Final answer:
The fluid within a nephron originates from the blood that enters the kidney through the renal artery and gets processed through a series of stages in the nephron components to ultimately form urine.
Step-by-step explanation:
The fluid within a nephron originates from the blood that enters the kidney through the renal artery. This blood is processed in the nephron – the functional unit of the kidney – where filtration, reabsorption, and secretion occur to form urine. Filtration begins as blood flows into the glomerular capillaries and due to blood pressure, water, and solutes are filtered out of the blood into Bowman's capsule as filtrate. Reabsorption and secretion then happen in the renal tubule of the nephron. Substances like glucose and amino acids get reabsorbed back into the blood, while wastes get secreted into the filtrate, which eventually becomes urine.
The nephron is composed of the renal corpuscle, which includes the glomerulus and Bowman's capsule, the renal tubule, and the associated capillary network. The initial filtrate formed is composed of water, salts, sugars, and urea. During its journey through the proximal convoluted tubule and the loop of Henle, substances are either returned to the blood or further processed into urine, which flows to the renal papilla and ureter and is stored in the bladder.