Homeostasis Unit

Endocrine Glands

A system of glands that release hormones directly into the bloodstream to regulate long-term body processes such as growth, metabolism, reproduction, and homeostasis

Hypothalamus

A region of the brain that links the nervous system to the endocrine system by producing hormones that control the pituitary gland; it uses neurons to send hormonal signals rather than nerve impulses

Pituitary Gland

Known as the “master gland,” this endocrine gland sits just below the brain and controls the activity of other endocrine glands through hormone release

Posterior Pituitary Lobe

The lobe of the pituitary that does not make its own hormones but stores and releases hormones produced by the hypothalamus, such as ADH and oxytocin

Anterior Pituitary Lobe

The lobe of the pituitary that produces its own hormones after receiving chemical signals from the hypothalamus and releases them to target organs

Antidiuretic Hormone (ADH)

A hormone released from the posterior pituitary that helps regulate water balance by increasing water reabsorption in the kidneys

Oxytocin

A hormone released from the posterior pituitary involved in childbirth contractions and social bonding behaviors

Thyroid Gland

An endocrine gland located in the front of the neck below the larynx, with two lobes on either side of the trachea, that regulates metabolism

Thyroxine (T4)

A thyroid hormone that increases the rate at which cells oxidize glucose and nutrients, raising overall metabolic rate

Triiodothyronine (T3)

A thyroid hormone that works with T4 to regulate metabolism and support growth and tissue development

Negative Feedback Loop (Metabolism)

A regulatory pathway where low metabolic rate triggers the hypothalamus to release TRH, stimulating TSH release from the pituitary, causing the thyroid to release T4 and increase metabolism

Calcitonin

A hormone released by the thyroid when blood calcium levels are high; it targets bones to promote calcium deposition and lower blood calcium levels

Goiter

An enlargement of the thyroid gland caused by iodine deficiency, leading to low thyroxine production and excessive TSH stimulation

Hyperthyroidism

A condition caused by excessive T4 production that results in symptoms like weight loss, heat intolerance, insomnia, high heart rate, and sometimes exophthalmos (bulging eyes)

Hypothyroidism

A condition where too little T4 is secreted, causing slowed metabolism, weight gain, cold intolerance, dry skin, and muscle weakness

Parathyroid Glands

Four small glands located near the thyroid that regulate calcium levels in the blood

Parathyroid Hormone (PTH)

A hormone released when blood calcium is low that targets bones, kidneys, and intestines to increase calcium levels in the bloodstream

Tetany

Muscle spasms of the hands and feet that can occur when parathyroid glands are damaged or removed, leading to dangerously low calcium levels

Growth Hormone (GH)

A hormone released by the anterior pituitary that stimulates protein synthesis, amino acid uptake, and bone elongation

Pineal Gland

A small gland near the center of the brain that secretes melatonin and helps regulate circadian rhythms based on light exposure

Melatonin

A hormone released during darkness that signals the hypothalamus to coordinate sleep-wake cycles and daily body rhythms

Digestive System

A continuous system from mouth to anus responsible for breaking down food and absorbing nutrients needed for energy, growth, and repair

Small Intestine

A long, folded tube (~6.7 m) where chemical digestion is completed and most nutrient absorption occurs through villi

Duodenum

The first section of the small intestine where bile, enzymes, and stomach acids mix to begin digestion and initial absorption

Jejunum

The middle portion of the small intestine that primarily absorbs carbohydrates, proteins, and increasing amounts of water

Ileum

The final section of the small intestine that absorbs fats, vitamin B12, and remaining water before waste enters the large intestine

Villi

Finger-like projections in the intestinal lining made of epithelial cells that increase surface area and release absorbed nutrients into the blood

Microvilli

Microscopic hair-like extensions on individual epithelial cells that further increase surface area for nutrient absorption

Large Intestine

A shorter (~1.5 m) organ that absorbs water and electrolytes, compacts stool, and stores waste before elimination

Cecum

The pouch connecting the small and large intestines where water and electrolyte absorption begins; the appendix is attached here

Ascending Colon

The part of the colon that removes water from stool, helping to harden it

Transverse Colon

The part of the colon where bacteria break down bilirubin and vitamins and water are absorbed

Descending Colon

The final storage area for stool before it moves to the rectum

Osmoregulation (Digestive System)

The regulation of water balance in the intestines using solute gradients and hormones like ADH and aldosterone

Mechanism of Water Absorption

A three-step process where Na⁺ and Cl⁻ are actively transported, creating an osmotic gradient that pulls water into intestinal cells and then into the bloodstream

Crohn’s Disease

An autoimmune inflammatory bowel disease that damages the intestinal lining, causing scar tissue and inflamed villi

Irritable Bowel Syndrome (IBS)

A disorder caused by disrupted brain–gut communication, leading to abnormal stool movement and food sensitivities

Sensory Receptors

Specialized cells that detect internal or external stimuli and convert them into nerve impulses for the brain to interpret

Mechanoreceptors

Receptors that detect mechanical energy such as pressure, movement, vibration, and body position

Photoreceptors

Receptors that detect light energy, allowing vision

Chemoreceptors

Receptors that detect chemical molecules or changes, such as taste, smell, and blood CO₂ levels

Thermoreceptors

Receptors in the skin that detect changes in temperature

Nociceptors

Pain receptors that detect tissue damage or harmful stimuli

Rods

Photoreceptors in the retina that detect brightness and shape and function best in dim light

Cones

Photoreceptors concentrated in the fovea that detect color and fine details in bright light

Cochlea

A fluid-filled structure in the inner ear where hair cells convert sound vibrations into nerve impulses

Semicircular Canals

Fluid-filled tubes in the inner ear that detect movement and help maintain balance

Taste Buds

Groups of chemoreceptors embedded in the tongue that detect sweet, sour, salty, bitter, and umami tastes

Olfactory Sense

The sense of smell, where airborne chemicals dissolve in nasal mucus and are detected by chemoreceptors that connect directly to the brain

Homeostasis

The maintenance of stable internal physical and chemical conditions necessary for life, including temperature, pH, glucose levels, and water balance, mainly within extracellular fluids.

Extracellular Fluid (ECF)

All body fluids outside of cells, including plasma and interstitial fluid, which are regulated to maintain homeostasis.

Interstitial Fluid

Fluid located between cells and tissues that allows exchange of nutrients, gases, and wastes.

Plasma

The liquid portion of blood that transports nutrients, hormones, gases, and metabolic wastes.

Homeostatic Mechanism

A system of receptors, control centers, and effectors that detects changes and restores internal balance.

Negative Feedback

A regulatory mechanism where a change in a variable triggers responses that reverse the change and return conditions to normal.

Positive Feedback

A mechanism where a change in a variable triggers responses that amplify the change instead of reversing it.

Insulin

A hormone released by pancreatic beta cells that lowers blood glucose by promoting glucose uptake and storage.

Glucagon

A hormone released by pancreatic alpha cells that raises blood glucose by stimulating glycogen breakdown.

Oxytocin

A hormone involved in positive feedback that intensifies uterine contractions and increases milk release.

Water Balance

The regulation of water intake and loss to maintain constant volume and solute concentration of extracellular fluids.

Osmosis

The passive movement of water across a selectively permeable membrane from high water concentration to low water concentration.

Hyperosmotic

A solution with higher solute concentration and lower water concentration than another solution.

Hypoosmotic

A solution with lower solute concentration and higher water concentration than another solution.

Iso-osmotic

Two solutions with equal solute and water concentrations, ideal for cell survival.

Osmoregulation

The active control of osmotic pressure in bodily fluids to maintain homeostasis.

Excretion

The elimination of metabolic wastes and foreign substances from the body.

Deamination

The removal of an amino group from amino acids during protein metabolism.

Ammonia

A highly toxic nitrogenous waste produced from deamination.

Urea

A water-soluble waste formed in the liver from ammonia that is excreted in urine.

Antidiuretic Hormone (ADH)

A hormone produced in the hypothalamus and released by the posterior pituitary that increases water reabsorption in the kidneys.

Excretory System

A system that filters blood, removes metabolic wastes, regulates ions, pH, and water balance, and maintains homeostasis.

Kidney

An organ that filters wastes from blood and regulates water and electrolyte balance.

Ureter

A tube that transports urine from the kidney to the bladder.

Bladder

A muscular organ that stores urine until excretion.

Urethra

A tube through which urine leaves the body.

Nephron

The functional unit of the kidney where urine formation occurs.

Filtration

The movement of water, ions, nutrients, and urea from blood into Bowman’s capsule.

Bowman’s Capsule

A cup-shaped structure that collects filtrate from the glomerulus.

Glomerulus

A network of capillaries where high pressure forces small molecules out of blood.

Reabsorption

The movement of essential substances from kidney tubules back into the blood.

Proximal Tubule

The nephron segment where most reabsorption of water, ions, glucose, and amino acids occurs.

Loop of Henle

A nephron structure that creates an osmotic gradient for water and salt reabsorption.

Distal Tubule

A nephron segment that fine-tunes jon balance and pH.

Aquaporins

Water channel proteins that allow rapid water movement during reabsorption.

Secretion

The transfer of wastes, toxins, drugs, and excess ions from blood into the nephron.

Collecting Duct

A structure that concentrates urine by reabsorbing water.

Renal Pelvis

A funnel-shaped area that collects urine and directs it to the ureter.

Endocrine System

A system of glands that secrete hormones into the bloodstream to regulate long-term processes.

Hormone

A chemical messenger produced in one part of the body that affects target cells elsewhere.

Exocrine Gland

A gland that releases substances outside the body or into body cavities.

Target Hormone

A hormone that acts only on specific target cells with matching receptors.

Non-target Hormone

A hormone that affects multiple types of cells.

Hypothalamus

A brain region that links the nervous and endocrine systems and controls the pituitary gland.

Pituitary Gland

The “master gland” that regulates other endocrine glands.

Steroid Hormone

A fat-soluble hormone made from cholesterol that enters cells and alters gene expression.

Protein Hormone

A water-soluble hormone that binds to cell membrane receptors and uses second messengers.

Second Messenger System

An intracellular signaling pathway that amplifies the signal of a protein hormone.

Neuron

A specialized nerve cell that transmits electrical and chemical signals.

Resting Potential

The electrical charge difference across a neuron’s membrane at rest, about −70 mV.

Polarization

The state where the inside of a neuron is negatively charged relative to the outside.