Objectives

By the end of this subtopic, you will be able to:

• state that the kidneys excrete urea and excess water and ions

• identify the kidneys, ureters, bladder and urethra.

• identify the structure of the kidney, limited to the cortex and medulla

• outline the structure and function of a nephron and its associated blood vessels

• describe the role of the liver including assimilation and deamination of amino acids

• state how urea is formed and why it needs to be excreted.

by removing excess water, urea, carbon dioxide, and other wastes from our blood.

How does excretion occur in the human body?

The liver

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The liver

Uptake of amino acids

Assimilation is the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells. The liver has a role in the assimilation of amino acids. It can convert them into proteins, including fibrinogen and other plasma proteins.

Protein in food is broken down in the stomach and small intestine to form amino acids.

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Excess amino acids are converted into urea in the liver.​

The process of converting amino acids into urea is called deamination

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Excess amino acids

Involves the removal of the nitrogen-containing (–NH ₂ group) part of amino acid molecules.

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The formula of amino acids is CHON; here we remove the nitrogen from the molecule, to get a carbohydrate. This is called deamination. Nitrogen is made into urea (CH4N2O)

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Deamination

Urea formation

Ammonia is a very soluble and highly toxic compound that is produced during deamination.

• It can be very damaging if allowed to build up in the blood

• This is avoided by converting ammonia to urea, Urea is less soluble and less toxic than ammonia

• Ammonia is combined with carbon dioxide to form urea

• ​The urinary system rids the blood of wastes produced by the cells.

• The urinary system also controls blood volume by removing excess water produced by body cells during respiration.

• Gets rid of excess salts

The urinary system

Homeostasis - kidneys and water balance

ADH and control of the water balance

The amount of water in the blood must be kept more or less the same all the time to avoid cell damage as a result of osmosis. There has to be a balance between the amount of water gained and the amount of water lost. This is achieved by the action of the hormone ADH (anti-diuretic hormone). How does it work?

The urinary system

On the next slide, you will observe the urinary track to learn about this system. As you observe, take notes of your thought about the differences between excretion and egestion.

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The structure of the kidney

How the kidney works

Each kidney contains about a million tiny structures called nephrons. A nephron has a cup-shaped Bowman's capsule leading into the renal tubule. On the next slide, you will explore a nephron diagram to learn about how the kidney balanced the blood. As you explore, pay attention to the composition of the of substances in the liquid as it filtered by the nephron

• Blood enters the kidneys through the renal artery.

• Blood enters the nephrons.

• Water, sugar, salt and wastes in the blood enter Bowman’s capsule in the nephron.

• Capillaries reabsorb most of the water, sugar, and salt.

• The renal vein returns purified blood to be circulated.

• Urine drains from each collecting tubule into funnel-shaped areas of the kidney

• Urine flows from the ureters to the urinary bladder

• Urine flows from the bladder through the urethra and out of the body.

The Nephron

Substances in blood plasma and urine compared

Summary

• Excretion is the removal of metabolic waste products created in the cell during respiration. These waste products include carbon dioxide, ions and urea.

• The lungs excrete carbon dioxide.

• The kidney excretes excess water, urea and ions out of the blood. These waste products make up the fluid urine. The urine is transported by the ureters to the bladder, where it is temporarily stored. The urine is transported from the bladder out of the body through the urethra.

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