4.2.1 Structure of Kidney

The kidneys are a pair of bean-shaped structures that filter blood and produce urine. They are located just below and posterior to the liver in the peritoneal cavity. The kidneys have three external layers and three internal regions, each with specific structures and functions.

Definition

Kidney: The kidneys are a pair of bean-shaped structures located just below and posterior to the liver in the peritoneal cavity. They filter blood and purify it. The filtrate coming out of the kidneys is called urine.

Main Idea

• Filter blood and purify it
• Produce urine as filtrate
• Have three external layers
• Have three internal regions
• Are located just below and posterior to the liver
• Have adrenal glands on top
• The right kidney is slightly lower than the left

Core Concept

• Kidneys = bean-shaped; filter blood
• Location = just below and posterior to the liver
• Adrenal glands = on top of each kidney
• Right kidney = slightly lower than left
• Three external layers = renal fascia, perirenal fat capsule, renal capsule
• Three internal regions = cortex, medulla, renal pelvis
• Hilum = concave part; entry and exit point
• Renal cortex = outer region; contains nephrons
• Renal medulla = middle region; contains renal pyramids
• Renal pelvis = inner region; leads to ureter
• Renal pyramids = 8 per kidney
• Renal columns = between pyramids
• Renal papillae = tips of pyramids
• Major calyces = 2–3 extensions
• Minor calyces = further branches

Detailed Explanation

Location of the Kidneys

• The kidneys are a pair of bean-shaped structures.
• They are located just below and posterior to the liver in the peritoneal cavity.
• The adrenal glands are on top of each kidney.
• Kidneys filter blood and purify it.
• The filtrate coming out of the kidneys is called urine.
• The reason why the right kidney is slightly lower than the left is due to the considerable space occupied by the liver on the right side.

Structure of the Kidney

External Layers: Externally, the kidneys are surrounded by three layers:

Internal Regions: Internally, the kidney has three regions:

• The hilum is the concave part of the bean-shape.
• It is where blood vessels and nerves enter and exit the kidney.
• It is also the point of exit for the ureters.

• The renal cortex is granular due to the presence of nephrons.
• Nephrons are the functional unit of the kidney.

• The medulla consists of multiple pyramidal tissue masses, called the renal pyramids.
• In between the pyramids are spaces called renal columns through which the blood vessels pass.
• The tips of the pyramids, called renal papillae, point toward the renal pelvis.
• There are, on average, eight renal pyramids in each kidney.
• The renal pyramids along with the adjoining cortical region are called the lobes of the kidney.

• The renal pelvis leads to the ureter on the outside of the kidney.
• On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces.
• These further branch into the minor calyces.
• The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.

Summary Table of Kidney Structure

Intermediate Level

External Layers of the Kidney

Internal Regions of the Kidney

Structures Within the Kidney

Relationship Between Structure and Function

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Kidneys are bean-shaped.
• 2. Located just below and posterior to the liver.
• 3. Adrenal glands are on top of each kidney.
• 4. Right kidney is slightly lower than left.
• 5. Right kidney is lower due to liver space.
• 6. Three external layers: renal fascia, perirenal fat capsule, renal capsule.
• 7. Three internal regions: cortex, medulla, renal pelvis.
• 8. Hilum is the concave part.
• 9. Hilum is entry and exit point.
• 10. Renal cortex is granular due to nephrons.
• 11. Renal medulla contains renal pyramids.
• 12. Renal columns are between pyramids.
• 13. Renal papillae are tips of pyramids.
• 14. About 8 renal pyramids per kidney.
• 15. Major calyces are 2–3 extensions.
• 16. Minor calyces are further branches.
• 17. Ureters exit from hilum.
• 18. Ureters empty into urinary bladder.

Remember

• - Kidneys = bean-shaped; filter blood
• - Location = just below and posterior to liver
• - Adrenal glands = on top of each kidney
• - Right kidney = slightly lower than left
• - Three external layers = renal fascia, perirenal fat capsule, renal capsule
• - Three internal regions = cortex, medulla, renal pelvis
• - Hilum = concave part; entry and exit point
• - Renal cortex = outer region; contains nephrons
• - Renal medulla = middle region; contains renal pyramids
• - Renal pelvis = inner region; leads to ureter
• - Renal pyramids = 8 per kidney
• - Renal columns = between pyramids
• - Renal papillae = tips of pyramids
• - Major calyces = 2–3 extensions
• - Minor calyces = further branches

Common Mistakes

• 1. Thinking the left kidney is lower — The right kidney is slightly lower.
• 2. Mixing up the external layers — Renal fascia (outer), perirenal fat capsule (middle), renal capsule (inner).
• 3. Mixing up the internal regions — Cortex (outer), medulla (middle), pelvis (inner).
• 4. Confusing renal pyramids and renal columns — Pyramids are tissue masses; columns are spaces between them.
• 5. Forgetting the number of pyramids — About 8 per kidney.
• 6. Mixing up major and minor calyces — Major calyces are 2–3 extensions; minor calyces are further branches.
• 7. Thinking the hilum is on the convex side — The hilum is the concave part.
• 8. Forgetting the role of the renal pelvis — It collects urine and leads to ureter.
• 9. Mixing up the functions — Cortex contains nephrons; medulla contains pyramids; pelvis collects urine.
• 10. Believing the adrenal glands are part of the kidney — They are separate glands on top of the kidneys.

Quick Revision

The kidneys are a pair of bean-shaped structures located just below and posterior to the liver in the peritoneal cavity. The adrenal glands are on top of each kidney. The right kidney is slightly lower than the left due to the space occupied by the liver. Externally, the kidneys are surrounded by three layers: the renal fascia (outermost), the perirenal fat capsule (middle), and the renal capsule (innermost). Internally, the kidney has three regions: the renal cortex (outer, granular, contains nephrons), the renal medulla (middle, contains renal pyramids), and the renal pelvis (inner, collects urine). The hilum is the concave part where blood vessels and nerves enter and exit, and where ureters exit. The renal medulla contains about 8 renal pyramids per kidney, with renal columns between them. The tips of the pyramids are called renal papillae. The renal pelvis branches into major calyces (2–3) and then minor calyces. Ureters exit the kidney and empty into the urinary bladder.

60-Second Revision

• Kidneys = bean-shaped; filter blood
• Location = just below and posterior to liver
• Adrenal glands = on top of each kidney
• Right kidney = slightly lower than left
• Three external layers = renal fascia, perirenal fat capsule, renal capsule
• Three internal regions = cortex, medulla, renal pelvis
• Hilum = concave part; entry and exit point
• Renal cortex = outer region; contains nephrons
• Renal medulla = middle region; contains renal pyramids
• Renal pelvis = inner region; leads to ureter
• Renal pyramids = 8 per kidney
• Warning: Right kidney is lower than left because of the liver

NEXT TOPIC

• Chapter:
• 4. Urinary System of Man

• Topic:
• 4.2.2 RELATIONSHIP BETWEEN STRUCTURE AND FUNCTION OF KIDNEY

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