Question

Discuss the physiology of urine formation

Answer 1

Having reviewed the anatomy and microanatomy of the urinary system, now is the time to focus on the physiology. Recall that the glomerulus produce a simple filtrate of the blood and the remainder of the nephron works to modify the filtrate into urine. You will discover that different parts of the nephron utilize three specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce of urine

Glomerular Filtration

Glomerular filtration occurs as blood passes into the glomerulus producing a plasma-like filtrate (minus proteins) that gets captured by the Bowman’s (glomerular) capsule and funneled into the renal tubule. This filtrate produced then becomes highly modified along its route through the nephron by the following processes, finally producing urine at the end of the collecting duct.

Tubular Reabsorption

As the filtrate travels along the length of the nephron, the cells lining the tubule selectively, and often actively, take substances from the filtrate and move them out of the tubule into the blood. Recall that the glomerulus is simply a filter and anything suspended in the plasma that can fit through the holes in the filtration membrane can end up in the filtrate. This includes very physiologically important molecules such as water, sodium, chloride, and bicarbonate (along with many others) as well as molecules that the digestive system used a lot of energy to absorb, such as glucose and amino acids. These molecules would be lost in the urine if not reclaimed by the tubule cells. These cells are so efficient that they can reclaim all of the glucose and amino acids and up to 99% of the water and important ions lost due to glomerular filtration. The filtrate that is not reasbsorbed becomes urine at the base of the collecting duct.

Tubular Secretion

Tubular secretion occurs mostly in the PCT and DCT where unfiltered substances are moved from the peritubular capillary into the lumen of the tubule. Secretion usually removes substances from the blood that are too large to be filtered (ex: antibiotics, toxins) or those that are in excess in the blood (ex: H+, K+). These substances secreted into the tubule are destined to leave the body as components of urine.

Answer 2

Answer:Urine is formed in three main steps- glomerular filtration, reabsorption and secretion. It comprises 95 % water and 5% wastes such as ions of sodium, potassium and calcium, and nitrogenous wastes such as creatinine, urea and ammonia. Osmoregulation is the process of maintaining homeostasis of the body.

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