4.1 Human Urinary System

Introduction

The human body produces waste products during metabolism. These waste products are harmful if they accumulate in the body. The urinary system is responsible for removing these waste products and maintaining the balance of water and salts in the body. This topic introduces the organs of the urinary system and their functions.

Definition

Excretion is the process by which metabolic waste products and toxic materials are removed from the body of an organism.

The urinary system is the organ system responsible for excretion of metabolic wastes (especially urea) and for osmoregulation (maintaining water and salt balance).

Main Idea

The products of metabolism which are not needed by the organism are called waste products. These are harmful if allowed to accumulate in the body. They must be either removed or deposited as harmless insoluble form.

• Urea
• Uric acid
• Toxic substances such as pesticides, drugs, food additives

Carbon dioxide and water vapour produced during respiration are also metabolic wastes.

• A pair of kidneys
• A pair of ureters
• Urinary bladder
• Urethra

Core Concept

Organs of the Urinary System

Detailed Explanation

1. Kidney

What it is: The kidney is a reddish-brown, bean-shaped organ.

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

Structure:

• It is enclosed in a tough, transparent membrane called the capsule.
• The right kidney is slightly lower than the left kidney.
• The concave side of the kidney faces the vertebral column.
• 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.

Blood supply:

• The kidney receives blood from the dorsal aorta via renal arteries.
• The renal veins return blood to the inferior vena cava.

Function:

• Filters blood to remove waste products (especially urea).
• Produces urine.
• Regulates water and salt balance (osmoregulation).
• Maintains blood pH and blood pressure.

Simple explanation: The kidneys are like filters that clean the blood and produce urine.

2. Ureter

What it is: From the kidney, a thin tube called the ureter comes out and extends downwards to join the urinary bladder.

Function:

- Carries urine from the kidney to the urinary bladder.

Simple explanation: The ureter is like a pipe that carries urine from the kidney to the bladder.

3. Urinary Bladder

What it is: Urine from each kidney passes through the ureter to the urinary bladder.

Structure:

- The bladder is an elastic muscular bag.

Function:

• Stores urine temporarily.
• Expands to hold urine and contracts to release it.

Simple explanation: The urinary bladder is like a storage tank that holds urine until it is ready to be removed.

4. Urethra

What it is: The urinary bladder has only one exit, a tube called the urethra which leads out of the body.

Function:

- Carries urine from the bladder out of the body.

Simple explanation: The urethra is the final tube through which urine leaves the body.

Summary of Urine Flow

Kidney → Ureter → Urinary bladder → Urethra → Outside body

Intermediate Level

The urinary system consists of:

Key points:

• The kidney is enclosed in a capsule.
• The right kidney is slightly lower than the left.
• The hilus is the depression where renal artery, renal vein, and nerves enter/exit.
• The kidney receives blood from renal arteries and returns blood via renal veins.
• The bladder is an elastic muscular bag.

Important Points

• - The products of metabolism which are not needed by the organism are called waste products.
• - Waste products are harmful if allowed to accumulate in the body.
• - The waste products include urea, uric acid, and toxic substances such as pesticides, drugs, and food additives.
• - Carbon dioxide and water vapour produced during respiration are also metabolic wastes.
• - The process by which metabolic waste products and toxic materials are removed from the body is called excretion.
• - The urinary system of man consists of a pair of kidneys, a pair of ureters, urinary bladder, and urethra.
• - The kidney is reddish-brown in colour.
• - The kidney is bean-shaped and enclosed in a tough transparent membrane called the capsule.
• - The right kidney is slightly lower than the left kidney.
• - From the kidney, a thin tube called the ureter comes out and extends downwards to join the urinary bladder.
• - Urine from each kidney passes through the ureter to the urinary bladder.
• - The bladder is an elastic muscular bag which stores urine.
• - The urinary bladder has only one exit, a tube called the urethra which leads out of the body.
• - 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 concave side of the kidney faces the vertebral column.

Remember

• - Excretion = removal of metabolic waste
• - Waste products = urea, uric acid, toxic substances
• - Urinary system = kidneys, ureters, urinary bladder, urethra
• - Kidney = filters blood, produces urine
• - Ureter = carries urine to bladder
• - Urinary bladder = stores urine
• - Urethra = carries urine out of body
• - Kidney = bean-shaped, enclosed in capsule
• - Right kidney = slightly lower than left
• - Hilus = depression where renal artery, renal vein, and nerves connect
• - Renal artery = from dorsal aorta
• - Renal vein = to inferior vena cava

Common Mistakes

• 1. Confusing the ureter and urethra.
• - Ureter = tube from kidney to bladder.
• - Urethra = tube from bladder to outside.
• 2. Thinking the kidney is only for urine production.
• - The kidney also regulates water balance, salt balance, blood pressure, and pH.
• 3. Forgetting that the right kidney is lower than the left.
• - The right kidney is slightly lower than the left kidney.
• 4. Mixing up the renal artery and renal vein.
• - Renal artery = brings blood to kidney.
• - Renal vein = takes blood away from kidney.
• 5. Believing the bladder produces urine.
• - The bladder only stores urine. Urine is produced by the kidneys.
• 6. Thinking excretion and egestion are the same.
• - Excretion = removal of metabolic waste (urea, CO₂).
• - Egestion = removal of undigested food (faeces).
• 7. Forgetting the role of the hilus.
• - The hilus is where the renal artery, renal vein, and nerves connect to the kidney.

Quick Revision

60-Second Revision

• Excretion = removal of metabolic waste
• Waste products = urea, uric acid, toxic substances
• Urinary system = kidneys, ureters, urinary bladder, urethra
• Kidney = filters blood, produces urine; bean-shaped; enclosed in capsule
• Ureter = carries urine to bladder
• Urinary bladder = stores urine
• Urethra = carries urine out of body
• Right kidney = slightly lower than left
• Hilus = depression where renal artery, renal vein, and nerves connect
• Warning: Do not confuse ureter (kidney to bladder) with urethra (bladder to outside).