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Anatomy and Physiology Unit 6: Digestive, urinary and immune systems

The digestive system breaks food into nutrients, the kidneys filter blood, and the immune system defends against pathogens.

Covers: Digestion and absorption, enzymes, liver and pancreas, kidneys and nephrons, innate and adaptive immunity

Study guide

The digestive system breaks food into molecules small enough to absorb. Food travels through the mouth, esophagus, stomach, small intestine and large intestine. Mechanical digestion (chewing, stomach churning) breaks food into smaller pieces, and chemical digestion uses enzymes to break molecules apart: salivary amylase starts on starch in the mouth, pepsin digests protein in the acidic stomach, and pancreatic enzymes (amylase, proteases and lipase) finish the job in the small intestine. Peristalsis moves food along.

Accessory organs help. The liver makes bile, which emulsifies fats into small droplets so lipase can work, and the gallbladder stores it. The pancreas releases digestive enzymes and bicarbonate to neutralize stomach acid, as well as insulin and glucagon.

Most nutrients are absorbed in the small intestine, whose villi and microvilli create a huge surface area. Sugars and amino acids enter the blood, and fats enter the lymph. The large intestine absorbs water and forms feces, with help from gut bacteria.

The urinary system filters the blood and removes wastes. The kidneys contain about a million nephrons each, the functional units. In a nephron, blood is filtered at the glomerulus, useful substances like glucose and most water are reabsorbed, and wastes are secreted, producing urine. Urine flows through the ureters to the bladder and out the urethra. The kidneys also regulate blood pressure, water balance and pH, with help from hormones like ADH.

The immune system defends against pathogens. Innate immunity is fast and nonspecific: skin and mucous membranes, inflammation, fever, and phagocytes like neutrophils and macrophages. Adaptive immunity is slower but specific and has memory. B cells become plasma cells that make antibodies, and T cells include helper T cells that coordinate the response and cytotoxic T cells that kill infected cells. Memory cells allow a faster response the next time, which is how vaccines work.

Lymphatic vessels collect excess tissue fluid (lymph) and return it to the blood, mainly through the thoracic duct. Lymph nodes filter lymph and house lymphocytes, the spleen filters blood and removes old red blood cells, and the thymus is where T cells mature (B cells mature in the bone marrow). When blood pressure drops, the kidneys release renin, which starts a cascade that produces angiotensin II. Angiotensin II constricts blood vessels and triggers aldosterone release. Aldosterone makes the kidneys reabsorb more sodium, and water follows, which raises blood volume and pressure. ADH acts on the collecting ducts to reabsorb more water.

Blood pressure in the glomerulus pushes water and small solutes into the glomerular (Bowman's) capsule. The proximal convoluted tubule reabsorbs most water, glucose, amino acids and salts. The loop of Henle sets up a salt gradient that lets the kidney concentrate urine, and the distal tubule and collecting duct fine-tune it under hormone control. Urea, made in the liver from protein breakdown, is the main nitrogen waste in urine. Active immunity means the body makes its own antibodies and memory cells, either naturally after an infection or artificially after a vaccine. Passive immunity is borrowed antibodies, received naturally through the placenta or breast milk or artificially through an antibody injection. Passive immunity works right away but fades within weeks.

Hormones coordinate digestion: gastrin stimulates stomach acid, secretin signals the pancreas to release bicarbonate, and CCK triggers enzyme release and gallbladder contraction. The stomach also makes intrinsic factor, which is needed to absorb vitamin B₁₂. Bacteria in the large intestine make vitamin K.

Key ideas

• Digestion: mouth → esophagus → stomach → small intestine → large intestine.
• Most nutrient absorption happens in the small intestine (villi).
• The liver makes bile; the pancreas makes digestive enzymes and insulin.
• Nephrons in the kidneys filter blood and make urine.
• Innate immunity is fast and general; adaptive immunity (B and T cells) is specific and has memory.

Formulas and rules

Common mistakes

• Thinking most absorption happens in the stomach. It's the small intestine.
• Mixing up B cells (antibodies) and T cells.

Words to know

Small intestine

Where most nutrients are absorbed.

Bile

Made by the liver; emulsifies fat into small droplets.

Nephron

The functional unit of the kidney.

B cell

Immune cell that becomes a plasma cell and makes antibodies.

Vaccine

Trains adaptive immunity and creates memory cells.

Digestive tract

Mouth → esophagus → stomach → small intestine → large intestine.

Mechanical digestion

Chewing and churning food into smaller pieces.

Chemical digestion

Enzymes breaking molecules apart.

Peristalsis

Waves of muscle contraction that move food.

Salivary amylase

Starts starch digestion in the mouth.

Pepsin

Digests protein in the stomach.

Villi

Finger-like folds that increase the small intestine's surface area.

Large intestine

Absorbs water and forms feces.

Glomerulus

Where blood is filtered in the nephron.

Innate immunity

Fast, nonspecific defense: skin, inflammation, phagocytes.

Adaptive immunity

Slower, specific defense with memory (B and T cells).

Antibody

A protein that binds a specific antigen.

Helper T cell

Coordinates the immune response.

Cytotoxic T cell

Kills infected cells.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.