Filtration, reabsorption, secretion flashcards

The renal exchange process

  1. describe the processes that allow for tubular reabsorption

  2. describe the role of secretion in the renal tubules

PROCESSES OF URINE FORMATION

(1) glomerular filtration

~ results in the formation of the filtrate

~ filtrate = plasma minus most of the plasma proteins

~ the glomerular membrane has 3 filtration barriers:

  • the fenestrated endothelium

  • the basal membrane

  • the surrounding epithelial cell layer

(2) tubular reabsorption

~ the peritubular capillaries reabsorb useful substances from the renal tubule cells such as:

  • water

  • glucose

  • amino acids

  • ions

~ some reabsorption is passive; most is active

~ most reabsorption occurs in the proximal convoluted tubule

ACTIVE REABSORPTION OF SOLUTE:

~ substances X and Y are both actively transported, but through different mechanisms

~ transported by mechanisms involving active transport across one membrane tubules

~ passive movement across the other membrane

WATER REABSORPTION

~ water diffusion is based on differences in osmolarity

~ solutes X and Y are actively reabsorbed, increasing the osmolarity of the plasma

~ decreasing the osmolarity of the tubular fluid

~ water diffuses into the plasma (an area of low water concentration)

TUBULAR REABSORPTION

~ transport maximum

  • concentration of substance when carriers are saturated

  • reflects the number of carriers

  • nutrients will normally be fully reabsorbed unless:

(1) they exceed the transport maximum (Tm)

(2) there is a pathological condition

~ renal threshold

  • the plasma concentration at which a specific substance or ion begins to appear in urine

~ different substances have different Tm and different renal threshold

  • glucose 180 mg/dL

  • amino acids 65 mg/dL

GLUCOSE REABSORPTION

  • freely filtered at glomerulus

  • normally 100% actively reabsorbed in proximal tubule

  • normally, no glucose appears in urine

(3) tubular secretion

  • molecules move from the plasma or peritubular capillaries into the renal tubules to becomes part of the filtrate

  • secretion follows the same basic processes as reabsorption and involves the same barriers, except that movement goes in the reverse direction

  • some substances diffuse from plasma into the filtrate, whereas others are actively transported

~ potassium ions and hydrogen ions

~ waste products, such as choline and creatinine

~ foreign substances such as the antibiotic pencillin

  • the end result of secretion is an increase in the quantity of solute excreted in the urine, which decreases the solute’s plasma concentration

EXCRETION

  • elimination of solute and water from the body in the form of urine

  • the rate at which a substance is excreted in the urine has a direct bearing on the volume and composition of the plasma

  • simple rule of excretion: MATERIAL THAT ENTERS THE LUMEN OF THE RENAL TUBULES IS EXCRETED UNLESS IT IS REABSORBED

  • any substance can enter renal tubules either by: filtration or by secretion

  • the quantity of a substance that is excreted in the urine is given by the expression:

amount excreted = amount filtered + amount secreted - amount reabsorbed

E = F + S - R

SUMMARY

  • reabsorption occurs via a mixture of diffusion and active transport

  • each segment of the renal tubule has a role in reabsorption and secretion