process called peristalsis, which is enabled by a layer of muscle that enables the walls of the GI tract to move and mix the contents within each organ.
• The digestive process starts in the mouth, where food is mixed with saliva that breaks down carbohydrates.
• When we swallow, the tongue pushes the food into the throat, and a small flap of tissue called the epiglottis folds over the windpipe to prevent choking.
• The food then passes into the esophagus, and the process becomes automatic, with the brain signaling the muscles of the esophagus and peristalsis begins.
• The lower esophageal sphincter relaxes and lets food pass into the stomach, which breaks down food and liquid into smaller parts.
• The stomach mixes the food with stomach acid and digestive enzymes, breaking down proteins and fats.

Kidney and Urinary System

• The kidney and urinary system helps the body eliminate liquid waste called urea, and keeps chemicals, such as potassium and sodium, and water in balance.
• Urea is produced when foods containing protein are broken down in the body.
• The kidneys remove urea from the blood through tiny filtering units called nephrons, which consist of a ball formed of small blood capillaries and a small tube called a renal tubule.
• The ureters carry urine from the kidneys to the bladder, and muscles in the ureter walls continually tighten and relax, forcing urine downward, away from the kidneys.
• The bladder is a triangle-shaped, hollow organ located in the lower abdomen, held in place by ligaments that are attached to other organs and the pelvic bones.
• The kidneys also regulate blood pressure, produce erythropoietin, which controls red blood cell production in the bone marrow, and maintain the acid-base balance and conserve fluids.