5.1.2 - Excretion as an example of homeostatic control Flashcards
What does excretion mean?
The process of removing metabolic waste products from the body
Which products are removed via excretion?
- Carbon dioxide
- Bile pigment
- Nitrogenous waste
What are the 3 blood vessels connected to the liver?
- Hepatic artery
- Hepatic portal vein
- Hepatic vein
What does the hepatic artery do?
Supplies oxygenated nutrient poor blood from the heart
What does the hepatic portal vein do?
Supplies deoxygenated nutrient rich blood from the digestive system
What does the hepatic vein do?
Carries deoxygenated blood out of the liver and to the heart
What is the liver primarily made up of?
Hepatocytes
What are hepatocytes surrounded by?
Capillaries called sinusoids
What are sinusoids connected to?
- Hepatic artery
- Hepatic portal vein
- Central vein
What does the central vein do?
Connects to hepatic vein
What are the 3 functions of the liver?
- Stores glycogen which helps control blood glucose levels
- Breaks down toxic substances such as alcohol
- Breaks down excess amino acids
How does the liver break down excess amino acids?
It removes the amine group from an amino acid in a process called deamination and converts this amine group into ammonia. The remainder of the amino acid is used in respiration.
How is urea formed?
Ammonia is highly toxic and highly soluble in blood so the liver combines it with carbon dioxide to form urea which is less toxic and soluble using a series of reactions called the Ornithine cycle. Once urea is produced it is excreted in urine.
What is the main role of the kidneys?
To filter blood and produce urine which removes harmful waste products and controls the water potential of the blood (osmoregulation)
What is the structure of a nephron?
- Glomerulus
- Bowman’s capsule
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
- Collecting duct
What is the glomerulus?
A mass of blood capillaries and it is supplied blood by the afferent arteriole.
What does the efferent arteriole do?
Carries blood away from the glomerulus and branches into a network of capillaries that surround the rest of the nephron which ensures the whole structure has a short diffusion pathway to the blood.
What is the first step of water being absorbed into the nephron?
Ultrafiltration
Where does ultrafiltration take place?
Glomerulus and Bowman’s capsule
What happens during ultrafiltration?
- The difference in diameters between the afferent and efferent arterioles causes high hydrostatic pressure in the glomerulus
- This forces water, urea and other small substances (glucose and ions) to pass through gaps in the capillary’s endothelium, the basement membrane and podocytes forming glomerular filtrate
What is the second step of water being absorbed into the nephron?
Selective reabsorption
Where does selective reabsorption take place?
Proximal convoluted tubule
What happens during selective reabsorption?
- Sodium ions in the epithelial cell are actively transported into the blood
- Sodium ions and other substances in the tubule lumen are then transported into the epithelial cell via facilitated diffusion using co-transport proteins
- These substances then travel down their concentration gradient into the blood
- Water is transported into the blood by osmosis
What adaptations does the proximal convoluted tubule have?
- Epithelial cells contain mitochondria to produce ATP for active transport
- Membrane contains a large number and variety of co-transport proteins
- Microvilli increase the surface area for diffusion
- Short diffusion distance through one epithelial cell
What happens in the loop of Henle?
- In the ascending limb sodium ions are actively pumped from the filtrate into the medulla
- This creates a water potential gradient and causes water to pass out of the descending limb via osmosis
- In the medulla sodium ions and water are reabsorbed into the blood
- This loss of sodium ions causes the water potential of the filtrate to gradually increase up the ascending limb
- The loss of water causes the water potential of the filtrate to gradually decrease down the descending limb
- This countercurrent mechanism maintains the water potential gradient along the length of the loop of Henle
How does the loop of Henle vary in length for organisms with limited access to water?
The loop is longer meaning the water potential gradient is maintained for a longer distance which allows more water to be reabsorbed giving more concentrated urine
What happens in the distal convoluted tubule and the collecting duct?
Water is reabsorbed into the blood depending on the permeability of the structures.
What do osmoreceptors do?
Detect changes in the blood’s water potential and produce ADH
Where are osmoreceptors found?
In the hypothalamus
Where is ADH stored?
In the posterior pituitary gland
What happens when the blood water potential decreases?
- Osmoreceptors detect the decrease
- They stimulate the posterior pituitary gland to release more ADH
- This increases the reabsorption of water by increasing the distal convoluted tubule and collecting duct’s permeability to water
- This increases the blood water potential and produces a smaller volume of more concentrated urine
What happens when the blood water potential increases?
- Osmoreceptors detect the increase
- They reduce stimulation of the posterior pituitary gland so it releases less ADH
- This decreases the reabsorption of water by decreasing the distal convoluted tubule and collecting duct’s permeability to water
- This decreases the blood water potential and produces a larger volume of less concentrated urine
What are the 3 main uses of urine samples?
- Pregnancy tests
- Drug tests
- Medical diagnosis
How are urine samples used in pregnancy tests?
- Pregnant women produce a hormone called hCG
- This hormone enters the urine and pregnancy tests give a positive result if they detect hCG
- The tip of the pregnancy test contains monoclonal antibodies attached to coloured beads and these antibodies are all complementary to hCG
- The sample of urine is taken at the tip and it moves across the length of the strip
- As it passes through the section with antibodies any hCG in the sample binds to the antibodies
- The next zone contains immobilised antibodies which are also complementary to hCG so the hCG antibody complexes bind to these as well and stop here creating a blue line
- Antibodies that aren’t bound to hCG move further down to a zone containing immobilised antibodies that are complementary to the unbound antibodies so these 2 antibodies bind together and this produces a 2nd line which is the control line showing that the test is working correctly
How are urine samples used in drug tests?
- In the body many drugs are broken down into substances that are excreted in urine
- Some of these substances can be identified using complementary antibodies and others are tested using gas chromatography
- Gas chromatography involves turning the urine into a gas to separate and identify the different substances in it
What is an example of a class of drugs identified using gas chromatography?
Anabolic steroids which are taken to increase muscle mass
How are urine samples used in medical diagnosis?
- A high level of glucose in the body can diagnose diabetes
- The presence of nitrites can diagnose a bacterial infection
- A lack of toxic substances such as urea can indicate kidney damage
What is the glomerular filtration rate?
The rate at which they filter the blood
How does kidney failure affect the glomerular filtration rate?
It decreases the GFR
How is the glomerular filtration rate estimated?
By measuring the concentration of creatine in the blood which is a waste substance used by the muscles that is normally excreted in the urine. Therefore a high concentration of creatine suggests the blood isn’t being filtered effectively which could indicate kidney damage
What are the factors affecting the glomerular filtration rate?
- It declines with age
- Men generally have more creatine in their blood than women which is because they tend to have a higher muscle mass
How does kidney failure affect the electrolyte balance?
The concentrations of ions and charged compounds are not maintained meaning there may be excess potassium ions in the body and incorrect sodium ion levels
What are the 2 types of dialysis?
Haemodialysis and peritoneal dialysis
How does haemodialysis work?
- They pass the patient’s blood through a dialysis machine
- This machine contains an artificial partially permeable membrane that separates the blood from the dialysis fluid
- This fluid has a similar composition to blood but it doesn’t contain urea which ensures glucose doesn’t diffuse out but that urea does
- To make sure all of the urea is removed the blood and dialysis fluid move in opposite directions which maintains a concentration gradient along the length of the machine
How does peritoneal dialysis work?
- Takes place inside the body and the dialysis fluid is inserted into the stomach
- Substances are exchanged through the lining of the abdomen which is called the peritoneal membrane
What are the advantages of kidney transplants as a treatment for kidney failure?
- The patient doesn’t have to go through dialysis which can have bad side effects
- This makes the treatment less expensive and the transplanted kidney can last for years
What are the disadvantages of kidney transplants as a treatment for kidney failure?
- There aren’t enough donors for transplants
- There is the risk that the patient’s body may reject the donor kidney
- Immunosuppressants increase the patient’s susceptibility to other diseases
