Urinary System Structures and Functions

• Lecture 10 covers structures of the urinary system, arterial supply, and venous drainage

The Urinary System

• Key components: Kidneys, ureters, urinary bladder, and urethra
• Two kidneys excrete urine
• Two ureters transport urine from the kidneys to the urinary bladder
• The urinary bladder collects urine
• The urethra passes urine outside of the body

Kidney Functions

• Kidneys filter nitrogenous wastes, toxins, and ions from the blood for excretion in urine
• They regulate blood volume and chemical composition, including water, salts, acids, and bases
• Kidneys produce regulatory enzymes, including Renin to regulate blood pressure and kidney function, and Erythropoietin to stimulate RBC production from bone marrow
• Kidneys metabolize vitamin D into its active form

Kidney Location

• Kidneys are reddish-brown and retroperitoneal, located in the superior lumbar region of the posterior abdominal wall
• Kidneys extend from the T12 to L3 vertebrae
• The right kidney is slightly lower than the left, by 1-2 cm, because of the presence of the liver
• Lateral surface is convex, medial surface is concave
• Kidneys have superior and inferior poles, and anterior and posterior surfaces.
• Kidneys have lateral and medial borders with the lateral border being convex and the medial border being concave
• The renal hilum is where blood vessels, nerves, and the ureter enter and exit
• Adrenal glands lie superior to each kidney

Kidney Anatomical Relations

• Right Kidney: related to the right adrenal gland, right lobe of the liver, duodenum, and right colic flexure
• Left Kidney: related to the left adrenal gland, stomach, spleen, body of the pancreas, left colic flexure, descending colon, and jejunum

Hilum of the Kidney

• Lies on the medial border
• Structures from anterior to posterior: renal vein, renal artery, and the renal pelvis of the ureter

Internal Anatomy of the Kidney

• Cortex: the superficial region that is light and granular
• Medulla: the deep central region of the kidney that has a deep and dark layer
• Renal pyramids: cone-shaped structures within the renal medulla with the the base against the cortex, and the apex forming the papilla
• Renal columns are extensions of the cortex that separate the renal pyramids.
• Collecting parts: renal papillae, minor calyces, major calyces, and the renal pelvis

Urine Collection

• Ducts from the renal papilla release urine into a minor calyx
• Minor calyx to major calyx
• Major calyx to renal pelvis
• Renal pelvis to ureter

Blood Supply

• The kidneys receive blood directly from the aorta via the renal arteries
• Filtered blood returns to the inferior vena cava via the renal veins

Blood Flow to Kidney - Arteries

• Renal arteries
• Segmental arteries
• Lobar arteries
• Interlobar arteries
• Arcuate arteries
• Interlobular arteries
• Afferent arterioles

Blood Flow from Kidney - Veins

• Blood flows from the nephron
• To the interlobular veins
• To the arcuate veins
• To the interlobar veins
• To the renal vein

The Nephron

• Nephrons are the functional units of the kidney responsible for urine formation
• Glomerular (Bowman's) capsule: a blind, cup-shaped end of a renal tubule that surrounds the glomerulus
• Glomerulus: A tuft of capillaries associated with a renal tubule
• Renal Tubules: Proximal Convoluted Tubule, Loop of Henle, and Distal Convoluted Tubules

Layers of Tissue Supporting the Kidney

• Renal capsule: A fibrous capsule that prevents kidney infection
• Adipose capsule: A fatty mass that cushions the kidney and helps it attach it to the body wall
• Renal fascia: An outer layer of dense fibrous connective tissue that anchors the kidney

Ureters

• The ureter is a continuation of the renal pelvis, consisting of a narrow, thick-walled tube, that transports urine from the kidneys to the urinary bladder
• Retroperitoneal location
• Approximately 25 cm long and 3 mm in diameter
• The ureter consists of abdominal and pelvic parts
• It runs medially within the posterior bladder wall before opening into the interior
• The entry into the posterolateral wall of the base of the urinary bladder assist in preventing backflow of urine

Ureter Characteristics

• Characterized by narrows, particularly at the junction with the renal pelvis, so renal calculi (stones) often get stuck there.
• Also narrows at the pelvic brim
• The ureter enters the bladder at the trigone in an angle that prevents backflow of urine, acting as a sphincter

Trigone

• A triangular region of the internal urinary bladder
• It is formed by the two ureteric orifices and the internal urethral orifice
• The trigone is very sensitive to expansion and signals the brain when the bladder needs to be emptied

The Urinary Bladder

• A bag that collects and stores urine until urination (micturition)

Urinary Bladder: Details

• An expandable muscular container, serving as a reservoir for urine, being located superior and posterior to the pubic symphysis
• In females: the urinary bladder is in contact with the uterus posterosuperiorly and with the vagina posteroinferiorly
• In males: the posterior surface is related to the rectovesical pouch, the seminal vesicle and vas deferens are related to the lower portions, and the prostate gland is inferiorly related
• When empty, the bladder exhibits an upside-down pyramidal shape

Micturition

• The urethral sphincter relaxes and the detrusor muscle contracts, to allow the bladder to empty with urination

The Urethra

• Fibromuscular tube exiting the urinary bladder through the urethral opening at the anteroinferior surface
• It conducts urine to the exterior of the body
• Two urethral sphincters are the Internal urethral sphincter that restricts the release of urine until pressure is high enough, and an External urethral sphincter with voluntary activities

Male Urethra

• Functions: passageway for both urine and semen, being approximately 18 to 20 centimeters long
• Partitioned into three segments:
  • Prostatic: Approximately 3 to 4 centimeters long, is the most dilatable portion, that extends through the prostate gland
  • Membranous extends from the inferior surface of the prostate gland through the urogenital diaphragm, being the shortest and least dilatable portion.
  • Spongy encases a cylinder of erectile tissue in the penis called the corpus spongiosum that extends out to the external urethral orifice (15 centimeters thus the longest part).

Female Urethra

• Single function of transporting urine from the urinary bladder to the vestibule outside the labia minora
• About 3 to 5 centimeters long and opens to the outside of the body at the external urethral orifice located in the female perineum

This lecture outlines the structures of the urinary system, including the kidneys, ureters, urinary bladder, and urethra. It details kidney functions such as filtering wastes, regulating blood volume, and producing regulatory enzymes. The lecture also covers the location and arterial supply of the kidneys.