5.1.2 - Excretion as an example of homeostatic control Flashcards
What is excretion ?
Excretion is the removal of the waste products of metabolism from the body
What are the main metabolic waste products in mammals ?
- Carbon dioxide
- Bile pigments
- Nitrogenous waste products (urea)
What is CO2 a waste product of ?
It is one of the waste products of cellular respiration which is excreted from the lungs
Where are bile pigments formed from ?
Formed from the breakdown of haemoglobin from old red blood cells in the liver
Where are bile pigments excreted from ?
liver into the small intestine via the gall bladder and the bile duct
What do the bile pigments do ?
They colour the urea
How is urea formed ?
- Formed from the breakdown of excess amino acids by the liver
Where is urea excreted from ?
Kidneys -> bladder -> urine
What nitrogenous waste products do fish produce ?
Ammonia
What nitrogenous waste products do birds and insects produce ?
Uric acid
What process is the liver involved in ?
Homeostasis
How much of the body mass does the liver make up ?
- 5% of the bodies mass
- It is the largest internal organ of the body
Where does the liver lie in the body ?
It lies just below the diaphragm and is made up of several lobes
What is special about the liver as an organ ?
The liver is very fast growing and damaged areas generally regenerate very quickly
How rich is the blood supply of the liver ?
- Very rich blood supply
- About 1dm3 of blood flows through it every minute
Which artery supplies oxygenated blood to the liver ?
Hepatic artery
Which vein removes blood from the liver to the heart ?
Hepatic vein
What is the name of the second blood vessel that supplies the liver with blood ?
Hepatic portal vein
What does the hepatic portal vein carry ?
It carries blood from the intestines to the liver
How much of the blood flowing to the liver does the hepatic portal vein carry ?
Up to 75% of the blood flowing through the liver comes via the hepatic portal vein
What are liver cells called ?
Hepatocytes
What is the structure of a hepatocyte like ?
- Large nuclei
- Prominent golgi apparatus
- Lots of mitochondria
- Indicates that they are metabolically active cells
What property do hepatocytes have that is very useful ?
They divide and replicate which allows the liver to regenerate
Where does the blood from the hepatic artery and the hepatic portal vein get mixed ?
They are mixed in spaces called sinusoids
What surrounds the sinusoids ?
They are surrounded by hepatocytes
What does the mixing of the blood from the hepatic artery and portal vein lead to ?
- It increases the oxygen content of the blood from the hepatic portal vein
- Supplying the hepatocytes with enough oxygen for their needs
What cells do the sinusoids contain ?
Kupffer cells
What do the kupffer cells act as ?
They act as resident macrophages of the liver, ingesting foreign particles and helping to protect against disease
What do the hepatocytes secrete ?
Bile from the breakdown of the blood
Where do the hepatocytes secrete the bile into ?
Into spaces called canaliculi
from these the bile drains into the bile ductules
which take it to the gall bladder
How many metabolic pathways are linked to the liver ?
Around 500 different pathways
What are the main functions of the liver ?
- Carbohydrate metabolism
- Determination of excess amino acids
- Detoxification
What form of homeostatic control are hepatocytes closely involved with ?
Homeostatic control of glucose levels in the blood by their interaction with insulin and glucagon
What do hepatocytes do when blood glucose levels rise ?
- Insulin levels rise and stimulate hepatocytes to convert glucose to glycogen
- About 100g of glycogen is stored in the liver
What do hepatocytes do when blood glucose levels fall ?
The hepatocytes convert the glycogen back to glucose under the influence of the hormone glucagon
How are hepatocytes and plasma proteins related ?
Hepatocytes synthesise most of the plasma proteins
What process do hepatocytes carry out, related to amines ?
Transamination
What is transamination ?
The conversion of one amino acid to another
Why is transamination important ?
Transamination allows for there to be a balance of amino acids in the body
What is the most important role of the liver in protein metabolism ?
Deamination
What is deamination ?
The removal of an amine group from a molecule
What would happen to excess ingested proteins if it was not for the role of hepatocytes ?
They would be excreted and therefore wasted if it were not for the action of the hepatocytes
What do the hepatocytes do to the excess ingested proteins ?
They deaminate the amino acids
then converting it first into ammonia which is very toxic
then convert it to urea
When is urea toxic ?
In high concentrations
How is urea secreted ?
Via the kidneys
What happens to the remainder of the amino acids ?
Fed into cellular respiration
Converted into lipids for storage
How is the ammonia produced in the deamination of proteins converted into urea ?
- In a set of enzyme controlled reactions
- Known as the ornithine cycle
What happens to the level of toxins in the body over time ?
They increase over time
What does the liver do to potentially poisonous substances ?
Liver is the site where most of these substances are detoxified and made harmless
What is an example of the liver breaking down toxic substances ?
The breakdown of hydrogen peroxide
What enzyme does a hepatocyte contain ?
Catalase
What does catalase do to hydrogen peroxide ?
Catalase breaks down hydrogen peroxide into oxygen and water
What is another example of the liver breaking down a poisonous substance ?
Liver detoxifying ethanol
What other enzyme does a hepatocyte contain ?
Alcohol dehydrogenase
What does alcohol dehydrogenase do ?
- It breaks down ethanol into ethanal
- Ethanal is then converted to ethanoate
Ethanoate can be used to build up fatty acids or used in cellular respiration
Where are the kidneys located ?
Attached to the back of the abdominal cavity
What are the kidneys surrounded by ?
Surrounded by a thick protective layer of fat and a layer of fibrous connective tissue
What roles do the kidneys have ?
- Excretion
- Osmoregulation
How are the kidneys involved in excretion ?
They filter nitrogenous waste products out of the blood, especially urea
How are the kidneys involved in osmoregulation ?
They help to maintain the water balance and pH of the blood, and hence the tissue fluid that surrounds the cell
What vessel supplies the kidneys with blood ?
Renal arteries that branch off from the abdominal aorta
What vessel removes blood that has circulated through the kidneys ?
Renal vein
What are the kidneys made up of ?
- Nephrons
- Millions of them
What do the nephrons act as ?
Filtering units
What liquid is produced by the kidney tubules ?
Urine
How does urine pass out of the kidney ?
It passes out of the kidney down tubes called ureters
Where is the urine collected once out of the kidney ?
The bladder
What happens when the bladder is full ?
The sphincter at the exit of the bladder opens and the urine passes out of the body down the urethra
What are the three main areas of the kidney ?
- Cortex
- Medulla
- Pelvis
What is the cortex ?
- The dark outer layer
- This is where the filtering of the blood takes place and it has a very dense capillary network carrying the blood from the renal artery to the nephrons
What is the medulla ?
- It is lighter in colour
- It contains the tubules of the nephrons that form the pyramids of the kidney and the collecting ducts
What is the pelvis ?
The central chamber where the urine collects before passing out down the ureter
What takes place in the nephrons ?
Ultrafiltration and selective reabsorption
What are the main structures of the nephrons ?
- Bowmans capsule
- Proximal convoluted tube
- Loop of Henle
- Distal convoluted tube
- Collecting duct
What is the function of the Bowmans capsule ?
Cup shaped structure that contains the glomerulus, a tangle of capillaries
What takes place at the glomerulus ?
More blood goes into the glomerulus than leaves it due to the ultrafiltration processes that take place
wider arteriole in and narrower arteriole out
What is the function of the proximal convoluted tube ?
- The first coiled region of the tubule after the Bowmans capsule, found in the cortex
- This is where many of the substances needed by the body are reabsorbed into the blood
What is the function of the loop of henle ?
It is a long loop of tubule that creates a region with a very high solute concentration in the tissue fluid deep in the kidney medulla.
What is the structure of the loop of henle like ?
- The descending loop runs down from the cortex through the medulla to a hairpin bend at the bottom of the loop
- The ascending limb travels back up through the medulla to the cortex
What is the function of the distal convoluted tube ?
- A second twisted tubule where the fine tuning of the water balance of the body takes place
- ADH affects the permeability of the walls to water
- Further regulation of the ion balance and pH of the blood also takes place in this tubule
What is the function of the collecting duct ?
- The urine passes down the collecting duct through the medulla to the pelvis.
- More fine tuning of the water balance takes place - the walls of this part of the tubule are also sensitive to ADH
What is the blood composed of when it leaves the kidney ?
- Greatly reduced levels of urea
- Levels of glucose and other substances such as amino acids needed by the body are the same
- Mineral ion concentration is at ideal levels
What are the two main functions of the nephrons ?
- Ultrafiltration
- Reabsorption
Where does ultrafiltration take place ?
In the Bowmans capsule
Why does ultrafiltration take place ?
As a result of the high blood pressure in the glomerulus
What is ultrafiltration in the kidney tubules ?
- It is the first step in the removal of nitrogenous waste and osmoregulation of the blood
- It is a specialised form of the process that results in the formation of tissue fluid in the capillary beds of the body
How is the glomerulus supplied with blood ?
It is supplied with blood by a relatively wide afferent (incoming) arteriole from the renal artery
How does the blood leave the glomerulus ?
- It leaves through a narrower efferent (outward) arteriole and causes larger pressure in the capillaries of the glomerulus
- This increased pressure forces the blood out of the capillary walls through fenestrations
What does the fluid pass through once it has left the glomerulus ?
It passes through the basement membrane
What is the basement membrane made out of?
It is made up of a network of collagen fibres and other proteins that make up a second ‘sieve’
What can and what cant pass through the basement membrane ?
- They can not pass through the fenestrations
Blood cells and plasma proteins can not pass through because of their size
Everything else can
What cells does the wall of the Bowmans capsule involve ?
Podocytes
What do the podocytes act as ?
They act as an additional filter
What special feature do the podocytes have on them ?
- They have extensions called pedicels that wrap around the capillaries
- Proteins that have got through the epithelial cells and the basement membrane do not get through into the tubule itself
What does the filtrate that enter the capsule contain ?
It contains glucose, salt, urea and many other substances