Location of the Renal Pyramids
The pyramids are located in the renal medulla (medulla renis), deep to the renal cortex (cortex renis).
The kidney (ren) is a paired, bean-shaped organ. It lies retroperitoneally (retroperitonealiter) on either side of the vertebral column, typically extending from the 12th thoracic (Th12) to the 3rd lumbar (L3) vertebra. The right kidney usually sits slightly lower than the left.
Anatomical landmarks of the kidney containing the pyramids:
- Surfaces — anterior and posterior (facies anterior et posterior).
- Borders — convex lateral (margo lateralis) and concave medial (margo medialis).
- Poles — superior and inferior (extremitas superior et inferior).
Renal capsules: fibrous capsule (capsula fibrosa), adipose capsule (capsula adiposa), and renal fascia (fascia renalis).
Structure and Urine Outflow Pathway
The structural and functional unit of the kidney is the nephron (nephron). Urine formation results from the interaction between the vascular network and the epithelial tubular system.
Path of urine from the renal pyramids to the excretory structures:
- Renal papillae (papillae renales) — apexes of the renal pyramids opening into the minor calyces.
- Minor renal calyces (calices renales minores) — structures receiving urine directly from the papillae.
- Major renal calyces (calices renales majores) — formed by the convergence of minor calyces.
- Renal pelvis (pelvis renalis) — formed by the confluence of major calyces.
The renal pelvis, along with blood vessels, nerves, and lymphatic vessels, passes through the renal hilum (hilum renale) located on the medial border. The hilum opens into the renal sinus (sinus renalis).
Blood Supply to the Renal Parenchyma
Renal circulation features unique hemodynamics: blood sequentially passes through two capillary beds. The first consists of the glomerular capillaries enclosed by Bowman's capsule, and the second surrounds the epithelial tubules.
Intrarenal circulation is supplied by the following vessels:
- Interlobar arteries and veins (aa. et vv. interlobares renis).
- Arcuate arteries and veins (a. et v. arcuata).
- Interlobular arteries (a. interlobularis).
The renal artery (a. renalis) branches within the renal sinus, and the renal vein (v. renalis) collects tributaries from the sinus area. The left suprarenal vein (v. suprarenalis) and left testicular/ovarian vein (v. testicularis) drain into the left renal vein.
Neonatal Renal Characteristics
In newborns, kidney morphology has several distinct features:
- Kidneys are relatively large, positioned lower, and exhibit fetal lobulation.
- The cortex is relatively thin, and the renal pelvis is largely intrarenal.
- The tubular system and loops of Henle are short with narrow lumens.
- Concentrating ability is reduced, leading to impaired sodium excretion, which readily predisposes infants to edema under water or salt loads.
- Ureters are wide and tortuous with an underdeveloped muscular and elastic framework, predisposing to vesicoureteral reflux.
- Daily urine output is about 250 mL with a high voiding frequency (20–25 times a day).
- There is a close lymphatic connection between the kidneys and the intestine, posing a risk of gut-derived infection translocation into the urinary tract.