0610 · 13.1Excretion in humans

Excretion and metabolic waste

0:00Imagine throwing away everything in your school bag just because some of it is rubbish. You would lose the useful things along with the waste. Your kidneys face a similar problem, because blood contains both substances your body needs and substances it must remove. Their solution is not simply to empty the blood. It is to filter out small substances, then take the useful ones back. By the end of this video, you will understand where urea comes from, why it must be removed, and how the kidneys produce urine without normally losing glucose.

0:27You will also be able to recognise the main parts of the urinary system and the important structures inside a kidney. We will connect everything into one journey, from waste production in the body to waste leaving the body. First, we need a little background. Metabolism means the chemical reactions taking place in the body, and some of those reactions produce waste. Blood transports dissolved substances between organs, including useful nutrients and waste products.

0:51Glucose is a useful substance that cells can use in respiration, while amino acids are the building blocks used to make proteins. Excretion is the removal of waste products of metabolism and substances in excess of requirements. The important point is that excretion is not just getting rid of anything unwanted. Egestion is the removal of undigested food as faeces, rather than the removal of waste produced by the body's chemical reactions.

1:14Urine and faeces therefore leave by different routes and represent different processes. One example of excretion happens whenever you breathe out. Carbon dioxide is produced during aerobic respiration, travels in the blood to the lungs, and is excreted through the lungs. The kidneys deal with a different set of substances: they excrete urea and excess water and ions. Ions are charged particles, such as those supplied by dissolved mineral salts, and the body needs appropriate amounts rather than unlimited amounts.

1:43Before we examine the kidneys, let us find out where urea comes from.

Urea and urinary system

1:47After proteins in food are digested, the resulting amino acids are absorbed into the blood. The liver has an important role in the assimilation of amino acids by converting them to proteins. Here, assimilation means using absorbed nutrients to make substances that become part of the body. Think of amino acids as building blocks delivered to a workshop. Joining them together makes proteins, which the body needs for growth, repair and many other functions.

2:10However, there may be more amino acids available than the body needs for making proteins. The excess cannot simply be stored as a reserve of amino acids, so the liver processes them. This involves deamination, the removal of the nitrogen-containing part of amino acids to form urea. The key idea is that excess amino acids contain nitrogen that must be dealt with. The liver removes that nitrogen-containing part and forms urea from it.

2:35You do not need the intermediate chemical reactions for this topic, but you must connect excess amino acids, deamination and urea formation. Urea is toxic if it accumulates, so it must be excreted rather than allowed to build up in the body. Once it has been formed in the liver, it is carried in the blood to the kidneys. This gives us an important distinction: the liver makes urea, while the kidneys remove it from the blood.

2:58Making a waste product and excreting it are not the same job. Now look at the urinary system as a whole. The kidneys are the two bean-shaped organs towards the back of the abdomen. Each kidney produces urine, which passes down a tube called a ureter to the bladder. The bladder stores urine until it is released through the urethra, the tube leading from the bladder to the outside of the body.

3:18The names ureter and urethra sound similar, so identify them by their connections rather than just their spelling. A ureter connects a kidney to the bladder, whereas the urethra connects the bladder to the outside. In a diagram or image, trace where each tube begins and ends. Also remember that the bladder stores urine; it does not filter blood or manufacture urine. If we cut through a kidney, we can identify two main regions.

3:42The cortex is the outer region, while the medulla is the inner region. In a sectioned kidney, the medulla often appears as several triangular areas pointing towards the centre. For this topic, those are the two kidney regions you need to identify, whether the question uses a drawing or an image of a real kidney. Inside the kidney are many microscopic filtering units called nephrons. Each nephron begins with a cup-shaped capsule surrounding a small knot of blood capillaries called a glomerulus.

4:09The capsule leads into a long tubule, and blood vessels run close to that tubule. This arrangement allows substances to move from the blood into the nephron, and useful substances to move back into the blood. Notice that there are two separate pathways here. Blood flows through the glomerulus and then through associated blood vessels, while filtered fluid flows inside the nephron tubule. The fluid inside the tubule is not the same thing as the blood beside it.

4:32Keeping these pathways separate in your mind makes filtration and reabsorption much easier to understand.

Filtration and reabsorption

4:38The first stage is filtration at the glomerulus. Blood pressure forces water and small dissolved substances out of the blood, through the filtration barrier, and into the nephron. These substances include water, glucose, urea and ions. Blood cells and large proteins stay in the blood because they are too large to pass through the filtration barrier. Here is the surprising part: useful glucose is filtered out too. The glomerulus does not recognise a molecule as useful or useless and then make a decision about it.

5:06Filtration separates mainly according to whether substances can pass through the barrier. That is why the fluid entering the nephron contains a mixture of useful substances and waste, rather than being finished urine. The second stage is reabsorption, which means taking substances from the nephron back into the blood. As the filtered fluid passes along the tubule, all of the glucose, some of the ions and most of the water are reabsorbed.

5:28The surrounding capillaries receive these substances and carry them away. The direction matters: reabsorption is from the nephron into the blood, not from blood into the nephron. Why recover these substances? Glucose is useful for respiration, so normally none of the filtered glucose should be lost in urine. Water is needed throughout the body, so losing all the filtered water would be a serious problem. Ions are also needed, but not all the filtered ions have to be retained.

5:54Reabsorption therefore keeps useful substances while allowing excess water and excess ions to be excreted. After filtration and reabsorption, the remaining fluid forms urine containing urea, excess water and excess ions. The kidneys have therefore achieved two jobs together: removing waste and avoiding unnecessary loss of useful substances. Remember our school bag analogy, but with one important difference. The kidney first filters a mixture of small substances, then recovers what the body needs, rather than selecting only rubbish at the first stage.

Predicting urine contents

6:25Let us work through an example rather than just memorising those lists. Suppose blood entering a healthy kidney contains glucose, urea, water, ions, blood cells and large proteins. We want to predict which of those will normally appear in the urine. To solve this, we need to follow each substance through both stages, because knowing what is filtered is not enough. Start with blood cells and large proteins. They remain in the blood during filtration, so they do not normally enter the nephron fluid or appear in urine.

6:52Next consider glucose: it passes out of the blood at the glomerulus because it is small enough to be filtered. However, all the filtered glucose is reabsorbed, so it is not normally present in the final urine. Now consider water and ions. Both enter the nephron during filtration, but most of the water and some of the ions return to the blood. The excess remains in the fluid that becomes urine.

7:14Urea is also filtered from the blood and is excreted in urine, allowing the body to remove the toxic waste formed from excess amino acids. This example shows why you should never treat filtration and urine formation as identical. Finding glucose in the first fluid entering the nephron is normal, because filtration includes glucose. Finding glucose in the final urine would not be the expected result for a healthy person in this model.

7:37Always ask whether the question refers to blood, newly filtered fluid or final urine before deciding what it contains. Another common mistake is saying that the kidneys turn amino acids into urea. That confuses the organ making the waste with the organ removing it. Deamination and urea formation occur in the liver, and blood carries the urea to the kidneys. Similarly, urine does not travel from the kidney to the bladder in a blood vessel; it travels along the ureter.

8:02A final wording mistake is saying that the kidneys remove all water and all ions. Both are useful, so the kidneys excrete excess water and excess ions, not the body's entire supply. Be equally precise about reabsorption: all of the glucose, some of the ions and most of the water return to the blood. Those words describe different outcomes, so they are not interchangeable.

Exam answers and recap

8:23In an examination, you could be asked to identify structures on a diagram, outline urine formation, or describe how urea is formed. For identification, use position and connections: cortex outside, medulla inside, glomerulus at the beginning of the nephron, and ureter leaving the kidney. If the question says outline, give the main stages without unnecessary chemical detail. If it says explain, make the reason or relationship clear. Try this exam-style question: outline how urine is formed in a nephron.

8:53Before answering, organise your thinking into filtration, reabsorption and what remains. This prevents you from listing substances without explaining where they move. A clear answer should name the glomerulus, give the substances filtered, describe their return to the blood, and state the contents of urine. At the glomerulus, water, glucose, urea and ions are filtered from the blood into the nephron. Blood cells and large proteins remain in the blood.

9:18All of the glucose, some of the ions and most of the water are reabsorbed from the nephron into the blood. Urine is formed containing urea, excess water and excess ions. Now try a second question: describe how urea is formed and explain why it must be excreted. A suitable answer is that excess amino acids are deaminated in the liver. Deamination removes the nitrogen-containing part of amino acids to form urea.

9:42Urea must be excreted because it is toxic if it accumulates in the body. Let us finish by joining the whole story together. The liver assimilates amino acids by converting them to proteins, and deaminates excess amino acids to form urea. Blood carries that urea to the kidneys, where filtration moves water, glucose, urea and ions into nephrons. Reabsorption then returns all glucose, some ions and most water to the blood.

10:08The remaining urine contains urea, excess water and excess ions, and passes along the ureters to the bladder before leaving through the urethra. Inside each kidney, the cortex is the outer region and the medulla is the inner region. Carbon dioxide follows a different route and is excreted through the lungs. Excretion is therefore not simply throwing things away: it removes waste while preserving substances the body still needs.