Final answer:
Urine volume is primarily determined by hydrostatic pressure and colloid osmotic pressure which affect GFR, and by the regulation of water reabsorption in the collecting ducts, which is influenced by ADH levels and plasma osmolarity.
Step-by-step explanation:
The two factors that determine urine volume are: hydrostatic pressure and colloid osmotic pressure, which influence the Glomerular Filtration Rate (GFR). The hydrostatic pressure in the glomerulus is affected by systemic blood pressure, autoregulatory mechanisms, sympathetic nervous activity, and paracrine hormones. This pressure is critical for the filtration process that occurs in the kidneys.
Additionally, urine volume can be greatly affected by water reabsorption in the collecting ducts, a process regulated by the hormone ADH (antidiuretic hormone or vasopressin). When plasma osmolarity rises, signaling that the blood is hyperosmotic, more water is reabsorbed to dilute the blood, leading to a decrease in urine volume. Conversely, when plasma osmolarity falls, signaling that the blood is hyposmotic, the collecting ducts reabsorb less water, resulting in an increase in urine volume.
Other factors that can affect urine volume include nutrient intake, exercise, environmental temperature, and the presence of conditions such as diabetes mellitus or diabetes insipidus. For example, diabetes mellitus can cause excess glucose in the blood to spill into the urine, attracting water due to its osmotic nature, thus increasing urine volume. Diabetes insipidus can lead to the production of large volumes of very dilute urine because of a lack of ADH or receptors for it, which results in reduced water reabsorption.