Secretion: movement of materials, such as ammonia and some drugs, from the blood back into the distal tubule

Sensory neurons: neurons that carry impulses from sensory receptors to the central nervous system; also known as afferent neurons

Sodium-potassium pump: an active transport mechanism that moves sodium ions out of and potassium ions into a cell against their concentration gradients

Spermatogenesis: process by which spermatogonia divide and differentiate into mature sperm cells

Steroid hormone: group of hormones, made from cholesterol, that includes male and female sex hormones and cortisol

Summation: effect produced by the accumulation of neurotransmitters from two or more neurons

Sympathetic nervous system: nerve cells of the autonomic nervous system that prepare the body for stress

Synapses: regions between neurons, or between neurons and effectors

Testosterone: male sex hormone produced by the interstitial cells of the testes

Thalamus: area of brain that coordinates and interprets sensory information and directs it to the cerebrum

Thermoregulation: maintenance of body temperature within a range that enables cells to function efficiently.

Threshold level: maximum amount of material that can be moved across the nephron

Threshold level: minimum level of a stimulus required to produce a response

Thyroid gland: a tow-lobed gland at the base of the neck that regulates metabolic processes

Thyroxine: iodine-containing hormone, produced by the thyroid gland, that increases the rate of body metabolism and regulates growth

Urea: nitrogen waste from two molecules of ammonia and one molecule of carbon dioxide.

Ureters: tubes that conduct urine from the kidneys to the bladder.

Urethra: tube that carries urine from the bladder to the exterior of the body.

Uric acid: waste product formed from the breakdown of nucleic acids.