Video Transcript
The liver is vital for excretion in humans. Which of the following best explains excretion? (A) Excretion is the removal of excess water and food from the body. (B) Excretion is the production of sweat from the sweat glands in the skin. (C) Excretion is the removal of the waste products of metabolism from the body. Or (D) excretion is the process by which waste products are converted into useful compounds for the body.
This question is asking about excretion, which involves several organs of the body, including the liver. So, let’s review some key facts about excretion and then look at each of these statements to narrow down our options and come up with the correct answer.
The process of excretion removes waste products that have been produced by cells. These wastes have resulted from different chemical reactions that have gone on inside the cell, including cellular respiration, protein and amino acid catabolism, and lactic acid fermentation in muscle cells.
The liver has many roles within the body. These include the conversion of lactic acid back into glucose; the detoxification of harmful substances such as drugs and alcohol; and the breakdown of hemoglobin, which enables iron to be recycled and the pigments to be excreted in bile. Amino acids are usually deaminated in the liver so that keto acids can be used in respiration. The toxic by-product ammonia can then be formed into the less toxic urea through the urea cycle. The urea is then released into the blood and excreted in urine via the kidneys.
So, let’s now look at the answer options. Looking at option (A), it states that excretion is the removal of excess water and food from the body. Although water is a waste of respiration, a process which occurs in cells, the option is incorrect. This is due to the second part of the statement, which highlights a common misconception. The removal of excess food from the body is carried out by the digestive system.
During digestion, the food is taken into the body. It is digested, the small soluble food molecules are absorbed, and any leftover food forms into feces which is egested out of the anus. This waste food has not entered any of the cells. It has just passed through the body through the esophagus, stomach, and intestines. Excretion involves wastes produced by processes inside cells.
Option (B) states that excretion is the production of sweat from the sweat glands. The main purpose of sweating is as a homeostatic mechanism to reduce body temperature. Sweat is made up of water and dissolved salts. So, it does contain substances produced by cells. But this is a minor role compared to its role in temperature regulation. It is just an example of excretion rather than an explanation.
Option (C) seems to be the correct answer. Metabolic reactions go on inside the cell and produce waste products, which then move out of the cell and are removed from the body.
But let us just look at the last statement before returning to the question. Although the liver converts lactic acid back into the very useful molecule glucose, this is not one of its excretory roles. Excretion is about removing wastes which are not useful to the body and are likely to harm the body if allowed to build up. This means that option (D) is, indeed, not the correct answer.
So, returning to our question, which asks for the best explanation of excretion. We now know the correct answer is option (C). Excretion is the removal of the waste products of metabolism from the body.
