4.2 Structure and Function of Kidney

Introduction

The kidneys are a pair of bean-shaped organs that play a vital role in maintaining the body's internal environment. They filter blood, remove waste products, and regulate water and salt balance. This topic explains the location, structure, and function of the kidney.

Definition

The kidney is a reddish-brown, bean-shaped organ that filters blood to remove waste products and produces urine. It is the main organ of the urinary system.

Main Idea

The kidneys are located at the back of the abdominal cavity, one on each side of the vertebral column. The concave side of the kidney faces the vertebral column. The kidney receives blood from the dorsal aorta via renal arteries, and the renal veins return blood to the inferior vena cava. At the centre of the concave surface is a depression called the hilus, where the renal artery, renal vein, and nerves are connected to the kidney.

Core Concept

External Structure of the Kidney

Internal Structure of the Kidney

In vertical section, the kidney shows the following:

Detailed Explanation

1. Location of the Kidney

The kidneys are located at the back of the abdominal cavity, one on each side of the vertebral column.

Simple explanation: The kidneys are at the back of your belly, one on the left and one on the right of your backbone.

Why this location is important:

• Protected by the lower ribs and muscles.
• Close to the major blood vessels (aorta and inferior vena cava).
• Behind the peritoneum (retroperitoneal).

2. External Structure

Shape: The kidney is bean-shaped.

Colour: The kidney is reddish-brown in colour.

Capsule: The kidney is enclosed in a tough, transparent membrane called the capsule.

Concave side: The concave side of the kidney faces the vertebral column.

Hilus: At the centre of the concave surface is a depression called the hilus, where the renal artery, renal vein, and nerves are connected to the kidney.

Right vs. left kidney: The right kidney is slightly lower than the left kidney.

Simple explanation: The kidney is like a bean with a dent on one side. The dent (hilus) is where blood vessels and nerves enter and exit.

Why the right kidney is lower:

• The liver occupies space on the right side of the abdomen.
• This pushes the right kidney slightly downward.

3. Blood Supply to the Kidney

The kidney receives blood from the dorsal aorta via renal arteries.

The renal veins return blood to the inferior vena cava.

Simple explanation: Blood comes into the kidney through the renal artery and leaves through the renal vein.

Why this is important:

• The kidney filters blood to remove waste products.
• It receives about 20–25% of the heart's output.

4. Internal Structure of the Kidney

In vertical section, the kidney shows the following:

(a) Renal Cortex

What it is: Renal cortex is the darker outer region.

Colour: It is dark brown in colour because of the large network of blood capillaries.

Function:

• Contains the glomeruli and convoluted tubules of nephrons.
• Site of filtration and reabsorption.

Simple explanation: The cortex is the outer layer of the kidney. It contains many tiny blood vessels and filters.

(b) Renal Medulla

What it is: Renal medulla is the light inner region.

Structure: The medulla consists of conical structures called renal pyramids.

Function:

• Contains the loops of Henle and collecting ducts.
• Site of concentration of urine.

Simple explanation: The medulla is the inner layer of the kidney. It contains triangular structures called pyramids.

(c) Renal Pelvis

What it is: Renal pelvis is the hollow chamber.

Structure: It is the enlarged end of the ureter within the kidney.

Function:

• Collects urine from the collecting ducts.
• Passes urine into the ureter.

Simple explanation: The pelvis is like a funnel that collects urine and sends it to the ureter.

Summary of Kidney Structure

Functions of the Kidney

• Filtration: Removes waste products (urea, uric acid, creatinine) from the blood.
• Reabsorption: Reabsorbs useful substances (glucose, amino acids, water, salts) back into the blood.
• Secretion: Secretes waste products and excess ions into the tubule.
• Osmoregulation: Regulates water and salt balance in the blood.
• pH regulation: Maintains acid-base balance.
• Blood pressure regulation: Produces renin.
• Hormone production: Produces erythropoietin (stimulates RBC production).
• Vitamin D activation: Converts vitamin D to its active form.

Simple explanation: The kidney is a multi-purpose organ. It filters blood, keeps the right amount of water and salts, and produces hormones.

Intermediate Level

The kidney has a specific structure adapted for its function:

External structure:

Internal structure:

Blood supply:

• Renal artery from dorsal aorta.
• Renal vein to inferior vena cava.

Functions:

• Filtration of blood.
• Reabsorption of useful substances.
• Secretion of waste products.
• Osmoregulation.
• pH regulation.
• Hormone production.

Important Points

• - The kidneys are located at the back of the abdominal cavity, one on each side of the vertebral column.
• - The concave side of the kidney faces the vertebral column.
• - The kidney receives blood from the dorsal aorta via renal arteries.
• - The renal veins return blood to the inferior vena cava.
• - At the centre of the concave surface is a depression called the hilus.
• - The hilus is where the renal artery, renal vein, and nerves are connected to the kidney.
• - In vertical section, the kidney shows renal cortex, renal medulla, and renal pelvis.
• - Renal cortex is the darker outer region.
• - Renal cortex is dark brown in colour because of the large network of blood capillaries.
• - Renal medulla is the light inner region.
• - The medulla consists of conical structures called renal pyramids.
• - Renal pelvis is the hollow chamber and it is the enlarged end of the ureter within the kidney.
• - The right kidney is slightly lower than the left kidney.
• - The kidney is enclosed in a tough transparent membrane called the capsule.

Remember

• - Kidney = bean-shaped, reddish-brown
• - Location = back of abdominal cavity, one on each side of vertebral column
• - Capsule = tough, transparent membrane
• - Hilus = depression where renal artery, renal vein, and nerves connect
• - Renal artery = from dorsal aorta
• - Renal vein = to inferior vena cava
• - Renal cortex = darker outer region
• - Renal medulla = light inner region with renal pyramids
• - Renal pelvis = hollow chamber, enlarged end of ureter
• - Right kidney = slightly lower than left

Common Mistakes

• 1. Confusing the renal cortex and renal medulla.
• - Cortex = outer region, darker.
• - Medulla = inner region, lighter, with pyramids.
• 2. Mixing up the renal artery and renal vein.
• - Renal artery = brings blood to kidney.
• - Renal vein = takes blood away from kidney.
• 3. Forgetting the role of the hilus.
• - The hilus is where the renal artery, renal vein, and nerves connect to the kidney.
• 4. Thinking the right kidney is higher than the left.
• - The right kidney is slightly lower than the left kidney.
• 5. Believing the renal pelvis produces urine.
• - The renal pelvis collects urine and passes it to the ureter.
• 6. Confusing the capsule with the cortex.
• - Capsule = tough, transparent membrane covering the kidney.
• - Cortex = outer region of the kidney tissue.
• 7. Thinking the kidney only filters blood.
• - The kidney also regulates water balance, salt balance, blood pressure, and pH.

Quick Revision

60-Second Revision

• Kidney = bean-shaped, reddish-brown
• Location = back of abdominal cavity, one on each side of vertebral column
• Capsule = tough, transparent membrane
• Hilus = depression where renal artery, renal vein, and nerves connect
• Renal artery = from dorsal aorta
• Renal vein = to inferior vena cava
• Renal cortex = darker outer region
• Renal medulla = light inner region with renal pyramids
• Renal pelvis = hollow chamber, enlarged end of ureter
• Right kidney = slightly lower than left
• Warning: Do not confuse renal cortex (outer, darker) with renal medulla (inner, lighter).