Specific gravity

The specific gravity of a substance refers to its density divided by (or relative to) the density of water. This is why specific gravity has no units, as it is based on the ratio of one density to another, so the units cancel each other out. This video from KhanAcademy explains the concept in more detail. The specific gravity of pure (distilled) water is therefore expressed simply as 1.000

In veterinary medicine we frequently want to know the specific gravity (S.G.) of liquids such as urine or colostrum because this information can be clinically useful. A refractometer is the instrument used to measure S.G.      

Urine typically has a S.G. in the range of 1.003 to 1.035. This means it is slightly denser than water. This makes sense as we know that urine consists of a mixture of excess body water and waste products of metabolism such as urea and creatinine, along with some crystals etc. Even very dilute urine contains some of these waste products so it will always have a S.G. >1.000. Very dilute urine is often seen in animals suffering from polyuria & polydipsia (PU/PD).

If the animal's kidneys are trying to conserve body fluids (regulated by the release of antidiuretic hormone or ADH from the posterior pituitary gland) most of the H2O molecules in the glomerular filtrate will be reabsorbed from the distal convuluted tubules in the renal nephrons. This will result in the production of lower volumes of concentrated urine, with a typical S.G. of at least 1.020. High urine S.G. is associated with dehydration or blood loss. 

The urine S.G. of normal animals therefore varies a lot, depending on their hydration status. Animals who have suffered severe renal damage may lose this ability to regulate the S.G. of their urine. Their urine S.G. will remain in the region of 1.008-1.012. This is known as isothenuria as it means that the urine has the same S.G. as plasma. In other words: the kidneys have lost their abilty to actively dilute or concentrate the urine being produced.