13. Excretion (13.1 & 13.2) 

13.1 Excretion 

1. Describe excretion as the removal of toxic materials and the waste products of metabolism from organisms 

2. State that carbon dioxide is a waste product of respiration, which is excreted through the lungs [water vapour is also excreted from the lungs] 

Other waste products : 

• Kidneys - urea, nitrogenous waste substances, water, ions, toxins, drugs, hormones 

• Skin - Water vapour in the form of sweat 

• Liver - Bile pigments 

3. State that urea is a toxic waste product produced in the liver from the breakdown of excess amino acids 

13.2 Urinary system 

1. Identify, on diagrams, the kidneys, ureters, bladder, and urethra, and state the function of each 

• Kidney: removing excess urea salts and water from the blood as urine 

• Bladder: stores urine, allowing urination to be controlled 

• Ureter: Transports urine from the kidneys to the bladder 

• Urethra: Transports urine from the bladder out of the body 

2. Explain the need for excretion, limited to the toxicity of urea 

• Some compounds, such as ammonia [made in the liver when excess amino acids break down], can become toxic if their concentration in the body increases. Hence, ammonia is converted to urea, which is a safer way to excrete nitrogenous waste substances 

3. Outline the structure of a nephron and its associated blood vessels, limited to: Bowman’s capsule, glomerulus, tubules, loop of Henle and collecting duct 

Bowman's Capsule: 

• Bowman's capsule is the cup-shaped structure at the beginning of a nephron. - 

• It surrounds the glomerulus and collects the filtrate(fluid) that enters the nephron. 

Glomerulus: 

• The renal artery divides into many arterioles and capillaries, mostly in the cortex. Each arteriole leads to a glomerulus. - 

• The glomerulus is a network of tiny blood vessels(capillaries)located within Bowman's capsule. - 

• It is where blood is filtered under pressure, allowing small molecules like water, ions, glucose, and waste products to pass into Bowman's capsule, forming the filtrate. 

Tubules: 

• Proximal Convoluted Tubule(PCT): After leaving Bowman's capsule, the filtrate enters the proximal convoluted tubule. - 

• Loop of Henle: The proximal tubule leads to the loop of Henle. The loop of Henle plays a crucial role in concentrating urine by reabsorbing water and salts from the filtrate. - 

• Distal Convoluted Tubule(DCT): The loop of Henle leads to the distal convoluted tubule, where further selective reabsorption and secretion of ions occur. 

Collecting Duct: 

• The distal convoluted tubule connects to the collecting duct, which collects urine from multiple nephrons. - 

• The collecting duct carries urine towards the renal pelvis, where it eventually drains into the ureter and is transported to the urinary bladder for storage. 

Associated Blood Vessels:

• Afferent Arteriole: The afferent arteriole brings blood into the glomerulus for filtration. - 

• Efferent Arteriole: The efferent arteriole carries blood away from the glomerulus. It plays a role in maintaining the high pressure within the glomerulus necessary for filtration. - 

• Capillaries: These are small blood vessels that surround the tubules throughout the nephron. They reabsorb useful substances, such as glucose and ions, from the filtrate back into the bloodstream and also supply oxygen and nutrients to the nephron cells. 

• A nephron is a single glomerulus with its Bowman’s capsule, renal tubule, and blood capillaries 

4. Outline the function of a nephron and its associated blood vessels, limited to: 

(a)The role of the glomerulus in the filtration of water, glucose, urea and ions 

• The blood pressure in the glomerulus is high because the afferent arteriole is wider than the efferent arteriole. The filtration process is driven by the pressure gradient between the blood in the glomerulus and the fluid in Bowman's capsule. The blood pressure in a glomerulus causes part of the blood plasma to leak through the capillary walls. This filtration is very fine and only very small molecules such as ions, glucose and urea dissolved in water are passed out as tissue fluid. Thered blood cells and plasma proteins that are too big to pass out. 

(b) The role of the nephron in the absorption of all of the glucose, some of the ions, and most of the water back into the blood 

• About 120 cm of filtrate is formed in the kidneys every minutecausing the body to lose too much water and other ions. If they were all drained out, the body would soon be dehydrated. The filtrate from the glomerulus collects in the renal capsule and passes down the renal tubule. As it does this, the capillaries that surround the tubule absorb those substances which the body needs back into the blood. Most of the water is reabsorbed by osmosis. Mineral ions and glucose are reabsorbed by active transport. It is a highly selective processand only the substances required by the body are reabsorbed readily. The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules 

(c) formation of urine containing urea, excess water, and excess ions (details of these processes are not required) 

• Ions not needed by the body are left to pass on down the kidney tubule, together with the urea and uric acid. The nitrogenous waste products, excess ionsand water continue down the renal tube into the pelvis of the kidney. From the fluid, now called urine, passes down the ureter to the bladder 

5. Describe the role of the liver in the assimilation of amino acids by converting them to proteins 

• The liver plays a very important role in assimilating amino acids. It removes amino acids from the plasma of the bloodstream and builds them up into proteins. Proteins are long chains of amino acids joined together by peptide bonds. These include plasma proteins like fibrinogen. 

6. Describe deamination in the removal of the nitrogen-containing part of amino acids, resulting in the formation of urea

• Excess amino acids cannot be stored in the body. They are broken down by the liver, removing the nitrogen-containing part to form ammonia. The rest of the molecule can be converted into carbohydrates and fats and used in respiration. The ammonia is converted to urea. This passes into the bloodstream and is filtered out by the kidneys. This process is called deamination. 

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School