Lesson 1: Overview of The Structure and Function of the Urinary System
By the end of this lesson, students will be able to:
- Identify the structure and function of the gross anatomical features of the urinary system.
- Identify the structures and functions of the internal anatomy of the kidney, including the renal cortex, renal medulla, calyces (minor, major), renal pyramids, renal columns, renal papillae, and renal pelvis.
Jump to Multilingual Approach Lesson Slides with translations
Introduction to the Urinary System
The urinary system plays a central role in maintaining homeostasis. Through waste removal and regulation of fluid balance, it directly influences blood pressure, electrolyte concentration, and acid–base balance. This chapter introduces the gross anatomy of the urinary system and examines the internal structure of the kidney, including its cortical and medullary organization, collecting structures, and blood supply.
Major Components of the Urinary System
The urinary system consists of four primary anatomical structures (Fig. 1):
-
Two kidneys
-
Two ureters
-
One urinary bladder
-
One urethra
The kidneys filter blood and produce urine. The ureters transport urine to the bladder for storage, and the urethra conveys urine from the bladder to the exterior.
Primary Functions of the Kidneys
The kidneys perform several essential physiological functions:
-
Removal of metabolic wastes, particularly nitrogenous wastes (like urea).
-
Regulation of fluid volume, influencing blood volume and blood pressure
-
Regulation of electrolyte concentration in body fluids
-
Maintenance of acid–base balance, working alongside the respiratory system
-
Regulation of red blood cell production, mediated by the hormone erythropoietin (released in response to low oxygen levels) which stimulates red blood cell production.
Anatomical Location of the Kidneys
The kidneys are positioned on the posterior wall of the abdominal cavity and are classified as retroperitoneal, meaning they lie behind the parietal peritoneum. They are located deep to the floating ribs and adjacent to the lumbar vertebrae.
This protected anatomical position shields the kidneys while allowing vascular access from the abdominal aorta and inferior vena cava.
External and Internal Structure of the Kidney
Externally, the kidney is bean-shaped. Internally, it contains a highly organized arrangement of cortical and medullary regions designed for efficient urine formation and collection (Fig. 3). At the medial indentation of the kidney lies the renal hilum, which houses blood vessels and urine-collecting structures.
The outer region of the kidney is the renal cortex, which surrounds the inner renal medulla. The medulla contains triangular structures called renal pyramids, separated by renal columns. The pyramids direct urine toward the collecting chambers at their apices. At the apex of each renal pyramid is a renal papilla, a small nipple-like projection where urine exits the medulla. From the renal pyramids, urine follows a defined pathway: Renal papilla → Minor calyx → Major calyx → Renal pelvis → Ureter. The renal pelvis serves as the funnel-shaped central chamber that collects urine before it exits the kidney through the ureter.
Renal Blood Supply: Arterial Pathway
The kidneys receive approximately 20% of resting cardiac output, reflecting their critical filtration function. Blood enters the kidney through the renal artery, a branch of the abdominal aorta, delivering hig-pressure blood to the arterial system of the kidney (Fig. 4). The arterial pathway proceeds as follows: Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Cortical radiate (interlobular) arteries → Afferent arterioles. These vessels deliver blood to the microscopic filtration units (the nephrons) of the kidney.
The venous return from mirrors the arterial pathway in reverse, with a single exception: Venous capillary beds near the nephron → Cortical radiate veins → Arcuate veins → Interlobar veins → Renal vein (NO SEGMENTAL VEINS).
Integration of Structure and Function
The internal architecture of the kidney reflects its physiological roles. The cortex and medulla are strategically organized to filter blood, concentrate urine, and regulate fluid and electrolyte balance. The collecting system ensures efficient transport of urine, while the extensive vascular network enables high-volume filtration.
Chapter Summary
The urinary system helps to maintain the body’s homeostasis through waste removal, fluid regulation, electrolyte balance, acid–base control, and hormonal signaling. The kidneys, located retroperitoneally in the posterior abdomen, contain a highly organized internal structure consisting of cortex, medulla, pyramids, columns, calyces, and renal pelvis. Their extensive vascular supply supports the essential function they play in filtration of the blood.
Multilingual Approach Lesson Slides with translations
Module Slide Decks in Eight Languages
Lesson Video
Watch this lesson video to hear an A&P instructor walk you through the lesson.
Practice Questions
Use these practice questions to assess your knowledge before you move on to the next section.
