Wastes

You learned back in the blood unit that wastes in the blood tend to be metabolic wastes... I think that now you will understand this a bit better. Especially after thinking about metabolism a bit in the last unit.

The metabolic processes that you learned about in the last unit were ones to convert macromolecules into sugars for cellular respiration (and also to convert sugars back into larger molecules for storage). During the breakdown of polysaccharides, there really isn't any waste made. But, during the breakdown of proteins and fats and nucleotides into sugars, there are some wastes. The main wastes come during the breakdown of proteins, and these are the nitrogenous wastes.

Remember, proteins contain the atom nitrogen, while sugars don't. So, when proteins are broken down into amino acids to be used to make sugars (gluconeogenesis), the nitrogen atoms have to be removed. This causes nitrogenous wastes to build up.

The main nitrogenous wastes are: urea and uric acid. Urea is formed from gluconeogenesis of amino acids, while uric acid is from gluconeogenesis of nucleotides. Our bodies tend to make more urea, because we have more protein available for catalysis than nucleic acids. Either way, these molecules are mainly removed from our bodies, keeping a low concentration of them in our blood. Urea is also used in the medulla to increase the amount of solute in the medullary interstitial fluid (making it more hypertonic)... this helps in the reabsorption of water. It should make sense to you that urea is both a waste product and useful in the kidney for water reabsorption-- if we have to get urea into the kidneys anyway for excretion, we may as well use this molecule for another kidney function while we're at it!

Urea and uric acid are the primary waste molecules of our cells. We excrete other molecules as well, and the excretion of these other molecules tends to maintain homeostasis in our bodies.

Other waste molecules are those that contain the atoms sulfur or phosphorus... both of these atoms can be found in amino acids (but not in glucose), and phosphorus is also present in high concentration in nucleotides. These waste molecules are typically the sulfate and phosphate ions.

Another common waste is creatinine and also creatine. These terms may seem familiar to you. We learned about creatine phosphate when we studied muscle. Look back at Figure 9.12 to get a reminder about how muscle uses creatine phosphate to store energy in muscle. Anyway, some of the creatine is lost into the blood, and most of it that enters the liver is converted into creatinine. These molecules are then excreted in the kidneys, because they are filtered out in the glomerulus, but then never reabsorbed.

You also will see that other electrolytes, especially Na⁺, K⁺, H⁺, and Cl^-, are also specifically lost in our urine to maintain homeostasis.