Structure of Urinary System

Overview of the Urinary System

A city needs both a clean water supply and an efficient waste-disposal network to function properly; your body relies on the urinary system for a similar waste-management role.
The urinary system has two main jobs: excretion (removing metabolic waste like urea) and osmoregulation (balancing the water and salt levels in your blood).
Macroscopically, the human urinary system is made up of four key components: two kidneys, two ureters, one bladder, and one urethra.
The system works sequentially to clean the blood. First, blood is filtered in the kidneys, then the resulting waste liquid (urine) travels down the ureters to be stored in the bladder before finally exiting the body via the urethra.

The Kidneys and Blood Supply

Despite being only the size of a computer mouse, your kidneys filter your entire blood volume dozens of times a day.
The kidneys are bean-shaped organs located at the back of the abdominal cavity, sitting just below the ribcage on either side of the spine.
Unfiltered, oxygenated blood is delivered directly from the aorta to the kidneys via the renal artery at very high pressure.
If you cut a kidney open, you would see three distinct internal regions:
- Cortex: The light-coloured outer layer where blood filtration begins.
- Medulla: The darker inner region containing pyramid-like structures.
- Renal pelvis: A central, funnel-like cavity that collects the newly formed urine.
Deep inside the cortex and medulla are roughly one million microscopic nephrons (kidney tubules). These are the functional units of the kidney where filtration and selective reabsorption actually occur.
Once the blood has been cleaned, it is now deoxygenated and leaves the kidney through the renal vein, traveling back to the vena cava.

The Pathway of Urine

What happens to the waste liquid once the kidneys have finished filtering the blood?
The collected urine—typically composed of $95\%$ water, $2\%$ urea, and $2.5\%$ mineral ions and other wastes—drains from the renal pelvis into the ureters.
The ureters are narrow, muscular tubes about $25$ to $30$ cm long. They do not rely on gravity; instead, they use active wave-like muscular contractions called peristalsis to squeeze urine down into the bladder.
The bladder is a hollow, muscular sac with elastic walls that acts as a temporary storage vessel, usually holding between $400$ and $600$ mL of urine.
When the bladder is full, stretch receptors in its wall send signals to the brain. Urine is kept inside by two sphincter muscles at its base: an internal involuntary one and an external voluntary one.
When these sphincters relax, urine is released into the urethra, a single tube leading to the outside of the body.
Note that male and female urethras differ: in females, the urethra is short and exclusively carries urine, whereas in males, it is longer and serves a dual purpose, carrying both urine and semen.

Interpreting Urine Production Rates

Doctors often use the rate of urine production to check if a patient's kidneys are functioning correctly.
In exams, you may be required to calculate flow rates by dividing the total volume of urine produced by the time taken.

A patient's kidneys produce $840\,cm^3$ of urine over a $14$ -hour period. Calculate the average rate of urine production in $cm^3/min$ .

Step 1: Identify the values and convert time into the correct units.

Volume $= 840\,cm^3$
Time in minutes $= 14 \times 60 = 840$ minutes

Step 2: Substitute into the rate formula.

Rate $= \frac{\text{Volume}}{\text{Time}}$

Step 3: Calculate the final answer with units.

Rate $= \frac{840}{840}$
Rate $= 1.0\,cm^3/min$

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Exam Tips

Common Mistake
Students frequently confuse the ureter and urethra. Remember this mnemonic: 'Ureter' has two 'e's (like the two tubes connecting the kidneys to the bladder), while 'Urethra' has an 'a' for 'away from the body'.
2
When asked to describe the blood in the renal vein, you must state that it is both 'deoxygenated' AND 'filtered' (or 'cleaned') to secure all the marks on an Edexcel mark scheme.
3
If you have to identify blood vessels on a diagram, look for the thickness of the walls; the renal artery will have thicker walls because it branches directly from the aorta and carries blood at a much higher pressure than the renal vein.
4
Remember that while glucose is present in the renal artery, it should be completely absent from the urine in a healthy individual because it is 100% selectively reabsorbed in the nephrons.

Key Terms(14)

Excretion: The removal of waste products of metabolism (e.g., urea, carbon dioxide) and substances in excess of requirements from the body.
Osmoregulation: The control of the water and salt (ion) balance in the blood and body fluids.
Kidney: A bean-shaped organ responsible for excretion and osmoregulation by filtering the blood.
Ureter: The muscular tube that transports urine from the kidney to the bladder.
Bladder: A muscular bag that stores urine until it is excreted.
Urethra: The tube through which urine passes from the bladder to the outside of the body.
Renal artery: The blood vessel carrying unfiltered, oxygenated blood from the aorta to the kidney at high pressure.
Renal vein: The blood vessel carrying filtered, deoxygenated blood from the kidney toward the vena cava.
Cortex: The light-coloured outermost region of the kidney where ultrafiltration begins.
Medulla: The darker inner section of the kidney containing pyramid structures.
Renal pelvis: A funnel-like cavity in the centre of the kidney that collects urine before it drains into the ureter.
Nephron: The microscopic functional unit of the kidney where filtration and selective reabsorption occur.
Urea: A toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids.
Sphincter muscles: Ring-shaped muscles at the base of the bladder that regulate the release of urine.

Previous Topic: Diabetes and ControlBody Mass and Diabetes Next Subtopic: Nephron Structure and FunctionNephron Structure and Ultrafiltration

Back to Structure of Urinary System

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Key Terms(14)

Excretion: The removal of waste products of metabolism (e.g., urea, carbon dioxide) and substances in excess of requirements from the body.
Osmoregulation: The control of the water and salt (ion) balance in the blood and body fluids.
Kidney: A bean-shaped organ responsible for excretion and osmoregulation by filtering the blood.
Ureter: The muscular tube that transports urine from the kidney to the bladder.
Bladder: A muscular bag that stores urine until it is excreted.
Urethra: The tube through which urine passes from the bladder to the outside of the body.
Renal artery: The blood vessel carrying unfiltered, oxygenated blood from the aorta to the kidney at high pressure.
Renal vein: The blood vessel carrying filtered, deoxygenated blood from the kidney toward the vena cava.
Cortex: The light-coloured outermost region of the kidney where ultrafiltration begins.
Medulla: The darker inner section of the kidney containing pyramid structures.
Renal pelvis: A funnel-like cavity in the centre of the kidney that collects urine before it drains into the ureter.
Nephron: The microscopic functional unit of the kidney where filtration and selective reabsorption occur.
Urea: A toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids.
Sphincter muscles: Ring-shaped muscles at the base of the bladder that regulate the release of urine.

Structure of Urinary System

Overview of the Urinary System

A city needs both a clean water supply and an efficient waste-disposal network to function properly; your body relies on the urinary system for a similar waste-management role.
The urinary system has two main jobs: excretion (removing metabolic waste like urea) and osmoregulation (balancing the water and salt levels in your blood).
Macroscopically, the human urinary system is made up of four key components: two kidneys, two ureters, one bladder, and one urethra.
The system works sequentially to clean the blood. First, blood is filtered in the kidneys, then the resulting waste liquid (urine) travels down the ureters to be stored in the bladder before finally exiting the body via the urethra.

The Kidneys and Blood Supply

Despite being only the size of a computer mouse, your kidneys filter your entire blood volume dozens of times a day.
The kidneys are bean-shaped organs located at the back of the abdominal cavity, sitting just below the ribcage on either side of the spine.
Unfiltered, oxygenated blood is delivered directly from the aorta to the kidneys via the renal artery at very high pressure.
If you cut a kidney open, you would see three distinct internal regions:
- Cortex: The light-coloured outer layer where blood filtration begins.
- Medulla: The darker inner region containing pyramid-like structures.
- Renal pelvis: A central, funnel-like cavity that collects the newly formed urine.
Deep inside the cortex and medulla are roughly one million microscopic nephrons (kidney tubules). These are the functional units of the kidney where filtration and selective reabsorption actually occur.
Once the blood has been cleaned, it is now deoxygenated and leaves the kidney through the renal vein, traveling back to the vena cava.

The Pathway of Urine

What happens to the waste liquid once the kidneys have finished filtering the blood?
The collected urine—typically composed of $95\%$ water, $2\%$ urea, and $2.5\%$ mineral ions and other wastes—drains from the renal pelvis into the ureters.
The ureters are narrow, muscular tubes about $25$ to $30$ cm long. They do not rely on gravity; instead, they use active wave-like muscular contractions called peristalsis to squeeze urine down into the bladder.
The bladder is a hollow, muscular sac with elastic walls that acts as a temporary storage vessel, usually holding between $400$ and $600$ mL of urine.
When the bladder is full, stretch receptors in its wall send signals to the brain. Urine is kept inside by two sphincter muscles at its base: an internal involuntary one and an external voluntary one.
When these sphincters relax, urine is released into the urethra, a single tube leading to the outside of the body.
Note that male and female urethras differ: in females, the urethra is short and exclusively carries urine, whereas in males, it is longer and serves a dual purpose, carrying both urine and semen.

Interpreting Urine Production Rates

Doctors often use the rate of urine production to check if a patient's kidneys are functioning correctly.
In exams, you may be required to calculate flow rates by dividing the total volume of urine produced by the time taken.

A patient's kidneys produce $840\,cm^3$ of urine over a $14$ -hour period. Calculate the average rate of urine production in $cm^3/min$ .

Step 1: Identify the values and convert time into the correct units.

Volume $= 840\,cm^3$
Time in minutes $= 14 \times 60 = 840$ minutes

Step 2: Substitute into the rate formula.

Rate $= \frac{\text{Volume}}{\text{Time}}$

Step 3: Calculate the final answer with units.

Rate $= \frac{840}{840}$
Rate $= 1.0\,cm^3/min$

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students frequently confuse the ureter and urethra. Remember this mnemonic: 'Ureter' has two 'e's (like the two tubes connecting the kidneys to the bladder), while 'Urethra' has an 'a' for 'away from the body'.
2
When asked to describe the blood in the renal vein, you must state that it is both 'deoxygenated' AND 'filtered' (or 'cleaned') to secure all the marks on an Edexcel mark scheme.
3
If you have to identify blood vessels on a diagram, look for the thickness of the walls; the renal artery will have thicker walls because it branches directly from the aorta and carries blood at a much higher pressure than the renal vein.
4
Remember that while glucose is present in the renal artery, it should be completely absent from the urine in a healthy individual because it is 100% selectively reabsorbed in the nephrons.

Key Terms(14)

Excretion: The removal of waste products of metabolism (e.g., urea, carbon dioxide) and substances in excess of requirements from the body.
Osmoregulation: The control of the water and salt (ion) balance in the blood and body fluids.
Kidney: A bean-shaped organ responsible for excretion and osmoregulation by filtering the blood.
Ureter: The muscular tube that transports urine from the kidney to the bladder.
Bladder: A muscular bag that stores urine until it is excreted.
Urethra: The tube through which urine passes from the bladder to the outside of the body.
Renal artery: The blood vessel carrying unfiltered, oxygenated blood from the aorta to the kidney at high pressure.
Renal vein: The blood vessel carrying filtered, deoxygenated blood from the kidney toward the vena cava.
Cortex: The light-coloured outermost region of the kidney where ultrafiltration begins.
Medulla: The darker inner section of the kidney containing pyramid structures.
Renal pelvis: A funnel-like cavity in the centre of the kidney that collects urine before it drains into the ureter.
Nephron: The microscopic functional unit of the kidney where filtration and selective reabsorption occur.
Urea: A toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids.
Sphincter muscles: Ring-shaped muscles at the base of the bladder that regulate the release of urine.

Previous Topic: Diabetes and ControlBody Mass and Diabetes Next Subtopic: Nephron Structure and FunctionNephron Structure and Ultrafiltration

Back to Structure of Urinary System

Put your knowledge into practice — try past paper questions for Biology

Key Terms(14)

Excretion: The removal of waste products of metabolism (e.g., urea, carbon dioxide) and substances in excess of requirements from the body.
Osmoregulation: The control of the water and salt (ion) balance in the blood and body fluids.
Kidney: A bean-shaped organ responsible for excretion and osmoregulation by filtering the blood.
Ureter: The muscular tube that transports urine from the kidney to the bladder.
Bladder: A muscular bag that stores urine until it is excreted.
Urethra: The tube through which urine passes from the bladder to the outside of the body.
Renal artery: The blood vessel carrying unfiltered, oxygenated blood from the aorta to the kidney at high pressure.
Renal vein: The blood vessel carrying filtered, deoxygenated blood from the kidney toward the vena cava.
Cortex: The light-coloured outermost region of the kidney where ultrafiltration begins.
Medulla: The darker inner section of the kidney containing pyramid structures.
Renal pelvis: A funnel-like cavity in the centre of the kidney that collects urine before it drains into the ureter.
Nephron: The microscopic functional unit of the kidney where filtration and selective reabsorption occur.
Urea: A toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids.
Sphincter muscles: Ring-shaped muscles at the base of the bladder that regulate the release of urine.