The Urinary System & Kidneys

• The urinary system's organs produce, store, and excrete urine, including the kidneys, ureters, urinary bladder, and urethra.

Kidneys

• Kidneys filter blood, removing water and small molecules, reabsorbing needed water and molecules.
• Paired kidneys are located at T12 and L3, with the right kidney more inferior due to the liver's presence.
• Kidneys are retroperitoneal, located behind the peritoneum with only the anterior surface covered, the posterior is adjacent to the diaphragm, 12th rib, psoas and quadratus lumborum muscles.
• Adrenal glands sit superior to the kidneys and have critical endocrine functions, making hormones like aldosterone, sex hormones, and corticosteroids.
• The fibrous renal capsule is the outermost layer, that gives the organ a smooth, firm surface.
• Thick perinephric fat supports and protects kidneys.
• The perinephric or renal fascia, a thick connective tissue, covers and supports the kidneys and perinephric fat.
• The right kidney is lower than the left because of the liver on the right side.

Ureters

• The ureters are 25-30 cm tubular structures, lined with smooth muscle, connect kidneys to the bladder.
• Ureters begin at the kidney hilum, descend anterior to the psoas muscles, cross the common iliac vessels, and enter the pelvis.
• In females, the ureter's pelvic segment crosses inferior to the uterine artery.
• Ureters connect to the bladder's posterior wall, entering at an angle without a valve.
• Bladder muscle contraction also contracts the ureteral orifice, preventing urine reflux.
• Kidney stones commonly get stuck in the ureters at the ureteropelvic junction, where the ureter crosses under the uterine artery, and at the ureteral insertion into the bladder.
• Stones cause hematuria and pain radiating down to the groin.

Urinary Bladder

• The bladder receives urine from the ureters, stores it, and expels it via micturition.
• The bladder is anterior to the pubic symphysis in both males and females.
• In males, the bladder is posterior to the rectum.
• In males, the levator ani muscle and prostate gland provide inferior support.
• In females, the bladder is posterior to the uterus.
• In females, the levator ani muscle and vagina provide inferior support.

Urethra in Males

• A bladder neck opening connects to the urethra, which carries urine outside.
• The male urethra is divided into segments based on location:
  • Preprostatic Urethra: From the bladder base to the prostate; periurethral glands here contribute to benign prostatic hyperplasia.
  • Prostatic Urethra: From the internal urethral sphincter at the bladder neck to the pelvic floor muscles; the ejaculatory duct opens here.
  • Membranous Urethra: From the prostate to the urethral bulb, penetrating the perineal membrane, encircled by the external urethral sphincter comprised of skeletal muscle, narrowest section.
  • Penile (Spongy) Urethra: Runs through the penis bulb and corpus spongiosum, opening at the urethral meatus.
• The membranous urethra in males is the shortest and narrowest part.

Urethra in Females

• In females, the urethra is very short and exits anterior to the vaginal opening.
• Females get more urinary tract infections because their urethra is shorter.

Kidney Vascular Supply

• The kidneys receive 25% of cardiac output and are well-perfused.
• Oxygenated blood flows from the aorta to the kidneys via renal arteries and drains into the inferior vena cava via renal veins.
• The renal artery splits into segmental arteries before entering the renal parenchyma.
• Segmental arteries branch into interlobar arteries, then arcuate arteries, then interlobular arteries and end in the glomerulus, as a capillary tuft to filter the blood.
• Glomerular filtration regulation occurs via arterial tone of the afferent (arriving) and efferent (exiting) arterioles.
• Kidneys have a second capillary network of peritubular capillaries supplied by the efferent arteriole, to supply tubules with oxygenated blood.
• Deoxygenated blood returns to the IVC via renal veins.
• The left renal vein is longer than the right because it crosses over the aorta.
• The left gonadal vein drains into the left renal vein, the right gonadal vein drains into the inferior vena cava.
• Obstructed left renal vein (e.g., by tumor or the superior mesenteric artery) causes high venous pressure, leading to a left-sided varicocele in males.
• The efferent arteriole drains the glomerulus.

Ureter Vascular Supply

• Ureters get blood from the renal arteries (upper third), aorta/common iliac/gonadal artery (middle third), and internal iliac artery (lower third).
• These arteries are paired with corresponding veins that drain into the inferior vena cava.

Urinary Bladder Vascular Supply

• The upper bladder is supplied by the superior vesical artery and vein; the lower bladder by the inferior vesical artery and vein.
• Superior vesical arteries branch from the umbilical arteries (from the internal iliac arteries).
• Superior and inferior vesical veins are tributaries to the internal iliac veins.
• In females, the uterine and vaginal arteries also supply the bladder.

Kidney Lymphatic Drainage

• Renal lymphatics run along arteries in the renal parenchyma.
• All lymphatics exit the kidney from the hilum, along with lymphatics from surrounding tissues and upper ureters.
• Kidney lymphatics drain into the right and left para-aortic and paracaval nodes.
• The lymphatic vessels drain into the para-aortic lymph nodes on the left and the paracaval lymph nodes on the right.

Ureter Lymphatic Drainage

• Ureter lymphatic drainage is divided:
  • Upper third (left) drains into the left para-aortic lymph nodes.
  • Upper third (right) drains into the paracaval lymph nodes.
  • Middle third drains into the common iliac lymph nodes.
  • Lower third drains into the common iliac, internal iliac, and external iliac lymph nodes.

Urinary Bladder Lymphatic Drainage

• Lymphatic vessels from the superior bladder drain into the external iliac lymph nodes and from the inferior bladder into the internal iliac nodes.

Kidney Innervation

• Kidneys are innervated by the autonomic nervous system (sympathetic and parasympathetic).
• Sympathetic nerves in the kidney cause arterial constriction, reduced renal blood flow, stimulated tubular sodium reabsorption, and renin release.
• Autonomic nerves arise from the renal nerve plexus, that consist of sympathetic and visceral afferent fibers.
• Sympathetic preganglionic neurons originate from T10-L1 spinal cord, and travel to the aorticorenal ganglion. Postganglionic fibers travel to the kidneys along the renal artery.

Ureter Innervation

• Autonomic innervation stimulates peristalsis, derived from adjacent autonomic plexuses (renal, aortic, superior/inferior hypogastric).
• Visceral afferent fibers conveying pain sensations follow sympathetic fibers back to spinal ganglia and cord segments T11-L5.

Urinary Bladder Innervation

• Micturition is a mix of voluntary and involuntary control, including the spinal micturition reflex.
• The bladder is innervated by sympathetic fibers (L1-L2) and parasympathetic fibers (S2-S4).
• Sympathetic fibers promote bladder relaxation and urine retention.
• Parasympathetic fibers promote bladder contraction and urine release.
• Sympathetic fibers to the bladder are conveyed to the vesical plexuses. They cause bladder relaxation and internal urethral sphincter contraction, preventing semen reflux during ejaculation in males.
• Parasympathetic fibers arise from sacral spinal cord levels and are conveyed by the pelvic splanchnic nerves and the inferior hypogastric plexuses. They stimulate detrusor muscle contraction and inhibit the male internal urethral sphincter, promoting urine expulsion.
• Sensory fibers in the bladder wall are stimulated by stretching, causing detrusor muscle contraction to empty the bladder.
• The involuntary internal urethral sphincter relaxes in males, allowing urine to flow into the urethra.
• The somatic pudendal nerve supplies motor innervation to the external urethral sphincter for voluntary control.
• Sympathetic stimulation causes bladder relaxation and therefore retention of urine.