Anatomy Terms Starting With U
Anatomy Glossary: U
Ulna
/ UL-nah / · Latin ulna, elbow or forearm
Ulna is the medial long bone of the forearm that forms the stable hinge of the elbow joint and runs parallel to the radius, whose rotation around the ulna produces forearm pronation and supination.
The olecranon process fits into the olecranon fossa of the humerus to create the hinge joint of the elbow, providing bone-on-bone stability for flexion and extension. Connecting the ulna to the radius, the interosseous membrane transmits compressive forces between the two bones and provides broad attachment surfaces for the deep forearm muscles. Fractures of the ulnar shaft are common defensive injuries, sustained when a raised forearm absorbs a direct blow, and are sometimes called nightstick fractures after the implement historically used to deliver such strikes.
In birds, the ulna is the larger of the two forearm bones and bears the quill knobs, small bony projections where the secondary flight feathers anchor; the number and spacing of these knobs vary enough among species that paleontologists use them to infer feather arrangement in feathered dinosaurs such as Velociraptor mongoliensis.
Fun Facts About the Skeletal System →The radius rotates around the ulna during forearm movement. During pronation and supination, the radius crosses over or uncrosses from the ulna, while the ulna itself remains relatively fixed at the elbow.
In birds, the ulna supports the attachment points of secondary flight feathers along its posterior border; in the bald eagle (Haliaeetus leucocephalus), these anchor points are visible as a row of raised quill knobs that can be counted on skeletal specimens to estimate the number of secondary feathers in life.
Ureter
/ yoo-REE-ter / · Greek oureter, from ourein, to urinate
Ureter is a muscular tube approximately 25 to 30 centimeters long that carries urine by peristalsis from the renal pelvis of each kidney to the posterior wall of the urinary bladder.
The ureteric wall contains three smooth muscle layers whose peristaltic contractions propel urine boluses at 2 to 6 centimeters per second, moving urine independently of body position or gravity. Each ureter enters the bladder obliquely through a tunnel in the detrusor muscle, creating a physiological valve that compresses the ureteric lumen during bladder filling and prevents urine reflux toward the kidneys. Ureteric stones are acutely painful because peristaltic contractions against an obstruction generate ureteric colic, one of the most severe pain syndromes encountered in emergency medicine, often requiring opioid analgesia.
Three anatomical narrowings along each ureter, at the ureteropelvic junction, where the ureter crosses the pelvic brim, and at the ureterovesical junction, are the sites where kidney stones most commonly lodge; the ureterovesical junction, with a lumen as narrow as 2 to 3 millimeters, is the most frequent site of impaction.
Urinary System Fun Facts →Ureters carry urine from the kidneys to the outside of the body. Ureters deliver urine only as far as the bladder; the urethra is the separate tube that carries urine from the bladder to the exterior.
In the Burmese python (Python bivittatus), the ureters are short relative to body length because the kidneys are positioned far posteriorly in the elongated coelomic cavity, and urine is delivered to a cloaca rather than a discrete bladder, a structural arrangement common to most reptiles.
Urethra
/ yoo-REE-thrah / · Greek ourethra, from ourein, to urinate
Urethra is the tube that carries urine from the urinary bladder to the exterior of the body, measuring approximately 3 to 4 centimeters in females and approximately 20 centimeters in males, with the male urethra also conducting semen during ejaculation.
The female urethra is straight and short, opening anterior to the vaginal orifice; its brevity reduces the distance bacteria must travel to reach the bladder, which is why women experience ascending urinary tract infections far more frequently than men. In males, the urethra passes through three segments, prostatic, membranous, and spongy, traversing the prostate gland and the external urethral sphincter before reaching the penile tip. Two sphincters govern urinary flow: the internal urethral sphincter, composed of involuntary smooth muscle, and the external urethral sphincter, composed of voluntary skeletal muscle that provides conscious control over the timing of urination.
The duck-billed platypus (Ornithorhynchus anatinus) has a single cloacal opening through which urine, feces, and reproductive products all exit, with no anatomically distinct urethra separate from the reproductive tract, a condition that contrasts sharply with the fully separated urethral and reproductive channels found in placental mammals.
Urinary System Fun Facts →The urethra and ureter are the same structure. The ureter is the tube connecting each kidney to the bladder, while the urethra is the separate tube that carries urine from the bladder to the outside of the body.
In male dogs (Canis lupus familiaris), the urethra passes through the os penis, a bone embedded in the penile tissue, and this narrow bony canal is a common site where urinary stones become lodged, causing obstruction that requires veterinary intervention.
Uterus
/ YOO-teh-rus / · Latin uterus, womb
Uterus is a hollow, thick-walled muscular organ in the female pelvis that receives and supports a fertilized ovum during implantation, sustains fetal development throughout gestation, and expels the fetus at parturition through coordinated myometrial contractions.
Three concentric layers form the uterine wall: the outer perimetrium, the middle myometrium composed of interlocking smooth muscle bundles, and the inner endometrium. Each menstrual cycle, rising estrogen drives endometrial proliferation, after which progesterone from the corpus luteum transforms the lining into a secretory tissue rich in glycogen-laden glands prepared for embryo implantation. When implantation does not occur, progesterone withdrawal triggers endometrial shedding as menstruation, resetting the cycle for the following month.
The uterus of the European hare (Lepus europaeus) supports superfetation, the simultaneous carrying of embryos at two different stages of development, because a second ovulation and fertilization can occur while a first litter is already gestating, a reproductive strategy not documented in humans.
How To Become A Neonatologist? →The uterus is only a hollow storage space. Myometrial smooth muscle generates the forceful, coordinated contractions that dilate the cervix and expel the fetus during labor, making the uterus one of the most mechanically active organs in the body.
Muscular System Facts →In placental mammals such as humans, the uterus supports the placenta during pregnancy; the placenta exchanges oxygen, carbon dioxide, nutrients, and metabolic wastes between maternal and fetal blood across a thin villous membrane without the two circulations directly mixing.
Reproductive System Fun Facts →