Gross Anatomy of the Kidney

Overview of the Urinary and Urogenital Systems

• Gross anatomy refers to the study of structures that can be observed with the unaided eye, without the assistance of a microscope.

• The kidneys are paired organs, and it is noted that the right kidney is positioned slightly lower than the left kidney.

• The kidneys lead into the ureters, which are tubes that transport fluid to the urinary bladder.

• The ureters enter the urinary bladder from the posterior aspect.

• The entry point for the ureters is at the edges of the trigone, which is a specific triangular area within the bladder.

• The trigone is distinguished by having a slightly different epithelium compared to the rest of the urinary bladder.

• The urinary system is composed of several structures working together:

  • The kidneys.

  • The ureters.

  • The urinary bladder.

  • The urethra.

• In males, this system is referred to as part of the urogenital system.

• In females, it is referred to specifically as the urinary system.

Internal Zonation: Renal Cortex and Renal Medulla

• The kidney can be divided into $2$ primary sections: the outer cortex and the inner medulla.

• The Cortex:

  • This is the outer portion or the "shell" of the kidney.

  • It has a spotted or speckled appearance to the unaided eye.

  • These specks are caused by the presence of glomeruli (or a single glomerulus), which are little tufts of capillaries.

• The Medulla:

  • This is the deeper, internal portion of the organ.

  • The medulla contains structures known as medullary pyramids (or simply renal pyramids).

The Medullary Pyramids and Urine Formation Pathway

• Medullary Pyramids:

  • The pyramids are shaped similarly to candy corn, the candy often distributed during Halloween.

  • The speaker notes that candy corn typically features a color sequence such as white, yellow, and orange.

  • In the kidney, these pyramids exhibit lines and a shaded character where the color changes across different sections.

  • The lines visible in the pyramids represent the collecting ducts.

• Physiological Process within the Pyramids:

  • Capillaries feed into a tubular network that drains into the collecting ducts.

  • The collecting ducts gather filtrate.

  • There is smooth muscle located at the end of the medullary pyramids.

  • This smooth muscle squeezes the fluid into a structure called a renal papilla.

  • The renal papilla is described as a "little nipple" structure.

  • The very end of the renal papilla is where the $1^{st}$ drop of urine arises from the filtrate.

Gross Structure of the Urine Collection System

• Once urine is formed at the renal papilla, it must be collected by a series of funnel-like structures:

  • Minor Calyx: This is a funnel-shaped structure that captures the urine immediately after it exits the renal papilla.

  • Major Calyx: Multiple minor calyces (the plural form of calyx) combine to form a major calyx.

  • Renal Pelvis: The kidney is a $3$-dimensional organ, so major calyces converge from multiple directions (including from "within the paper" in the diagram and from above). These major calyces all collect into the renal pelvis.

• The renal pelvis then feeds into the ureter, which carries the urine to the bladder for storage and eventual excretion through the urethra.

Renal Vasculature: Arterial Blood Supply

• Blood enters the kidney via the renal arteries, which are paired and branch off the descending aorta.

• The renal arteries are positioned anterior to the renal pelvis.

• Sequential Arterial Branching:

  • Renal Artery.

  • Lobar Arteries.

  • Segmental Arteries.

  • Interlobar Arteries: These travel up through the renal columns, which are the areas located between the renal pyramids.

  • Arcuate Arteries: These are formed as the interlobar arteries divide and arch over.

  • Cortical Radiate Arteries: These radiate out into the cortex.

• The cortical radiate arteries feed into the glomerulus, a capillary bed that is the functional unit of the kidney (visible only as specks in gross anatomy).

Renal Vasculature: Venous Drainage Systems

• The drainage of blood from the kidney follows a path that largely reverses the arterial flow, with one notable exception.

• At the efferent end of the glomerular capillary beds are efferent arterioles (which are too small to be seen with the unaided eye).

• These feed into $2$ types of networks:

  • Peritubular Capillaries: These surround the renal tubules in the cortex.

  • Vasa Recta: These are "straight veins" (or straight vessels) that dive deep down into the medulla and then re-ascend before returning to the venous system.

• Sequential Venous Drainage:

  • Cortical Radiate Veins.

  • Arcuate Veins.

  • Lobar Veins.

  • Note: There are no segmental veins in the kidney; the drainage goes directly from the arcuate/cortical radiate area into the lobar veins.

  • Renal Vein.

• The left renal vein is described as being long, while the right renal vein is short.

• Both renal veins drain into the inferior vena cava, which is located on the right side of the body.