EXCRETION AS AN EXAMPLE OF HOMEOSTATIC CONTROL
(a) The term excretion and its importance in maintaining metabolism and homeostasis
● To include reference to the importance of removing metabolic wastes, including carbon
dioxide and nitrogenous waste, from the body.
Excretion is the removal of metabolic waste—unwanted products of cell metabolism—from the
body. They need to be removed so they do not build up and inhibit enzyme activity or become
toxic. The main excretory products are C02 from respiration, nitrogenous compounds, such as
urea, and other compounds, such as bile pigments in the faeces.
CO2 dissociates into hydrogen carbonate ions, and ammonia is similarly acidic. They reduce the
pH of the cytoplasm, so if they build up, they would denature enzymes. Other metabolic products
may act as inhibitors and reduce the activity of essential enzymes. CO 2 also inhibits the action of
haemoglobin forming haemoglobinic acid or carbaminohemoglobin.
CO2 is diffuses out of all cells in respiring tissues into the bloodstream, where it is transported,
mostly as hydrogencarbonate and hydrogen ions, to the lungs, where carbon dioxide diffuses
into the alveoli and is excreted in expiration. The liver has many metabolic roles and the waste
products are passed into the bile for excretion in the faeces, for example bilirubin. It is involved
in converting deaniminting excess amino acids, producing ammonia. This is a highly soluble and
toxic molecule, so is converted to the less soluble and toxic compound, urea. The urea is passed
into the bloodstream and carried in solution to the kidney and excreted in the urine.
(b) (i) The structure and functions of the mammalian liver
● To include the gross structure and histology of the liver
● the roles of the liver in storage of glycogen, detoxification and the formation of urea (the
ornithine cycle covered in outline only).
Structure of the liver
It is important for the liver to have a good blood supply so hepatocytes can carry of hundred of
metabolic processes, that play a role in excretion and homeostasis. A good blood supply is
maintained by the hepatic artery, and the hepatic portal artery. The arrangement of the lover into
lobes, further divided into lobules each supplied by an intralobular-branch of the hepatic and
hepatic portal veins, ensures that the greatest possible contact is maintained between the the
blood and the liver cells. Oxygenated blood travels from the aorta into the liver in the former,
supplying a high pO2 of oxygen to sustain a high rate of aerobic respiration, to sustain active
processes in hepatocytes. The hepatic portal vein supplies deoxygenated blood from the
digestive system. This provides hepatocytes with high concentrations of respiratory substrates
from digestion, but mainly is needed so that the toxic compounds not absorbed by the intestine
can have their concentrations adjusted in the liver before they circulate around the body.
The blood is mixed in the sinusoid to provide both the products of digestion, and oxygen needed
to carry out the metabolic functions, such as protein synthesis, transformation and storage of
carbohydrates, synthesis of cholesterol and bile salts, detoxification, etc in the dense cytoplasm
of the cuboidal hepatocytes. They have microvilli to increase the surface area in contact with
blood in the sinusoid. Kupffer cells, specialised macrophages, are mobile within the sinusoid,
breaking down and recycling old red blood cells, the by-product of which is the pigment
bilirubin.
Blood then flows out through the hepatic vein, via an intra-lobular vessels. Bile made in the
hepatocytes is released into bile canaliculi, which join to form the bile duct, which carries the
