Anatomy of the Kidney

Anatomy of the Kidney

External Anatomy

• Located retroperitoneally on the PAW (posterior abdominal wall)
• Found on either side of the vertebral column at the T12-L3 vertebrae
• Living kidneys are reddish-brown
• Approximate kidney measurements:
  • 10 cm in length
  • 5 cm in width
  • 5.5 cm in thickness
• Hilum of the left kidney is near the transpyloric plane
• The kidneys are bean-shaped in appearance
• Each kidney has:
  • Anterior and posterior surfaces
  • Medial and lateral margins
  • Lateral margins have a convex shape
  • Medial margins are concave and include the renal hilum, renal sinus, and renal pelvis
  • Superior and inferior poles.

Renal Hilum

• The point at which structures serving the kidneys enter and exit the renal sinus
  • E.g., vessels, nerves, and urine-draining structures
• Structures on the renal hilum (anterior to posterior):
  • Renal vein (larger and wider than the renal artery)
  • Renal artery
  • Renal pelvis
  • Flattened, funnel-shaped expansion of the superior end of the ureter
  • Apex joins the ureter

Laterality

• For determining the laterality of kidneys, note that the renal pelvis or ureter is on the posterior portion of the hilum

Vasculature

• Right renal artery is longer due to the aorta's location on the left
• Left renal vein is longer (6-10 cm) because the IVC is on the right side
• Renal arteries branch into 5 segmental end arteries (1 posterior, 4 anterior)
• Left renal vein receives 3 tributaries: inferior phrenic, gonadal, and lumbar veins
• Segmental arteries are end arteries, thus there is no collateral circulation (if one is blocked, the supplied tissue dies), while there is extensive collateral circulation around the infundibula and perinephric area
• Blood flow:
  • Renal artery to segmental end arteries
  • To lobar, interlobar, arcuate, and interlobular arteries
  • Then to the afferent arteriole, glomerulus, and efferent arteriole
  • Finally to the vasa recta and secondary capillary networks, which drain via interlobular, arcuate, interlobar, lobar, and segmental veins into the renal vein

Arteries

• The longer right renal artery passes posterior to the IVC
• Kidney has 5 vascular segments (aka renal segments), each supplied by an end artery
  • Renal segments are independent and surgically resectable units
  • END arteries have no collateral circulation
  • Blockage leads to ischemia
• Renal arteries branch into 5 Segmental End Arteries

• Note: In Moore, anterosuperior and antero-inferior segmental arteries were used; in the KUB anatomy lecture, upper/superior segmental end artery instead of anterosuperior segmental artery and middle segmental end artery instead of antero-inferior segmental artery was used

Superior Polar Artery

• An unusual branching pattern of the renal artery
• Superior polar renal artery has different sources of origin:
  • Abdominal aorta as an accessory renal artery
  • Direct branch from the main renal artery
  • Superior hilar renal artery
  • A segmental branch of the renal artery
• Superior polar artery as an accessory renal artery arising from the abdominal aorta
  • During development, the kidneys lie in the pelvic cavity and as the embryo grows they ascend to reach the lumbar region
  • They take their blood supply from branches of iliac arteries, and as they ascend, their blood supply also shifts from the iliac arteries to the abdominal aorta
  • Accessory arteries are due to the persistence of embryonic vessels which are formed during the ascension of the kidneys

Internal Anatomy

• Coverings of the Kidney
• Perinephric Fat
  • A pad of fat that surrounds the kidneys and their vessels with extensions into the renal sinuses
• Renal Capsule
  • A tough, fibrous, connective tissue that closely envelopes each kidney and gives support to the soft tissues inside

Spaces/Areas

• Renal Sinus
  • Space within the kidney
• Renal Hilum
  • Is the entrance to the renal sinus

Parenchyma of the Kidney

• Renal Cortex
  • Outer region inner to the capsule that surrounds the medulla
• Renal Columns
  • aka Columns of Bertin
  • Extension of the renal cortex parenchyma
  • Separates renal pyramids and provides a passage for blood vessels
  • Contains fibrous connective tissue contributing to the stroma
• Renal Medulla
  • A series of renal pyramids that appear striated as they contain tubular structures and blood vessels
• Renal Pyramids
  • Are triangular
  • Consist of numerous collecting ducts that condense at the apex of each pyramid (the renal papilla)
  • Form the kidney lobes that are visible on the external surface

Drainage of Urine from Parenchyma

• Renal Papilla
  • The apex of the renal pyramid, from which urine is excreted and drained into the ureter
• Calyces
  • Major calyces are cup-shaped cavities, receiving urine from the renal pelvis
  • Each is divided into two or three minor calyces
  • Each minor calyx is indented by a renal papilla
  • In living persons, the renal pelvis and its calices are usually collapsed or empty
• Renal Pelvis
  • A flattened, funnel-shaped expansion of the superior end of the ureter
  • Apex joins the ureter
• Ureter
  • Two narrow tubes that convey urine produced by the kidneys to the urinary bladder located in the pelvis

Flow of Urine

• Renal Pyramid/Medulla to Renal Papilla to Minor Calyces (2 or 3) to Infundibula to Major Calyces to Renal Pelvis

• Urine Formation

  • Begins when a large amount of fluid that is free of protein is filtered from the glomerular capillaries into Bowman’s capsule
  • As filtered fluid leaves the Bowman’s capsule and passes through the tubules, it is modified by reabsorption of water and specific solutes back into the blood, then secretion of other substances from the peritubular capillaries into the tubules occurs In summary, urine excretion results from (1) glomerular filtration, (2) tubular reabsorption, and (3) tubular secretion Formula for urinary excretion rate: Filtration rate - Reabsorption rate + Secretion Rate.

• Substance A

  • Only freely filtered by glomerular capillaries; neither reabsorbed nor secreted; excretion rate equals filtration rate (e.g., creatinine)

• Substance B

  • Freely filtered but partly reabsorbed from tubules to blood; urinary excretion is less than filtration rate (e.g., electrolytes like sodium and chloride)

• Substance C

  • Freely filtered at glomerular capillaries, but not excreted into urine because all filtered substance is reabsorbed from tubules back into blood (e.g., nutritional substances like amino acids and glucose)

• Substance D

  • Freely filtered at glomerular capillaries and not reabsorbed
  • Additional quantities secreted from peritubular capillary blood into renal tubules; excretion rate is filtration rate plus tubular secretion rate (e.g., organic acids and bases)

Anatomy of the Bladder

Related Spaces (Female Pelvis)

• Utero-vesical pouch
  • Is found between the anterior surface of the uterus and superior wall of the bladder; is a pelvic cavity formed by a peritoneal fold
• Recto-uterine space
  • Bounded by the uterus (posterior surface) and the rectum (anterior surface)
  • Is also known as the posterior cul-de-sac
  • Considered a potential space where natural fluid (e.g., free peritoneal fluids, blood due to retrograde menstruation, ovarian secretions) can accumulate
• Retropubic space
  • Also known as Space of Retzius or Perivesical space
  • Located between the bladder and pubic bone
• Male Pelvis:
  • Retropubic Space
  • Retrovesical Space homolog to the recto-uterine space

Median Umbilical Ligament

• Is a fibrous remnant of the urachus
• Represents a structure rising from the apex of the urinary bladder, directed cranially or superiorly to the umbilicus
• Covered by the median umbilical fold
• Is a fibrous cord that runs from the apex of the fetal bladder to the umbilicus

Urachus

• Is an extraembryonic membrane that stores urinary wastes and mediates gas exchange
• Initially protrudes from the embryo's endodermal surface and links to the yolk sac
• Transforms into the umbilical cord
• The juncture of the allantois and developing embryo forms part of the hindgut, developing into the bladder
• Allantois to urachus to median umbilical ligament
• Patent Urachus
• Is due to persistent remnants of the urachus Clinical presentation varies: periumbilical discharge, umbilical mass, abdominal/periumbilical pain, dysuria. Presentation depends on the persistent remnant’s size and location; various remnants include umbilical polyps, sinuses, fistulae, cysts, and bands

Parts of the Urinary Bladder

• Apex
  • Points toward the superior edge of the pubic symphysis when the bladder is empty, opposite the fundus
• Body
• Major portion of the bladder between the apex and the fundus
• Fundus
  • Located opposite the apex and formed by the posterior wall
• Neck -
  • Located inferiorly (below the internal urethral orifice)
  • Emerges from the union of the right and left inferolateral surfaces and connects to the urethra

Bladder Bed

• Structures in direct contact with the bladder
• On each side, the pubic bones and the superior obturator internus muscle lie in contact with the inferolateral surfaces of the bladder
• Only the superior surface is covered by peritoneum
• Males:
  • the fundus is separated from the rectum only by the fascial rectovesical septum centrally and by the seminal glands and ampullae of the ductus deferens laterally.

• Females:
  • The fundus is directly related to the superior anterior wall of the vagina

Detrusor Muscle

• Is the main component of the bladder walls in the bladder's walls (smooth muscle) under autonomic control
  • Parasympathetic: Contraction (+ relaxation of internal urethral sphincter) → bladder emptying
  • Sympathetic (ejaculation): Contraction of internal urethral sphincter → prevents semen reflux into the bladder

Internal Urethral Sphincter (Involuntary)

• The muscle fibers (of detrusor muscle) is formed at the neck of the male bladder
• Contracts during ejaculation to prevent semen reflux into the bladder
• Internal Urethral Orifice
  • Opening assisted by fibers that run radially in the bladder
  • Males: muscle fibers of the neck are continuous with the fibromuscular tissue of the prostate
  • Females: muscle fibers are continuous with the muscle fibers in the wall of the urethra

Trigone of the Bladder

• The smooth, triangular area bearing three openings inside the fundus of the bladder; defined by two superior ureteric orifices and the inferior internal urethral orifice
• Ureteric Orifices
  • Encircled by loops of detrusor muscle that tighten when the bladder contracts to prevent the reflux of urine into the ureter
• Interureteric Ridge/Crest/Bar (Bar of Mercier)
  • A submucosal ridge running transversely between the two ureteric orifices
• Uvula of the Bladder
  • Is a slight elevation of the trigone, more prominent in men due to the posterior lobe of the prostrate