Question

1.  Describe the formation of urine within a nephron (the functional unit of the kidney).  Starting with the glomerulus provide an explanation for how plasma is filtered and concentrated to form urine.  Be sure to describe both the structures that the filtrate travels through, and the processes that are occurring.

Answer 1

Urinary excretion rate= Filtration rate − Reabsorption rate + Secretion rate

Urine formation begins when a large amount of fluid is filtered from the glomerular capillaries into Bowman’s capsule. Most substances in the plasma, except proteins, are freely filtered, so their concentration in the glomerular filtrate in Bowman’s capsule is almost the same as in the plasma. As filtered fluid leaves Bowman’s capsule and passes through the tubules, it is modified by reabsorption of water and specific solutes back into the blood or by secretion of other substances from the peritubular capillaries into the tubules.

Tubular reabsorption is more important than tubular secretion in the formation of urine, but secretion plays an important role in determining the amounts of potassium and hydrogen ions and a few other substances that are excreted in the urine. Most substances that must be cleared from the blood, especially the end products of metabolism such as urea, creatinine, uric acid, and urates, are poorly reabsorbed and are therefore excreted in large amounts in the urine. Certain foreign substances and drugs are also poorly reabsorbed but, in addition, are secreted from the blood into the tubules, so their excretion rates are high. Electrolytes, such as sodium ions, chloride ions, and bicarbonate ions, are highly reabsorbed, so only small amounts appear in the urine. Certain nutritional substances, such as amino acids and glucose, are completely reabsorbed from the tubules and do not appear in the urine even though large amounts are filtered by the glomerular capillaries.

About 65 percent of the filtered load of sodium and water and a slightly lower percentage of filtered chloride are reabsorbed by the proximal tubule before the filtrate reaches the loops of Henle. The descending part of the thin segment is highly permeable to water and moderately permeable to most solutes, including urea and sodium. The ascending limb, including both the thin and the thick portions, impermeable to water, and helps for concentrating the urine, the ascending limbs are capable of active reabsorption of sodium, chloride, and potassium. Distal tubule reabsorbs most of the ions, including sodium, potassium, and chloride, but is virtually impermeable to water and urea, so it is referred to as the diluting segment because it also dilutes the tubular fluid. The medullary collecting ducts reabsorb less than 10 percent of the filtered water and sodium, they are the final site for processing the urine and, therefore, play an extremely important role in determining the final urine output of water and solutes.

When there is excess water in the body and extracellular fluid osmolarity is reduced, the secretion of ADH by the posterior pituitary decreases, reducing the permeability of the distal tubule and collecting ducts to water, which causes increased amounts of more dilute urine to be excreted. Thus, the rate of ADH secretion determines, to a large extent, whether the kidney excretes dilute or concentrated urine.