Bio 132 - Urinary System Part 2

Nephron

- Consists of renal tubule and renal corpuscle

- > 1 million per kidney

Renal Tubule

-Long tubular passageway

-Begins at renal corpuscle

Renal Corpuscle consists of

-Bowman's (Glomerular) capsule

-cup-shaped chamber

-capillary network (glomerulus)

Glomerulus consists of

50 intertwining fenestrated, highly porous capillaries

Glomerulus blood is delivered via

afferent arteriole

Glomerulus blood leaves in

efferent arteriole:

-flows into peritubular capillaries

-which drain into small venules

-and return blood to venous system

Filtration occurs in the

renal corpuscle

Filtration: blood pressure forces

water and dissolved solutes out of glomerular capillaries into capsular space

Filtration: blood pressure produces

protein-free solution (filtrate) similar to blood plasma

3 Functions of Renal Tubule

1.Reabsorb useful organic nutrients that enter filtrate

2.Reabsorb more than 90% of water in filtrate

3.Secrete waste products that failed to enter renal corpuscle through filtration at glomerulus

Segments of Renal Tubule located in the cortex

-Proximal convoluted tubule (PCT)

-Distal convoluted tubule (DCT)

Segments of Renal Tubule separated by loop of Henle (nephron loop)

-U-shaped tube

-extends at least partially into medulla

3 Processes of the DCT

1.Active secretion of ions, acids, drugs, and toxins

2.Selective reabsorption of sodium and calcium ions from tubular fluid

3.Selective reabsorption of water: concentrates tubular fluid

Traveling along tubule

filtrate (tubular fluid) gradually changes composition

Each Nephron empties into the

collecting system

Collecting System

A series of tubes that carries tubular fluid away from nephron

Collecting ducts

receive fluid from many nephrons

Each collecting duct

-begins in cortex

-descends into medulla

-carries fluid to papillary duct that drains into a minor calyx

2 types of Nephrons

-cortical nephron

-juxtamedullary nephron

Cortical Nephrons

-85% of all nephrons

-Located mostly within superficial cortex of kidney

-Loop of Henle (Nephron loop) is relatively short

In Cortical Nephrons efferent arteriole delivers

blood to a network of peritubular capillaries:

-which surround entire renal tubule

-Low-pressure, porous capillaries adapted for absorption of
water and solutes

Cortical Nephrons perform most of the

reabsorptive and secretory functions of the kidney

Juxtamedullary Nephrons

-15% of nephrons

-Have long loops of Henle that extend deep into medulla

Juxtamedullary Nephrons have capillaries connected to

vasa recta:

-long, straight capillaries parallel with loop of Henle

Juxtamedullary Nephrons function

making concentrated urine

Each renal corpuscle

-is 150-250 µm in diameter

-includes Bowman's capsule and glomerulus

Bowman's Capsules

-Is connected to initial segment of renal tubule

-Forms outer wall of renal corpuscle

-Encapsulates glomerular capillaries

Bowman's (Glomerular) Capsule outer wall is lined by

simple squamous parietal epithelium:

-Continuous with visceral epithelium which covers glomerular capillaries

Bowman's Capsule Parietal & Visceral Epithelial

-Separated by capsular space

-Continuous where glomerular capillaries connect to afferent and efferent arterioles

Visceral Epithelium consists of

large cells (podocytes):

-with complex foot processes (pedicels) that wrap around specialized lamina densa of glomerular capillaries

Filtration Slits

-Are narrow gaps between adjacent pedicels

-Materials passing out of blood at glomerulus:
~ must be small enough to pass between filtration slits

Glomerular Capillaries are

fenestrated capillaries:

-endothelium contains large-diameter pores

Lamina Densa

May encircle more than 1 capillary

Special supporting (Mesangial) cells of Bowman's Capsule

-between adjacent capillaries

-control diameter and rate of capillary blood flow

-Angiotensin II will cause contraction

Filtration Membrane consists of

-fenestrated endothelium

-lamina densa

-filtration slits

Fenestration (pore) of glomerular endothelial cell

prevents filtration of blood cells but allows all components of blood plasma to pass through

Basal lamina of glomerulus

prevents filtration of larger proteins

Slit membrane between pedicels

prevents filtration of medium-sized proteins

Filtration: blood pressure forces

water and small solutes across the membrane into capsular space

Glomerular filtration

larger solutes, including all but the smallest plasma proteins, are excluded

Filtration at Renal Corpuscle

-Is passive

-Solutes enter capsular space

Solutes that enter capsular space

-metabolic wastes and excess ions

-glucose, free fatty acids, amino acids, and vitamins

Reabsorption

process where useful materials are recaptured before filtrate leaves kidneys

Reabsorption occurs in

proximal convoluted tubule

Proximal Convoluted Tubule (PCT)

-Is the first segment of renal tubule

-Entrance to PCT lies opposite the point of connection of afferent and efferent arterioles with glomerulus

Epithelial Lining of PCT

-Is simple cuboidal

-Has microvilli on apical surfaces (brush boarder)

-Functions in reabsorption

-Secretes substances into lumen

Tubular Cells

-Absorb organic nutrients, ions, water, and plasma proteins from tubular fluid

-Release them into peritubular fluid (interstitial fluid around renal tubule)

Loop of Henle is also known as

nephron loop

Nephron Loop: renal tubule turns toward renal medulla leading to

the loop of Henle

Nephron Loop: Each limb contains

-thick segment

-thin segment

Thick Descending Limb

Has functions similar to PCT

Ascending Limbs of juxtamedullary nephrons in medulla

create high solute concentrations in peritubular fluid

Thin Segments

-simple squamous epithelium

-Are freely permeable to water: not to solutes

-Water movement helps concentrate tubular fluid

Thick Ascending Limb

-Cuboidal to columnar cells

-Ends at a sharp angle near the renal corpuscle: where DCT begins

Distal Convoluted Tubule (DCT)

third segment of renal tubule

-Initial portion passes between afferent and efferent arterioles

-Has a smaller diameter than PCT

-Cuboidal cells with very few microvilli