Chapter 25

Function of kidneys

Function of kidneys

• regulate total water volume

• regulate ion concentration

• excrete metabolic wastes, toxins, and drugs

• ensure long term acid base balance

• regulates BP

Ureters

transport urine from kidneys to urinary bladder

urinary bladder

temporary storage reservoir for urine

urethra

transports urine out of the body

renal cortex

granular appearing superficial region

Renal medulla

deep to cortex, composed of cone-shaped renal pyramids

Papilla

tip of pyramids, points internally

renal columns

inward extensions of cortical tissue

Lobe

medullary pyramid and its surrounding cortical tissues; 8 per kidney

Renal pelvis

funnel shaped tube continuous with ureter

two parts of the renal pelvis

• minor calyces

• major calyces

Minor calyces

cup shaped areas that collect urine draining from pyramidal papillae

major calyces

collects urine from minor calyces and empties it into renal pelvis

Urine flow

• renal pyramid

• minor calyx

• major calyx

• renal pelvis

• ureter

Nephrons

structural and functional units that form urine

Two parts of nephrons

• renal corpuscle

• renal tubule

What is in filtrate?

• water

• small proteins

• glucose

• wastes

How is GFR maintained at 125mL/min

• nervous system

• hormones

• intrinsic regulation

Myogenic mechanism

increase pressure to afferent arterioles

Bowmans capsule

captures filtrate

How much water is reabsorbed?

65%

Which is thicker? the afferent arteriole or the efferent arteriole

afferent arteriole

Renal arteries

deliver about 1/4 of cardiac output to kidneys each minute

Arterial flow

• renal

• segmental

• interlobar

• arcuate

• cortical radiate

Venus flow

• cortical radiate

• arcuate

• interlobar

• renal veins

Glomerulus

• tuft of capillaries composed of fenestrated endothelium

• capillaries are specialized for filtration

• different from other capillary beds because they are fed and drained by arteriole

Renal tubule

Consists of single layer epithelial cells, but each region has its own unique histology and function

Major parts of the renal tubule

• proximal convoluted tubule

• nephron loop

• distal convoluted tubule

Proximal convoluted tubule

• cuboidal cells with dense microvilli that form brush border

• functions in reabsorption and secretion

• confined to cortex

Nephron loop

• formerly called loop of Henle

• U shaped structure consisting of two limbs

Distal convoluted tubule

• cuboidal cells with very few microvilli

• function more in secretion than reabsorption

• confined to cortex

2 cell types in collecting ducts

• principal cells

• intercalated cells

Principal cells

• sparse with short microvilli

• maintain water and sodium balance

Intercalated cells

• cuboidal cells with abundant microvili

Cortical nephrons

• makes up 85% of nephrons

• almost entirely in cortex

Peritubular Capillaries

• low pressure, porous capillaries adapted for absorption of water and solutes

• arise from efferent arterioles

• cling to adjacent renal tubules in cortex

• empty into venules

Vasa recta

• long thin walled vessels parallel to long nephron loops of juxtamedullary nephrons

• arise from efferent arterioles serving juxtamedullary nephrons

• function in formation of concentrated urine

Macula densa

• tall closely packed cells of ascending limb

• contain chemoreceptors that sense sodium chloride content of filtrate

Granular cells (JG cells)

• enlarged smooth muscle cells of arteriole

• act as mechanoreceptors to sense blood pressure in afferent arteriole

• contain secretory granules that contain enzyme renin

Extraglomerular mesangial cells

• located between arteriole and tubule cells

• interconnected with gap junctions

• may pass signals between macula densa and granular cells

Glomerular filtration

produces cell and protein free filtrate

tubular reabsorption

selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts

Tubular secretion

• selectively moves substances from blood to filtrate in renal tubules and collecting ducts

• occurs completely in PCT

Intrinsic controls

• maintain nearly constant GFR when MAP is in range of 80-180mmHg

Two types of renal autoregulation

• myogenic mechanism

• tubuloglomerular feedback mechanism

Passive tubular reabsorption of solutes

• solute concentration in filtrate increases as water is reabsorbed

• sat soluble substances, some ions, and urea will follow water into peritubular capillaries down their concentration gradients

Descending limb

water can leave, solutes cannot

Ascending limb

water cannot leave, solutes can

ADH hormone in the renal system

• released by posterior pituitary gland

• causes principal cells of collecting ducts to insert aquaporins in apical membranes, increasing water reabsorption

Aldosterone

• targets collecting ducts and distal DCT

• promotes synthesis of apical sodium and potassium channels and basolateral sodium, potassium and ATP for sodium reabsorption

• little sodium leaves body

Atrial natriuretic peptide

• reduces blood sodium resulting in decreased blood volume and blood pressure

• released by cardiac atrial cells if blood volume or pressure elevated

Parathyroid hormone

acts on DCT to increase calcium reabsorption

Importance of tubular secretion

• disposing of substances such as drugs that are bound to plasma proteins

• eliminating undesirable substances that were passively reabsorbed

• ridding body of excess potassium

Countercurrent multiplier

Interaction of filtrate flow in ascending and descending limbs of nephron loops of juxtamedullary nephrons

Countercurrent exchanger

Blood flow in ascending/ descending limbs of vasa recta

Trigone

• smooth triangular area outlined by openings for ureters and urethra

• infections tend to persist in this region