25.4 Microscopic Anatomy of the Kidney

what is the term forming urine

will be used hereafter to describe the filtrate as it is modified into true urine

what is the principle task of the nephron population

is to balance the plasma to homeostatic set points and excrete potential toxins in the urine

what is the three principle functions of the nephrons

filtration, reabsorption, and secretion

what are the secondary functions of nephrons that exert control in three areas:

blood pressure (via prod of renin), rbc prod (via hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, active form of vitamin D)

what is renin

enzyme produced by juxtaglomerular cells in response to decreased blood pressure or sympathetic nervous activity; catalyzes the conversion of angiotensinogen into angiotensin I

what does the renal corpuscle consists of

a tuft of capillaries called the glomerulus that is largely surrounded by Bowman’s (glomerular) capsule

what is the glomerulus

a high-pressure capillary bed between afferent and efferent arterioles

what forms the lumen

the bowman’s capsule surrounding the glomerulus

what is PCT

proximal convoluted tubules

what does the lumen do

captures and directs filtrate to the PCT (proximal convoluted tubules)

what is the outermost part of the bowman’s capsule, the parietal layer, covered in

a simple squamous epithelium

what does the simple squamous epithelium (parietal layer) of the bowman’s capsule transition onto

the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule

what is the characteristic of the visceral layer of bowman’s capsule

cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like arms (pedicels) to cover the glomerular capillaries

what are podocytes

cells forming finger-like processes; form the visceral layer of Bowman’s capsule; pedicels of the podocytes interdigitate to form a filtration membrane

what are pedicels

finger-like projections of podocytes surrounding glomerular capillaries; interdigitate to form a filtration membrane

what are filtration slits

formed by pedicels of podocytes; substances filter between the pedicels based on size

what forms the filtration splits

podocytes and pedicels

what do the filtration slits do

leave small gaps between the digits to form a sieve

What percentage of plasma is filtered as blood passes through the glomerulus

Between 10 to 20 percent of the plasma is filtered

Where does the filtered plasma go after passing through the glomerulus

It is captured by Bowman’s capsule and funneled to the proximal convoluted tubule

What creates the filtration slits in the glomerulus

The filtration slits are formed by the spaces between the interdigitating podocyte projections or pedicels

What separates the lumen of the capillaries and the lumen of Bowman’s capsule

They are separated by a shared basement membrane

What are the three components of the filtration membrane

The fenestrated endothelium of the glomerular capillaries the shared basement membrane and the filtration slits between podocyte projections

What is the function of the filtration membrane

It allows very rapid movement of filtrate from the capillaries into Bowman’s capsule

How small are the pores in the filtration membrane

The pores are about 70 nanometers in diameter

pedicels on one podocyte always

interdigitate with the pedicels of another podocyte

what is fenestrations

small windows through a cell, allowing rapid filtration based on size; formed in such a way as to allow substances to cross through a cell without mixing with cell contents

what does fenestration do in the glomerular caps

prevent filtration of blood cells or large protiens but allow most other constituents through

What size substances can easily cross the filtration membrane

Substances smaller than 4 nanometers cross readily and most substances up to 8 nanometers can pass freely

How does electric charge affect filtration across the membrane

Negatively charged pores repel negatively charged substances and allow positively charged substances to pass more easily

What does the basement membrane block from being filtered

It prevents the filtration of medium to large proteins such as globulins

What is the role of mesangial cells in the filtration membrane

They can contract to help regulate the rate of filtration in the glomerulus

What factors regulate filtration at the glomerulus

Fenestrations in the capillary endothelium podocyte filtration slits the charge of the membrane and the basement membrane between cells

What is the final composition of the filtrate

It does not contain cells or large proteins and has a slight predominance of positively charged substances

what are mesangial cells

contractile cells found in the glomerulus; can contract or relax to regulate filtrate rate

where is the juxtaglomerular apparatus (JGA) located

located at the juncture of the DCT and the afferent and efferent arterioles of the glomerulus

what is the JGA

juxtaglomerular apparatus (JGA

what is the juxtaglomerular apparatus (JGA)

plays a role in the regulation of renal blood flood and GFR (glomerular filtration rate)

what is the DCT

distal convoluted tubule

what is the macula densa

cuboidal cells found in the part of the DCT forming the JGA; sense Na⁺ concentration in the forming urine

what does the macula densa do

This cluster of cuboidal epithelial cells monitors the fluid composition of fluid flowing through the DCT

How do these cuboidal cells respond to sodium ion concentration in the fluid

They release paracrine signals in response to the concentration of sodium ions

What kind of cilium do these cuboidal cells have and what does it detect

They have a single nonmotile cilium that detects the rate of fluid movement in the tubule

What paracrine signals are released by these cuboidal cells

They release adenosine triphosphate ATP and adenosine

what is the juxtaglomerular cell

specialized smooth muscle cells that are part of the juxtaglomerular apparatus

what is the second cell type in the juxtaglomerular apparatus (JGA)

juxtaglomerular cell

what are the cell types that makes up the juxtaglomerular apparatus (JGA)

macula densa and juxtaglomerular cells

where are juxtaglomerular cells located

found in the walls of the afferent arteriole near the glomerulus

what does the juxtaglomerular cells do

play a role in regulating blood flow and blood pressure

how do juxtaglomerular cells respond to chemical signals

They contract or relax in response to ATP or adenosine released by the macula densa cells in the distal convoluted tubule

What is the effect of juxtaglomerular cell contraction on glomerular filtration rate GFR (glomerular filtration rate)

Contraction narrows the afferent arteriole which reduces blood flow to the glomerulus and decreases the glomerular filtration rate

what is GFR

glomerular filtration rate

what is hyperosmotic

solution, fluid, or environment has a greater concentration of dissolved substances (solute) compared to water (solvent)

When do juxtaglomerular cells contract

They contract when the filtrate is hyperosmotic meaning it has a high solute concentration which reduces urine formation and helps retain water in the body

What happens when the filtrate is hypoosmotic or too dilute

The juxtaglomerular cells relax increasing blood flow into the glomerulus raising GFR and allowing more water to be lost in the urine which raises blood osmolarity

How does this regulation help the body - hypoosmotic

It helps keep the rate of filtration and the osmolarity of the blood relatively stable by adjusting urine output based on solute concentration

What additional role do macula densa cells play in this system

They help regulate the release of renin from juxtaglomerular cells based on sodium concentration and fluid flow in the distal convoluted tubule

What is renin and what is its function

Renin is an enzyme made of 304 amino acids that starts the renin angiotensin system by cleaving angiotensinogen to form angiotensin I

What is angiotensinogen

Angiotensinogen is a large protein produced by the liver that circulates in the blood and is the inactive precursor to angiotensin I

What is angiotensin I

Angiotensin I is a short peptide formed when renin acts on angiotensinogen but it is not biologically active on its own

What is angiotensin converting enzyme or ACE

ACE is an enzyme found mainly in the lungs that converts angiotensin I into angiotensin II the active form

What is angiotensin II and what does it do

Angiotensin II is a powerful vasoconstrictor that narrows blood vessels to raise blood pressure it also stimulates the adrenal cortex to release aldosterone

What effect does aldosterone have on the kidneys

Aldosterone causes the kidneys to reabsorb more sodium and water follows the sodium increasing blood volume and blood pressure

What is the overall purpose of the renin angiotensin aldosterone system

To regulate blood pressure fluid volume and electrolyte balance by adjusting kidney function blood vessel tone and hormone levels

what is PCT

proximal convoluted tubule

where does filtered fluid collected by bowman’s capsule travel to

the PCT

what kind of cells forms the tubule and its features

simple cuboidal cells with prominent microvilli on the luminal surface

what is a brush border

fuzzy appearance formed by microvilli on the surface of certain cuboidal cells

where is the brush border found in the kidneys

in the PCT

what does the brush border do in context of the kindeys

increases surface area for absorption

what are the solutes that are absorbed and secreted by the nephrons

Sodium (Na+), chlorine (Cl-), glucose, etc

what do the simple cuboidal cells in the PCT do

These cells actively transport ions across their membranes, so they possess a high concentration of mitochondria in order to produce sufficient ATP

What is the loop of Henle also known as

It is sometimes called the nephron loop

How are the descending and ascending limbs of the loop of Henle arranged

They are continuations of the same tubule and run adjacent and parallel to each other after making a hairpin turn at the bottom of the loop

What is the structure of the descending limb of the loop of Henle

It has a short thick portion made of simple cuboidal epithelium followed by a long thin portion made of simple squamous epithelium

What is the structure of the ascending limb of the loop of Henle

It begins with a short thin portion made of simple squamous epithelium and is followed by a long thick portion made of simple cuboidal epithelium

What type of cells are found in the thick descending portion of the loop of Henle

Simple cuboidal epithelial cells similar to those in the proximal convoluted tubule

What type of cells are found in the thin portions of the loop of Henle

Simple squamous epithelial cells

What type of cells are found in the thick ascending portion of the loop of Henle

Simple cuboidal epithelial cells similar to those in the distal convoluted tubule

Why are these structural differences in the loop of Henle important

Because different parts of the loop have different permeabilities to water and solutes which plays a key role in concentrating urine

what does DCT stand for

distal convoluted tubule

like the PCT, what is the DCT formed by

also simple cuboidal epithelium

what is different about the DCT and PCT in terms of lengths

DCT is shorter than PCT

what is different about the DCT and PCT in terms of cells

DCT cells are not as active as PCT therefore DCT has fewer microvilli on the apical surface

what must the cells in the DCT to

pump ions against their concentration gradient, so you will find of large numbers of mitochondria, although fewer than in the PCT

Are collecting ducts part of the nephron

They are continuous with the nephron but not technically part of it

What does each collecting duct do

Each collecting duct collects filtrate from several nephrons for final modification

What happens to collecting ducts as they descend into the medulla

They merge to form about 30 terminal ducts which empty at a renal papilla

What type of cells line the collecting ducts

They are lined with simple cuboidal or columnar epithelium

What hormone do the epithelial cells of the collecting duct respond to

They respond to antidiuretic hormone or ADH

What happens when collecting duct cells are stimulated by ADH

They insert aquaporin channel proteins into their membranes to allow water to move from the filtrate into the surrounding tissues

What is an aquaporin

a channel protein that allows water to pass through the cell membrane while blocking other molecules due to its very small pore size

How does water move once aquaporins are inserted into the collecting duct cells

Water passes through the aquaporin channels into the interstitial fluid and is then absorbed by the vasa recta

What is the result of aquaporin activity in the presence of ADH

Large amounts of water are recovered from the filtrate and returned to the blood

What happens in the absence of ADH

Aquaporin channels are not inserted so water remains in the filtrate and is excreted as dilute urine

Do other cells in the body contain aquaporins

Yes most if not all body cells contain aquaporin channels

How many types of aquaporins are found in the human body and how many are in the kidney

There are at least 10 types of aquaporins in humans and six of them are found in the kidney

What is the main function of all aquaporins

Their main function is to allow water to move across the hydrophobic cell membrane