excretion

what is excretion

the removal of metabolic waste from the body e.g. carbon dioxide is a waste product of respiration and is removed from the body by the lungs

where is carbon dioxide produced

it is produced in every cell of the body as a product from respiration

why does carbon dioxide need to be removed from the body

• if there is too much CO2 in the blood, it can reduce oxygen transport

• it combines directly with haemoglobin to form carbaminohaemoglobin. this molecule has a low affinity for O2 than normal haemoglobin

• excess CO2 can cause respiratory acidosis

role of 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

role of the hepatic vein

takes deoxygenated blood away from the liver

role of hepatic portal vein

brings blood from the duodenum and ileum (parts of the small intestine) which is high in products needing to be digested. meaning any ingested harmful substances are filtered out and broken down straight away

role of the bile duct

takes bile (produced by the liver to emulsify fats) to the gall bladder to be stored

what are and what is the function of liver lobules?

• cylindrical structures made up of hepatocytes (cells) that are arranged in rows radiating out from the centre

• each lobule has a central vein in the middle that connects to a hepatic vein

• many branches of the hepatic artery, hepatic portal vein and bile duct are also found connected to each lobule

how is the hepatic artery and hepatic portal vein connected to the central vein

by capillaries called sinusoids

where are and what are the roles of Kupffer cells?

• they are attached to the walls of the sinusoids

• they remove bacteria and break down old red blood cells

describe the blood flow along the sinusoids

• as blood flows along the sinusoids, exchange can take place. fenestrations (small pores) in walls of sinusoids increase the rate of exchange

• blood flows past the hepatocytes that remove harmful substances and O2 from the blood

• the harmful substances are broken down by the hepatocytes into less harmful substances that then re-enter the blood

how is the bile duct connected to the central vein

by tubes called canaliculi

why do hepatocytes have microvilli

as they increase it’s surface area

describe how excess amino acids are broken down in the liver

• firstly, the nitrogen-containing amino groups (-NH2) are removed from any excess amino acids, forming ammonia (NH3) and keto acid. (this process is called deamination)

• the keto acid can be used directly in respiration to give ATP or converted to carbohydrate and stored as glycogen

• ammonia is too toxic for mammals to excrete directly, so it is combined with CO2 in the ornithine cycle to create urea and water

• the urea is released from the liver into the blood, the kidneys then filter the blood and remove the urea as urine, which is excreted from the body

where does ultrafiltration take place

in the glomerulus and bowman’s capsule

what is the afferent arteriole

the arteriole which takes blood into each glomerulus

what is the efferent arteriole

the arteriole which takes the filtered blood away from the glomerulus

why has the efferent arteriole got a smaller diameter than the afferent arteriole

• means the blood in the glomerulus is under a higher pressure

• this high pressure forces liquid and small molecules in the blood out of the capillary and into the bowman’s capsule

describe the process of ultrafiltration

• blood arrives at the glomerulus through the afferent arteriole and leaves the glomerulus via the efferent arteriole

• the high hydrostatic pressure from the efferent arteriole forces small molecules out of the blood (through pores in the capillary endothelium)

• the small molecules move through the basement membrane, which has collagen fibres, preventing large molecules from entering the bowman’s capsule

• the molecules then move through slits in the endothelium formed by pedicels, which make up podocytes (specialised epithelial cells)

• blood arrives in the bowman’s capsule to form the glomerular filtrate

give some examples of substances that are filtered into glomerular filtrate

water, salts, glucose, urea

give some examples of substances that remain in the blood (too large to diffuse into bowman’s capsule)

blood cells, platelets, proteins

which substances are selectively reabsorbed

ALL glucose, amino acids and small proteins

SOME water and ions

where does selective reabsorption take place

in the proximal convoluted tubule

describe some key adaptations of the epithelial cells in the PCT

• microvilli - they increase the surface area for reabsorption

• basal infoldings - further increase the surface area for moving substances into surrounding blood capillaries

• numerous mitochondria - provide ATP for the active transport processes involved in reabsorption

• co-transporter proteins in the plasma membrane - allow co-transport of substances from the filtrate into epithelial cells

describe the process of selective reabsorption at the PCT

• sodium-potassium pump uses ATP from the mitochondria to pump sodium ions out of the epithelial cells (lining the PCT) into the blood

• so there is a low concentration of sodium ions in the cytoplasm of epithelial cells

• therefore, sodium and glucose bind to carrier molecules on the membrane of the PCT, and they are transported into the cell of the PCT

• the carrier molecule and substances dissociate into the cell

• glucose and sodium diffuse to the basal end of the cell

• sodium and glucose are actively transported out of the cell and then they diffuse into the capillary, sodium travels down the electrochemical gradient

• the movement of ions creates an osmotic gradient due to the changes in water potential, so water moves out of the PCT by osmosis

describe water reabsorption at the loop of Henle

• the descending limbs walls are permeable to water so water leaves the filtrate via osmosis into the interstitial space

• more water is lost as the filtrate moves down the descending limb, reaching it’s lowest water potential in the medulla

• water that is lost, is reabsorbed by the surrounding capillaries and carried away in the blood

• the ascending limb is impermeable to water, but is permeable to sodium and chloride ions

• this increases the concentration of ions in the interstitial space of the medulla, making it’s water potential very low

• these ions are actively transported out of the ascending limb as the filtrate moves upwards, increasing the water potential

• this creates a water potential gradient in the ascending limb, with the higher water potential in the cortex and the lower water potential in the medulla

describe reabsorption when the filtrate enters the collecting duct

• water moves out of the collecting duct by osmosis and enters the capillaries

• urine leaving the collecting duct has a very low water potential, as most water has been reabsorbed into the blood

• the urine passes down the collecting duct to the pelvis