Human Anatomy - Kotsan I. Ya. 2009

Urogenital apparatus
Urinary organs
Age-related features of the urinary system organs

In newborns and infants, the Kidneys are relatively large and have a lobulated, bumpy surface due to the underdevelopment of the renal cortex. This lobulation persists up to 2–3 years of age. By 5 years, the boundaries between the lobes gradually smooth out, and the outer surface of the kidneys becomes smooth.

At birth, the Kidney has a length of 4.2 cm and a mass of 12 g. Renal growth occurs in several stages. During the first year of life, the kidney grows rapidly, whereas from the second to the seventh year, growth slows down. In the first three years, the renal mass triples, after which growth remains minimal until the age of thirteen. Significant renal growth takes place at 13–14 years of age. Although the kidney reaches adult average size by age 20, it continues to grow until 30–40 years of age.

The renal cortex in newborns is very thin (1.5–2.0 mm) due to insufficiently developed proximal convoluted tubules. It is only by the age of 8–10 years that its Structure approximates that of an adult kidney.

The renal pelvis in newborns is broad and ampullary in shape.

The fibrous capsule of the kidney becomes distinct by 5 years of age, and by 10–14 years, its structure closely resembles that of the adult renal fibrous capsule.

The adipose capsule (renal fat) is absent in newborns and begins to form only during early childhood (4–7 years), subsequently continuing to thicken gradually. By 40–50 years of age, the thickness of the adipose capsule reaches its maximum, whereas in old age, it thins out and sometimes disappears entirely.

The renal fascia is very thin in newborns and thickens progressively as the child grows older.

The Topography of the kidneys changes with age due to their descent. For instance, in newborns, the upper pole of the kidney is projected at the level of the upper border of the 12th thoracic vertebra. In infancy (up to 1 year), it is already at the level of the middle of the 12th thoracic vertebra. The lower pole of the kidney in a newborn is located at the level of the lower border of the 4th lumbar vertebra, whereas in a one-year-old child, it is half a vertebra higher, which is associated with the rapid growth of the spine. After 5–7 years, THE POSITION OF the kidney relative to the spine approaches that of an adult. Over the age of 50, particularly in elderly and lean individuals, the kidneys may lie lower than in youth.

Due to bodily growth, not only the position of the kidney changes, but also the relative length of its artery and vein (the renal pedicle). In newborns, the renal pedicle is relatively long, and the vessels are positioned obliquely, with the Water/144.html">Origin of the renal artery and the opening of the vein located higher than the renal hilum. Subsequently, the renal pelvis gradually assumes a horizontal position, and after 50 years of age, due to some downward Displacement of the kidneys, the length of the renal pedicle increases, directed inferiorly.

The Ureters in newborns are spindle-shaped and tortuous. The length of the ureters in neonates is 5–7 cm. Ureters grow rapidly, increasing in length to 13–15 cm during the first two years of life. The muscular layer of the Ureter is poorly developed in early childhood.

The Urinary Bladder in newborns is spindle-shaped, in children of the first few years of life it is pear-shaped, at 8–12 years it is egg-shaped, and in adolescents, it acquires the adult shape. The bladder capacity in a newborn is 50–80 cm³. By 5 years of age, bladder capacity reaches 180–200 cm³, and by 12 years—250 cm³. Because the pelvic cavity is underdeveloped in newborns, the urinary bladder is located within the Abdominal cavity. At 1–3 years of age, the Cytology/practical/108.html">Fundus of the bladder lies at the level of the upper margin of the Pubic Symphysis; in adolescents, it is at the level of the middle; and in young adults, at the level of the lower margin of the pubic symphysis. The topography and STRUCTURE OF THE urinary bladder are finally established by 15–17 years of age.

The male Urethra in newborns is relatively longer (5–7 cm) than in other age periods because it originates from a high-positioned urinary bladder. The urethra grows slowly until adolescence, after which its growth accelerates.

The urethra in newborn females is 2.3–3 cm long, relatively wide, and curved in its lower part, forming an obtuse angle open anteriorly. The muscular coat of the urethra and the external sphincter develop during childhood (up to 12–13 years of age).