Module 5.2 - Excretion As An Example Of Homeostatic Control Flashcards
Define excretion
The removal of metabolic waste from the body
Define metabolic waste
Waste substances that may be toxic or are produced in excess by the reactions inside cells
Describe CO2 as a waste product
Dissolves in blood to form carbonic acid, dissociates to form hydrogencarbonate ions and H+ ions, making blood acidic
Chemoreceptors sense the change and the respiratory centre of the medulla oblongata causes an increase in heart and breathing rate to get rid of excess CO2
Why is excess CO2 toxic?
Too much results in less oxygen carriage
If blood pH drops too low, respiratory acidosis can occur (caused by lung diseases such as asthma/airway blockage, causes slow breathing, headaches, confusion, rapid heart rate)
Describe nitrogenous waste as a waste product
From excess amino acids which can’t be stored
Excreted as urea in urine
Transported dissolved in plasma
Kidneys remove it from blood and turn it into urine
Why is nitrogenous waste toxic?
Amino acids turned into ammonia in liver, which is highly toxic and highly soluble
Ammonia is turned into urea in liver before travelling in blood to kidneys to be removed and put into urine in the bladder
Functions of the liver
Formation of urea from excess amino acids
Synthesis of bile, plasma proteins (fibrinogen), cholesterol
Storage of vitamins A, D and B12, iron, glycogen
Detoxification
Breakdown of hormones
Destruction of red blood cells
Adaptations of hepatocytes
Many mitochondria (very metabolically active) Many ribosomes (protein synthesis) Many/larger Golgi apparatuses
Role of afferent arteriole
Brings blood to glomerulus
Role of proximal convoluted tubule
Where selective reabsorption occurs
Role of the loop of Henle
Reduce water potential of medulla tissue
Role of distal convoluted tubule
Concentrations of some salts are adjusted by active transport
Role of collecting duct
Water is reabsorbed into surrounding tissue fluid in medulla
Fluid is now urine and will be taken by the duct through the pelvis to the bladder via the ureter
What is ultrafiltration
Filtration at molecular level Smaller molecules (urea, water, glucose, amino acids, ions) are filtered out of blood into lumen of the Bowman’s capsule
Points about basement membrane
Mesh of glycoproteins and collagen acts as molecular filter
Nothing with molecular mass over 69,000 can pass through (e.g. proteins and blood cells)
Allows plasma to escape containing dissolved molecules
What are podocytes?
Allow filtrate to pass beneath them in ultrafiltration
How are molecules ‘selected’ to be reabsorbed in the PCT?
Specific carrier proteins
What is reabsorbed in selective reabsorption?
All glucose
All amino acids
Most water
Some ions
Process of selective reabsorption
1) Sodium potassium pumps sodium out into blood and potassium into cells by an active process (the cells have lots of mitochondria)
2) Many co-transporter proteins attached to cell surface membrane for facilitated diffusion of glucose/amino acids with sodium ions
3) Movements of ions causes a lower water potential in the PCT cells. Water moves from lumen to cells via osmosis and will follow ions into the blood
4) Glucose and amino acids diffuse into tissue fluid and then the blood
Process of water reabsorption in loop of Henle
Purpose is to cause a decrease in water potential in tissue fluid of medulla
Ions are actively transported out of ascending limb into medulla tissue fluid
Water potential of tissue fluid surrounding loop of Henle and collecting duct is lower than inside them
Descending limb and collecting duct are permeable to water
Water is absorbed from descending limb and collecting duct into tissue fluid and then into blood
Process of ADH release
Osmoreceptors in hypothalamus monitor water potential of blood
Neurosecretory cell bodies in the hypothalamus produce anti-diuretic hormone (ADH)
ADH travels down axon of neurosecretory cells to the terminal bulb in posterior pituitary gland to be stored
If osmoreceptors sense blood water potential is too low, they lose waste by osmosis
Triggers action potential down axon
Stimulates release of ADH from terminal bulb in posterior pituitary gland
