excretion, homeostasis and the liver

Question 1

excretion in mammals

Answer 1

The main metabolic waste products in mammals are:
• Carbon dioxide - one of the waste products of cellular respiration which is excreted from the lungs.
-Bile pigments - formed from the breakdown of haemoglobin from old red blood cells in the liver. They are excreted in the bile from the liver into the small intestine via the gall bladder and bile duct.
They colour the faeces.
-Nitrogenous waste products (urea) - formed from the breakdown of excess amino acids by the liver. All mammals produce urea as their nitrogenous waste. Fish produce ammonia while birds and insects produce uric acid. Urea is excreted by the kidneys in the urine.

Question 2

the liver

Answer 2

The liver is one of the major body organs involved in homeostasis.
It is a reddish-brown organ which makes up about 5% of the total body mass - the largest internal organ of the body. It lies just below the diaphragm and is made up of several lobes. The liver is very fast growing and damaged areas generally regenerate very quickly.
The liver has a very rich blood supply - about 1 dm of blood flows through it every minute. Oxygenated blood is supplied to the liver by the hepatic artery and removed from the liver and returned to the heart in the hepatic vein. The liver is also supplied with blood by a second vessel, the hepatic portal vein. This carries blood loaded with the products of digestion straight from the intestines to the liver and this is the starting point for many metabolic activities of the liver (Figure 1). Up to 75% of the blood flowing through the liver comes via the hepatic portal vein.

Question 3

the structure of the liver

Answer 3

The liver carries out many different complex functions but the cells are surprisingly simple and uniform in appearance. Liver cells or hepatocytes have large nuclei, prominent Golgi apparatus, and lots of mitochondria, indicating that they are metabolically active cells (Figure 1). They divide and replicate - even if around 65% of the liver is lost, it will regenerate in a matter of months.
The blood from the hepatic artery and the hepatic portal vein is mixed in spaces called sinusoids which are surrounded by hepatocytes. This mixing increases the oxygen content of the blood from the hepatic portal vein, supplying the hepatocytes with enough oxygen for their needs. The sinusoids contain Kupffer cells, which act as the resident macrophages of the liver, ingesting foreign particles and helping to protect against disease. The hepatocytes secrete bile from the breakdown of the blood into spaces called canaliculi, and from these the bile drains into the bile ductules which take it to the gall bladder.

Question 4

functions of the liver-carbohydrate metabolism

Answer 4

Hepatocytes are closely involved in the homeostatic control of glucose levels in the blood by their interaction with insulin and glucagon.
When blood glucose levels rise, insulin levels rise and stimulate hepatocytes to convert glucose to the storage carbohydrate glycogen.
About 100g of glycogen is stored in the liver. Similarly, when blood sugar levels start to fall, the hepatocytes convert the glycogen back to glucose under the influence of the hormone glucagon.

Question 5

functions of the liver-deamination of excess amino acids

Answer 5

The liver plays a vital role in protein metabolism where hepatocytes synthesise most of the plasma proteins. Hepatocytes also carry out transamination - the conversion of one amino acid into another. This is important because the diet does not always contain the required balance of amino acids but transamination can overcome the problems this might cause.
The most important role of the liver in protein metabolism is in deamination - the removal of an amine group from a molecule. The body cannot store either proteins or amino acids. Any excess ingested protein would be excreted and therefore wasted if it were not for the action of the hepatocytes. They deaminate the amino acids, removing the amino group, and converting it first into ammonia which is very toxic and then to urea. Urea is toxic in high concentrations but not in the concentrations normally found in the blood. Urea is excreted by the kidneys (you will learn about the role of the kidneys in excretion and water balance in Topics 15.5 and 15.6).
The remainder of the amino acid can then be fed into cellular respiration or converted into lipids for storage.
The ammonia produced in the deamination of proteins is converted into urea in a set of enzyme-controlled reactions known as the ornithine cycle. Removing the amino group from amino acids and converting the highly toxic ammonia to the less toxic and more manageable compound urea involves some complex biochemistry. This is simplified and summarised in Figure 3.

Question 6

functions of the liver-detoxification

Answer 6

The level of toxins in the body always tends to citruline
increase. Apart from urea, many other metabolic pathways produce potentially poisonous substances.
NH3
We also take in a wide variety of toxins by choice such as alcohol and other drugs. The liver is the site where most of these substances are detoxified and made harmless.
One example is the breakdown of hydrogen peroxide, a by-product of various metabolic pathways in the body. Hepatocytes contain the enzyme catalase, one

of the most active known enzymes, that splits the hydrogen peroxide into oxygen and water. Another
example is the way in which liver detoxifies the ethanol - the active drug in alcoholic drinks. Hepatocytes contain the enzyme alcohol dehydrogenase that breaks down the ethanol to ethanal. Ethanal is then converted to ethanoate which may be used to build up fatty acids or used in cellular respiration.