A urinary bladder is the organ that stores urine from kidneys prior to excretion (urination) in most animals. The bladder is a muscle that expands and contracts as urine enters (expansion) from the kidneys and exits (contraction) through the urethra. Frequency of urination and volume of the bladder vary greatly among species and even within species. For example, most male human's bladders hold less than .7 liters of urine. Women's bladders hold slightly less urine, differing also in location of the bladder based on other physiological differences, as the female urethra is not a part of the genital tract. In either sex, the sensation or trigger to urge urination usually begins at about 25 percent capacity and grows rapidly as the bladder exceeds 50 percent capacity.
Common conditions that affect bladders include the urge to urinate more frequently than is necessary ("overactive bladder"), incontinence (inability to prevent urination that occurs without urge) and bed wetting (sometimes associated with nocturia). Far more serious conditions including bladder cancer, calcium deposits (bladder stones) and bladder spasms can occur. For the most part, adult healthy urination is under voluntary control.
In animals, urination serves additional social functions in addition to excretion of waste (urine) from the body. Many animals mark territory by releasing small amounts of urination at multiple locations; a common example is a dog being walked around a block who would stop at each yard and make a brief splash on a bush or lawn adornment. In vertebrates, the urinary bladder is often voided as a sign of fear or submission, especially among wild animals. Animals also use urine in "social" contexts and in mating rituals.
At least one class of animals hardly has a urinary bladder at all. In fact, most bird species do not have urinary bladders; only the ostrich and related families distinctly release urine from feces.