Final answer:
The process that reduces the concentration of urea in the blood of humans is Excretion, which is facilitated by the kidneys filtering urea from the bloodstream and through urine. In cases where the kidneys fail, dialysis is used to remove urea using a dialysate and a semipermeable membrane.
Step-by-step explanation:
The process that reduces the concentration of urea in the blood of humans is A) Excretion. Urea is a waste product of protein catabolism, synthesized in the liver, and it is essential to remove this water-soluble, non-toxic compound from the blood to prevent the accumulation to toxic levels. This process is facilitated by the kidneys filtering the urea out of the bloodstream and excreting it through urine. On average, an adult excretes about 25 to 30 grams of urea in urine per day. If the kidneys are not functioning properly, such as in patients with kidney illnesses, dialysis machines are utilized as an artificial means to remove harmful urea from the blood. During dialysis, blood is separated from a solution called the dialysate. The dialysate has a lower solute concentration compared to the blood, which promotes the diffusion of urea out of the blood through a semipermeable membrane into the dialysate, thus cleansing the blood of excess urea.