Module 5: Excretion. Flashcards
What is excretion? Give an example.
Excretion is the removal of the waste products of metabolism from the body.
For example, carbon dioxide is excreted by the lungs, when we breathe out and nitrogenous waste is excreted in the urine. These substances would cause tissue damage if allowed to build up, so excretion maintains homeostasis by keeping substances within a healthy range.
Describe the structure and function of the liver.
It breaks down metabolic waste products and other substances that can be harmful such as drugs and alcohol.
- The hepatic artery supplies the liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration, providing plenty of energy.
- The hepatic vein takes deoxygenated blood away from the liver.
- The hepatic portal vein brings blood from the duodenum and ileum (parts of the small intestine), so it is rich in the products of digestion. This means any ingested harmful substances are filtered out and broken down straight away.
- The bile duct takes bile (a substance produced by the liver to emulsify fats) to the gall bladder to be stored.
Describe the histology of the liver
- It is made up of liver lobules which are cylindrical structures made of cells called hepatocytes that are arranged in rows radiating out from the centre.
- Each lobule has a central vein in the middle that connects to the hepatic vein. Many branches of the hepatic artery, hepatic portal vein and bile duct are also found connected to each lobule.
Explain how excretion takes place in the liver.
- The hepatic artery and the hepatic portal vein are connected to the central vein by capillaries called sinusoids.
- Blood runs through the sinusoids, past the hepatocytes that remove harmful substances and oxygen from the blood.
- The harmful substances are broken down by the hepatocytes into less harmful substances that then re-enter the blood.
- The blood runs to the central vein, and the central veins from all the lobules connect up to form the hepatic vein.
- Cells called Kupffer cells are also attached to the walls of the sinusoids. (they are a type of white blood cells that carries out phagocytosis). They remove bacteria and break down old red blood cells.
- Hepatocytes produce bile and secrete it into tubes called bile canaliculi. These tubes drain into bile ducts. The bile ducts from all the lobules eventually connect up and leave the liver.
What would you see when you examine a liver tissue under a microscope?
- Large white circular shape is the central vein.
- White spaces are the sinusoids
- The red dots are the nuclei
- Cells that radiate out from the central vein are hepatocytes.
Why does deamination in the liver occur?
There is excess amino acids produced by eating and digesting protein. Amino acids contain nitrogen, which cant be stored by the body.
This means excess amino acids can be damaging to the body, so they are used by the body to make proteins or to be broken down and excreted.
Describe the process of how excess amino acids are broken down in the liver.
1) First, the nitrogen-containing amino groups (-NH2) are removed from any excess amino acids, forming ammonia (NH3) and organic acids- this process is called deamination.
Amino acids ——-(-deamination)——-> ammonia + organic acids.
2) The organic acids can be respired to give ATP or converted to carbohydrate and stored as glycogen.
3) ammonia is too toxic for mammals to excrete directly, so it is combined with CO2 in the ornithine cycle to create urea and water
Ammonia + CO2 ———-> urea + water
4) The urea is released from the liver into the blood and then the kidneys filter the blood and remove the urea as urine, which is excreted from the body.
What is the ornithine cycle?
Ammonia is too toxic for mammals to excrete directly, so it is combined with CO2 in the ornithine cycle to create urea and water
What role does the liver play in detoxification?
The liver breaks down harmful substances, like alcohol, drugs and unwanted hormones.
They are broken down into less harmful compounds that can then be excreted from the body - detoxification.
Give 3 examples of harmful products the liver breaks down and how?
Alcohol - it is a toxic substance that can damage cells. It is broken down by the liver into ethanal, which is then broken down into a less harmful substances called acetic acid. Excess alcohol over a long time can lead to cirrhosis of the liver - this is when the cells of the liver die and scar tissue blocks blood flow.
Paracetamol - Broken down in the liver. Excess paracetamol in the blood can lead to liver and kidney failure.
Insulin - Is a hormone that controls blood glucose concentration. It is broken down as excess insulin can cause problems with blood sugar levels.
Describe the role of the liver into glycogen storage.
The liver converts excess glucose in the blood to glycogen in a process called glycogenesis.
The glycogen is then stored as granules in the liver cells until the glucose is needed for energy.
State the components of the kidney.
Renal artery.
Renal vein.
Ureter
Bladder
Urethra
(Inside kidneys)
Cortex
Medulla
Renal calyx
Renal capsule.
What is a nephron?
They are long tubules along with the bundle of capillaries where the blood is filtered.
Around one million nephrons in each kidney.
Describe the location and structure of one nephron.
Renal artery.
Afferent arteriole.
Glomerulus.
efferent arteriole.
Renal vein
Ureter
Bowmans capsule
Loop of Henle
Proximal convoluted tubule
Distal convoluted tubule.
What is ultrafiltration (not the process)
Blood from the renal artery enters smaller arterioles in the cortex.
Each arteriole splits into a structure called the glomerulus - which is a bundle of capillaries looped inside a hollow ball called the Bowmans capsule (this is where ultrafiltration takes place)
As the blood passes through the capillaries, substances are filtered out of the blood and into long tubule’s (nephrons) that surround the capillaries.
Describe the process of ultrafiltration.
Afferent arteriole = takes blood into each glomerulus.
Efferent arteriole = takes the filtered blood away from the glomerulus.
The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure and this high pressure forces liquid and small molecules in the blood out of the capillary and into the Bowmans capsule.
The liquid and small molecules pass through three layers to get into the Bowman’s capsule and enter the nephron tubule - which are the capillary endothelium, basement membrane and the epithelium of the Bowmans capsule (made up of podocytes cells)
Larger molecules like proteins and blood cells cannot pass through and stay in the blood. The liquid and small molecules, now called filtrate, pass along the rest of the nephron and useful substances are reabsorbed along the way.
Finally the filtrate flows through the collecting ducts and passes out of the kidney along the ureter.
What is selective reabsorption?
It is when useful substances ,such as glucose, are reabsorbed back into the blood from the tubules in the medulla and cortex.
Describe the process of selective reabsorption.
Takes place as the filtrate flows along the proximal convoluted tubule (PCT), through the loop of Henle, and along the distal convoluted tubule (DCT). Useful substances leave the tubules of the nephrons and enter the capillary network that is wrapped around them.
The epithelium of the wall of the Proximal convoluted tubule has microvilli to provide a large surface area for the reabsorption of useful materials from the filtrate (in the tubules) into the blood (in the capillaries)
Useful solutes like glucose and amino acids, vitamins and some salts are reabsorbed along the PCT by active transport and facilitated diffusion. some urea is also reabsorbed by diffusion.
Water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate. Water is reabsorbed from the loop of Henle, DCT and the collecting duct. The filtrate that remains is urine, which passes along the ureter to the bladder.
What is Urine made up of?
Usually made up of water and dissolved salts, urea and other substances such as hormones and excess vitamins.
Doesn’t contains protein or blood cells as they are too big to be filtered out of the blood.
Glucose, amino acids and vitamins are reabsorbed back into the blood, so are not found in urine.
Where does regulation of the water potential of the blood take place in the kidney?
Takes place in the middle and last parts of the nephron - the loop of Henle, the distal convoluted tubule and the collecting duct.
The volume of water reabsorbed is controlled by hormones.
What happens if the water potential of the the blood is too low or too high?
if the water potential of the blood is too low (the body is dehydrated). This means more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more concentrated, so less water is lost during excretion.
If the water potential of the blood is too high (the body is too hydrated) and therefore less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute, so more water is lost during excretion.
What is the loop of Henle made up of and what mechanism does it involve?
It is made up of two limbs - the descending limb and the ascending limb.
They help set up a mechanism called the counter current multiplier mechanism -and this helps to reabsorb water back into the blood.
Describe how the countercurrent multiplier mechanism works in the Loop of Henle.
- Near the top of the ascending limb, Na+ and Cl- ions are actively pumped out into the medulla. The ascending limb is impermeable to water, so the water stays inside the tubule and this creates a low water potential in the medulla, because there is a high concentration of ions.
- Because there is a lower water potential in the medulla than in the descending limb, water moves out of the descending limb into the medulla by osmosis. This makes the filtrate more concentrated (the ions cannot diffuse out - the descending limb isn’t permeable to them). The water in the medulla is reabsorbed into the blood through the capillary network.
- Near the bottom of the ascending limb Na+ and Cl- ions diffuse out into the medulla, further lowering the water potential in the medulla. (the ascending limb is impermeable to water, so stays in the tubule)
- The first three stages massively increase the ion concentration in the medulla, which lowers the water potential. This causes water to move out of the collecting ducts by osmosis. As before, the water in the medulla is reabsorbed into the blood through the capillary network.
