Chapter 23: Urine Formation 2: Tubular Reabsorption and Secretion

proximal convoluted tubule

reabsorbs most of glomerular filtrate and removes substances from blood and secretes them into tubular fluid for disposal in urine

structure of PCT

long, microvilli, abundant mitochondria

tubular reabsorption

process of reclaiming water and solutes from the tubular fluid and returning them to the blood

two routes of reabsorption

transcellular route and paracellular route

transcellular route

substances pass through the cytoplasm of the PCT epithelial cells and out their base

paracellular route

substances pass through gaps between the PCT epithelial cells

solvent drag

as water passes, it carries a variety of dissolved substances

peritubular capillaries

takes up reabsorbed fluid

two ways reabsorption occurs

osmosis and solvent drag

three factors promote osmosis and solvent drag

high interstitual fluid pressure, low blood hydrostatic pressure in peritubular capillaries, high colloid osmotic pressure

high interstitual pressure

due to accumulation of reabsorbed fluid in extracellular space

low blood hydrostatic pressure in peritubular capillaries

due to narrowness of efferent arterioles

high colloid osmotic pressure

due to prescence of proteins that were not filtered

transport maximum

maximum rate of reabsorption for a solute, which is reached when all transport proteins are saturated

if all transporters are occupied...

any excess solute passes by and appears in urine

each solute...

has its own transport maximum

tubular secretion

process in which renal tubule extracts chemicals from capillary blood and secretes them into tubular fluid

purpose of secretion in PCT and nephron loop

acid-base balance, waste removal, clearance of drugs and contaminants

acid-base balance

secretion of hydrogen and bicarbonate ions to regulate pH of body fluids

waste removal

urea, uric acid, bile acids, ammonia and creatinine are secreted into the tubule

clearance of drugs and contaminants

morphine, penicillin, aspirin

nephron loop function

generate osmotic gradient that enables collecting duct to concentrate the urine and conserve water

fluid arriving in the DCT

very dilute, water and salts

two kinds of cells in the DCT and collecting duct

principal cells and intercalated cells

principal cells

most abundant, have receptors for hormones and involved in salt and water balance

intercalated cells

involved in acid-base balance by secreting into tubule lumen and reabsorbing

aldosterone

hormone that stimulates reabsorption of sodium and secretion of potassium, "salt-retaining hormone"

triggers for aldosterone

blood concentration falling or rising, drop in BP triggers renin

what does aldosterone act on?

thick segment of ascending nephron loop, DCT and collecting duct

natriuretic peptides

secreted by heart in response to high BP, excretes salt and water in urine to reduce BP

roles of natriuretic peptides

dilates afferent arteriole, constricts efferent arteriole, inhibits renin and aldosterone, ADH and NaCl reabsorption

antidiuretic hormone (ADH)

stimulates water retention by the kidney

triggers of ADH

dehydration, loss of blood volume

roles of ADH

makes the collecting duct more permeable to water so that water stays in the body rather than being lost in the urine

parathyroid hormone (PTH)

secreted due to calcium deficiency

roles of PTH on PCT

increase phosphate excretion

roles of PTH on DCT

increase calcium reabsorption

increased phosphate content

lowers calcium in urine and calcium stays in the circulation

obligatory water reabsorption

in PCT when water is reabsorbed independent of hormones and at a constant rate