Excretory Products and Their Elimination · Lesson 2 of 10
Human Urinary System and the Internal Anatomy of the Kidney
“Locate the major urinary organs and describe kidney layers, the renal pelvis, calyces and route of urine.”
• Identify the two kidneys, ureters, bladder and urethra. • Explain how kidney location, vessels and drainage connect with function. • Distinguish capsule, cortex, medulla, pyramids and renal columns. • Trace urine from collecting ducts to the external opening. • Explain the difference between ureter and urethra.
A filtering system with a separate storage and drainage route
The kidneys continuously adjust blood composition by removing waste solutes and controlling the amount of water and ions returned to circulation. Yet filtering the blood is only the beginning of eliminating waste from the body. Urine created within the kidneys must be collected, transported, stored and ultimately expelled. The urinary system therefore includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
The two kidneys lie near the posterior abdominal wall on either side of the vertebral column, roughly from the last thoracic to the upper lumbar vertebral levels. Each adult kidney is reddish brown and bean-shaped, with approximate dimensions of 10–12 cm long, 5–7 cm broad and 2–3 cm thick, weighing about 120–170 g. Dimensions vary by person and should be understood as illustrative anatomical values rather than exact measurements of every healthy kidney.
What enters and leaves each kidney?
The concave medial surface contains a notch called the *hilum*. This acts as a gateway for the renal artery, renal vein, nerves, lymphatic vessels and the renal pelvis/ureter connection. The renal artery delivers blood containing dissolved wastes along with needed nutrients and ions. The renal vein returns processed blood towards the circulation. The urine drainage route is separate: urine emerging from the kidney flows to the ureter, not into the renal vein.
The indented region on a kidney's inner border through which major blood vessels, nerves and urine-draining structures pass.
From the capsule towards the central pelvis
A tough connective-tissue capsule covers the kidney. Immediately underneath lies the outer renal cortex, which contains numerous renal corpuscles and convoluted nephron segments. Deeper lies the renal medulla, organised into conical renal pyramids whose tips, called papillae, project into small receiving spaces known as minor calyces. These merge into larger collecting spaces and eventually the funnel-shaped renal pelvis.
The renal cortex also extends down between adjacent medullary pyramids as renal columns, historically called columns of Bertini. The medulla is not just a solid block; it is arranged into many collecting ducts and parallel tubular structures important in concentrating urine. The arrangement creates a pathway: nephrons generate fluid, collecting ducts carry it inward, papillae deliver it to calyces, and the pelvis joins the ureter.
| Part | Location | Functional significance |
|---|---|---|
| Fibrous capsule | Outer covering | Protects and supports kidney tissue |
| Cortex | Outer internal zone | Contains renal corpuscles and convoluted tubules |
| Medulla | Inner zone | Contains loops and collecting structures involved in concentration |
| Renal pyramids | Cone-shaped medullary regions | Drain collecting ducts towards calyces |
| Renal columns | Cortical tissue between pyramids | Separates neighbouring medullary masses |
| Calyces and renal pelvis | Central collecting spaces | Receive formed urine and direct it to the ureter |
The ureters transport; the bladder stores; the urethra releases
Each ureter is a muscular tube that propels urine from the renal pelvis towards the urinary bladder. Peristaltic contractions assist transport. The urinary bladder has expandable muscular walls so that urine can accumulate until micturition occurs. The urethra is the tube that carries urine from the bladder to the outside. Students often interchange ureter and urethra because the names look similar, but they perform different positions in a single ordered pathway.
The distinction between urine formation and urine storage is equally important. Nephrons form urine continuously, but a healthy person does not continuously release it because the bladder stores urine. The mechanism of bladder emptying includes neural reflexes and voluntary control, which will be explained after urine formation and its regulation.
Why is kidney structure arranged in cortex and medulla?
The cortical region contains structures that begin filtering blood and much of the subsequent fine adjustment of filtrate. The medulla forms an elongated environment through which loops of Henle and collecting ducts pass. A progressively concentrated medullary interstitium allows water to be reclaimed efficiently from suitable tubule segments. Thus the visible gross division of kidney tissue anticipates its physiological function: the kidney is not merely a sieve but a controlled filtration and recovery system.
Blood: renal artery → small renal vessels → kidney microcirculation → renal vein. Urine: collecting ducts → calyces → renal pelvis → ureter → bladder → urethra. These must never be drawn as a single continuous tube.
Quick Quiz
Quiz
Where are the kidneys primarily located?
Which structures carry urine from the kidneys to the bladder?
Which region contains the renal pyramids?
The tissue projecting between medullary pyramids is called...
What is the direct function of the urinary bladder?
What is the correct order of urine drainage after collecting ducts?
Practice Problems
- Identify the four major organ types in the human urinary system and their functions.
- Trace urine through kidney collection structures and down to the bladder.
- Describe capsule, cortex, medulla, pyramids and columns in a kidney cross section.
- Differentiate ureter and urethra, including number in a typical urinary system.
- Why are kidney medullary pyramids important for urine handling?
Kidneys form urine and regulate blood composition; ureters transport urine; bladder stores it; urethra releases it outside.
Key Takeaways
• The urinary system has two kidneys, two ureters, a urinary bladder and a urethra. • Kidneys lie near the posterior abdominal wall and receive blood through renal vessels. • The outer cortex and inner medulla have different structural and physiological roles. • Medullary pyramids drain into calyces, which converge into the renal pelvis. • Ureters transport urine to the bladder; the urethra releases stored urine from the body.