Excretion as an example of homeostatic control Flashcards
What is excretion?
Excretion is the removal of metabolic waste from the body. Metabolic waste is unwanted product of cell metabolism.
What can happen if metabolic waste builds up?
They may inhibit enzymes reducing enzme activity or become toxic. E.g. the build up of CO2 and ammonia can be toxic – they can alter the PH interfering with other enzyme-controlled activities slowing down the metabolism.
What are the main excretory products?
- Carbon dioxide from respiration
- Nitrogen containing compounds such as urea (i.e. nitrogenous waste)
- Other compounds such as bile pigments found in faeces
Name the excretory organs?
- Lungs
- Liver
- Kidneys
- Skin
What do the lungs excrete?
- Carbon dioxide produced from respiration.
- It is transported in the blood stream in the form of mainly hydrogencarbonate ions before diffusing into the alveoli to be excreted during exhalation.
What does the liver excrete?
- The liver is involved in many metabolic reactions.
- Metabolic waste produced in these reactions pass into the bile to be excreted with the faeces for example the pigment bilirubin (produced when liver breaks down old erythrocytes)
- Also breaks down excess amino acids into urea - amino acids are deaminated to remove the nitrogen- containing part of the molecule, the amino group, which is then combined with carbon dioxide to make urea.
What does the kidneys excrete?
- Urea made by the liver is transported to the kidney via the blood stream (dissolved in the plasma).
- In the kidneys, the urea is removed from the blood to become a part of the urine.
- The urine is then stored in the bladder before being excreted via the urethra.
What does the skin excrete?
- Not the primary function.
- However, it excretes sweat which contains salts, urea, uric acid, water and ammonia.
- Urea, uric acid and ammonia are all excretory products.
- The loss of water and salt is due to homeostasis.
Why is the build up of CO2 toxic?
- CO2 is transported in the blood as hydrogencarbonate ions. However alongside the formation of these hydrogencarbonate ions (dissociation of carbonic acid), hydrogen ions are also released.
- these H+ ions decrease the pH of the cytoplasm in RBCs, and interact with the bonds in haemoglobin(Hb) disrupting unchanging it’s 3D shape.
- This reduces the affinity of Hb for oxygen, So less is transported in the blood.
- The H+ ions Produced can also combine with Hb, forming haemoglobinic acid.
- CO2 that is not converted to hydrogencarbonate ions, can directly combine with haemoglobin to form carbaminohaemoglobin.
- Both haemoglobinic acid and carbaminohaemoglobin unable to combine with oxygen is normal reducing oxygen transport further
Excess hydrogen can reduce the pH of plasma. how is this minimised?
Proteins in the blood act as buffers to resist change in pH.
If the change in pH is small only small (due to CO2), what happens in the body?
The extra hydrogen ions or detected by the respiratory centre in the medulla oblongata of the brain. this causes an increase in the breathing rate to help remove the excess carbon dioxide. This returns the pH of the blood back to normal.
What happens if the blood pee H drops below 7.35?
Respiratory acidosis:
- May cause headaches, drowsiness, restlessness , tremor and confusion.
- Increase in heart rate and changes in blood pressure.
- It can also be caused by conditions which affect the lungs, or if the airway is blocked.
What happens when amino acids build up in the body?
Amino group of AA are toxic , so it cannot stored, but the AA molecuule stores a lot of energy so so it is wasteful to excrete directly, therefore the AA is treated using 2 processes: deamination + ornithine cycle, converting excess amino acids into urea.
Word equation for deamination and ornithine cycle?
Deamination: amino acid + oxygen –> keto acid + ammonia
ornithine cycle: ammonia + carbon dioxide –> urea + water
IMPORTANT THAT YOU KNOW THESE EQUATIONS
What are liver cells known as?
Hepatocytes
What are the shapes of liver cells?
Cuboidal, with many microvilli on their surface
Why does the liver have an important role in homeostasis?
Because it carries out many metabolic process is needed for homeostasis
What kind of metabolic processes does the liver carry out?
- Control of blood glucose/amino acid/lipid levels
- Synthesis of bile, plasma proteins + cholestrol
- Synthesis of RBCs in the fetus
- Storage off vitaminations e.g. B12, iron + glycogne
- Detoxification of alcohol, drugs
- Breakdown of RBCs.
Because the liver is involved in many metabolic processes, what does it require?
Each hepatocyte has a very good blood supply to return substances to the blood to ensure concentrations are maintained and to remove excess or unwanted substances from the blood. It is also needed to provide sufficient oxygen and glucose needed for aerobic respiration. The structure of the liver ensures this.
Why does the liver contain lots of mitochondria?
Mitochondria carry out aerobic respiration to generate lots of ATP which is then hydrolysed to released lots of energy needed for all these metabolic processes.
Which two sources supplies the liver with blood?
- The hepatic artery
- The hepatic portal vein
What type of blood does the hepatic artery supply and where does it come from?
- Oxygenated blood from the aorta of the heart enter the liver via the hepatic artery.
- Only this blood vessel supplies OXYGEN to the liver.
What type of blood does the hepatic portal vein and where does it come from?
- Deoxygenated blood from the digestive system enter the liver via the hepatic portal vein.
- This blood is rich in the products of digestion and the concentration of various substances will be uncontrolled as they have just entered the body (and been digest in the intestines). The blood may also contain toxic compounds that have been absorbed from the intestine. It is important that the concentration of these substances have been adjusted and toxic substance have been removed before they circulate around the body.
Which blood vessel does the blood leave the liver by?
- The hepatic vein then carries blood back to the heart via the vena cava (and the normal body circulation occurs).
- The concentration of substances in the blood at this point have now been modified and regulated.