Excreatory System

 

Introduction

Learn all about the human excretory system. 

There are a total of 3 learning objectives, each learning objective contains information and questions to help prepare you for the upcoming lesson.

 

 

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1. Describe the structure of the urinary system

Urinary system

The urinary system is one of the human body’s waste disposal units and its filtration unit.

Composed of:

  • Kidneys
  • Ureters (tubes connecting kidneys to the bladder)
  • Bladder and urethra

Function

Helps to empty the body of potentially harmful waste substances, like urea and alcohol. It does so through filtration and excretion.

What is the function of the kidneys?

Kidney

Structure:

  • Bean-shaped organ, about 11cm long.
  • Humans have two of them
  • Positioned on the posterior wall of the abdomen, either side of the spine
  • Made up of over a million twisted tubes called nephrons, which do the kidneys’ work of filtration and excretion.

Functions of the kidneys include filtration of impurities and metabolic waste from blood, regulation of water and salt balance, formation of urine and regulation of blood pressure and volume

  • filtering of impurities from the blood
  • formation of urine
  • regulation of water and salt balance
  • all of the above

The blood filtration unit inside a kidney is known as?

Two distinct parts:

  • the cortex on the outside and
  • the medulla on the inside

Hilum is the concave centre of the kidney is known as the hilum and it is at this point that blood vessels, lymphatic vessels, nerves and the ureter enter the organ

Renal Pelvis is a funnel-shaped cavity which connects the medulla to the ureter. It collects urine from the tubules in the medulla and passes it into the ureter.

Medulla leads into an area called the renal pelvis

  • hilus
  • medulla
  • nephron
  • renal pyramid

What is the function of the ureter?

Ureters

Function: to take urine from the kidneys to the bladder. The presence of urine inside them stimulates a mechanical contraction which propels the fluid forwards.

  • filter impurities
  • propel urine from the bladder to the exterior
  • propel urine from the kidneys to the bladder
  • store urine

Where is the bladder is situated?

Bladder

Function: A reservoir for urine. When about 200ml of urine has been collected the presence of the liquid stimulates the autonomic nerve endings in the bladder wall and the walls contract.

The bladder has an internal sphincter which relaxes when the walls contract, thus opening and emptying the urine into the urethra.

  • in the pelvic cavity behind the symphysis pubis
  • in the abdominal cavity behind the intestines
  • on the posterior of the abdominal cavity
  • behind the urethra

Urethra

Structure: A narrow tube passing from the bladder to the outside of the body. It has an external sphincter which is voluntarily controlled by the central nervous system. It is shorter in women, thus making them more susceptible to infection.

Function: to take urine from inside the body (the bladder) to outside. In men, the urethra is also the passage for semen.

2. Describe 3 stages of urine production of the urinary system

Inside the kidney, the renal artery splits into a network of capillaries are called?

Urine Production

Urine is produced as filtrate moves through renal tubules.

The 3 Stages:

  1. Filtration
  2. Re-absorption
  3. Secretion

  • afferent arteriole
  • glomerulus
  • efferent arteriole
  • hilus

Blood is filtered inside what section of the kidney?

Filtration in the Bowman’s capsule

Blood enters the kidneys via the afferent arterioles. These tiny blood vessels become the glomerulus, a tangle of capillaries surrounded by the glomerular capsule, also known as the Bowman’s capsule.

The blood in the capillaries is under pressure and since the capillary walls are permeable to water and other substances these pass through into the capsule, whilst blood cells and protein remain in the blood vessel. The Bowman’s capsule thus serves as the collection point for the waste products carried in the blood. However, at this point the capsule has also collected other substances which are not waste and these will be reabsorbed as they pass through the nephron.

  • Bowman’s capsule
  • Loop of Henle
  • Glomerulus
  • Proximal Convoluted Tubule

Re-absorption

Re-absorption in the convoluted tubules

Once the filtered substances have been collected by the capsule they are passed into a system of twisted tubes, known as convoluted tubules. The tubes of the nephron which lead away from the Bowman’s capsule are known as the proximal convoluted tubules. These straighten out into a long loop, called the Loop of Henle, which passes into the medulla and back to the cortex. Finally, there is another series of twists called the distal convoluted tubules.

Re-absorption takes place in the tubules. Cells in the lining of the tubules are able to absorb any water, glucose, salts and ions which the body needs that must not be disposed of as waste. Only 1% of the liquid filtered into the Bowman’s capsule is actually excreted as urine. The rest is re-absorbed.

Secretion

Collection in the pelvic calyces

The nephron straightens out into a collecting tube in the medulla.

These collecting tubes form masses called pyramids of the medulla, the tops of which stick up into the renal pelvis.

The branches of the pelvis, or calyces, connect with the tops of these pyramids and collect the waste liquid, funnelling it back into the pelvis, from where it will empty into the ureter.

Composition of Urine

What is excreted: Composition of urine

The liquid that results from the processes of filtration and re-absorption is known as urine. This amber-coloured liquid is composed of 96% water, 2% urea, and 2% other substances, such as ammonia, sodium, potassium, phosphates, chlorides, sulphates and excess vitamins. The salts must be excreted in order to maintain the correct balance of fluids and electrolytes in the body.

The colour of urine comes from bilirubin, a bile pigment. Normal urine is acidic, but this varies depending on diet and body temperature, emotions and blood pressure.

Urine Production

About 1.5 litres of urine is produced every 24 hours, which is only a small percentage compared to the amount of liquid filtered from the blood in the glomerulus. Urine production is increased by liquid intake and cold weather and decreased by drinking less and any activity or state that increases sweating (hot weather, exercise). Humans need a minimum of 0.5 litres of water per day for waste removal.

Video of Urinary System

3. Describe diseases and disorders of the urinary system.

Cystitis

Inflammation of the bladder, causing pain when urinating.

Sometimes caused by infections. Very common in women due in part to the shorter length of the female urethra.

Kidney Stone

Calcium deposits of substances found in urine which form solid stones within the renal pelvis, bladder or ureters. Extremely painful and often removed by surgery.

Nephritis

Inflammation of the kidney, resulting from causes other than infection. Often used to refer to a wide range of different inflammatory disorders.

Revision

The blood filtration unit inside a kidney is known as?

  • hilus
  • medulla
  • nephron
  • renal pyramid

What is the function of the kidneys?

  • filtering of impurities from the blood
  • formation of urine
  • regulation of water and salt balance
  • all of the above

Fill in the blanks

The functions of ureter include taking urine from the  to the .

Why the condition cystitis commonly affects females?

  • women tend to have a weaker bladder than men
  • women have a shorter urethra
  • women have smaller kidneys
  • all of the above

Which of the following factors does not affect fluid balance?

  • blood pressure
  • body temperature
  • diet
  • time of day

Inside the kidney, the renal artery splits into a network of capillaries are called?

  • afferent arteriole
  • glomerulus
  • efferent arteriole
  • hilus

Where kidney stones may be deposited in?

  • ureter
  • bladder
  • renal pelvis
  • all of the above

What are the main composition of urine?

  • Urea
  • Blood
  • Water
  • Salt
  • Enzymes
You can choose more than one answer

Identify the parts

  • Kidney
  • Ureters
  • Bladder
  • Urethra

What is the function of the ureter?

  • filter impurities
  • propel urine from the bladder to the exterior
  • propel urine from the kidneys to the bladder
  • store urine

Where is the bladder is situated?

  • in the pelvic cavity behind the symphysis pubis
  • in the abdominal cavity behind the intestines
  • on the posterior of the abdominal cavity
  • behind the urethra