Chapter 14: Kidneys

functions of kidney

• excretion of wastes

• ! maintenance of ion and fluid balance

• ! hormone production

• homeostatic regulation of pH

• urea

• uric acid

• creatinine

• toxic substances

excretion of wastes includes: (kidney function)

maintenance of ion & fluid balance includes: (kidney function)

• Na+

• K+

• Ca2+

• osmolarity

• water

hormone production includes: (kidney function)

• renin (blood pressure)

• erythropoietin (hemoglobin)

percentage of nephrons that are cortical:

85%

percentage of nephrons that are juxtamedullary

15%

juxtamedullary nephron

• loops descend deep into medulla & establish osmotic gradient to concentrate urine

renal corpuscle includes:

• glomerulus

• bowman’s space in bowman’s capsule

renal tubule includes:

• proximal tubule

• loop of Henle

• distal convoluted tubule

• collecting duct system

components of proximal tubule

• proximal convoluted tubule

• proximal straight tubule

components of loop of Henle

• descending limb

• thin segment of ascending limb

• thick segment of ascending limb

collecting duct system includes

• cortical collecting duct

• medullary collecting duct

Big overview of nephron tubule system

1. renal corpuscle

2. renal tubule

3. distal convoluted tubule

4. collecting duct system

small overview of nephron tubule system

1. glomerulus

2. bowman’s space

3. proximal convoluted tube

4. proximal straight tubule

5. descending limb of loop of Henle

6. thin segment of ascending of loop of Henle

7. thick segment of ascending limb of loop of Henle

8. distal convoluted tube

9. cortical collecting duct

10. medullary collecting duct

11. renal pelvis

• peritubular capillaries

• vasa recta capillaries

what are tubules surrounded by as things move the tubule system

the deeper in the nephron loop and collecting duct

to greater the osmolarity of the medulla

renal corpuscle

= glomerular capillaries + bowman’s capsule

renal corpuscle is:

• blood flows into the glomerulus through the afferent arterioles

• leaves the glomerulus through the efferent arterioles

• proximal tubule exits Bowman’s capsule

podocytes and capillaries

• podocytes of bowman’s capsule surround capillaries

• filtration slits between the podocytes to allow fluid to pass into bowman’s capsule

• glomerulus is composed of capillary endothelium that is fenestrated

• surrounding epithelial cells is a basement membrane

glomerular filtration membrane includes:

• substances in blood are filtered through capillary pores between endothelial cells

• filtrate then passes across basement membrane and through filtration slit between foot processes

• enters the capsular space

• filtrate us transported to the lumen of the proximal convoluted tubule

filtration membrane composed of:

• fenestrations in glomerular capillaries + podocytes

small

what size of molecule passes through the filtration slits between podocytes?

unfiltered molecules include:

large molecules such as

• Proteins

• RBCs

negatively charged molecules

• repelled by negative charges on basement membrane

problems

what do atypical molecules in urine mean:

glomerular filtration rate (GFR) =

net filtration pressure (NFP) x filtration coefficient (Kf)

arterial capillary hydrostatic pressure is

• uncharacteristically high in the glomerular capillary

Bowman’s osmotic pressure

• =

• none flows through filtration membrane

main point of GFR

• proportional to NFP

resistance

what causes the loss of pressure as fluids experience?

change of glomerular capillary pressure

• changing afferent arteriole diameter

• changing efferent arteriole diameter

Na+ drives molecules by

• cotransport

• countertransport

proximal convoluted tubule

where does most reabsorption and secretion occur?

cotransport

• symport

• paired movement in same direction

countertransport

• antiport

• paired with movements in opposite direction

secondary active transport

• paired with cotransport and countertransport

• moves substances against their concentration gradient

vasopressin/antidiuretic hormone

• ADH

• translocates aquaporin proteins from vesicles to apical membrane to increase water reabsorption

juxtaglomerular apparatus includes

• macula densa cells

• juxtaglomerular cells

macula densa cells

• cells at beginning of distal tubule monitor Na+ content

• low fluid volume

• tells JG cells to release renin

what does low Na+ suggest in macula densa cells?

juxtaglomerular cells

• intrarenal baroreceptors (pressure censors)

renin

what does the JG cells release if blood pressure drops?

blood pressure

what do changes in total body water influence?

• go up

the more water retained impacts blood pressure to?

Na+ regulation

• this is indirectly regulated via blood pressure/volume

• lose H2O

what happens when you lose a bunch of Na+

• drops

what happens to your blood pressure when you lose blood volume?

• decreases

  • less fluid pushed through, the less you have to lose

what happens to GFR when you have lower blood pressure? and why?

higher GFR

how does the body produce more filtrate which increases fluid loos via urine?

by altering the GFR

how does the body keep mean arterial pressure (MAP) in an acceptable window?

functions of kidney

1. regulation of water, inorganic ion balance, and acid-base balance

2. removal of metabolic water

3. removal of foreign chemicals

4. gluconeogenesis

5. production of hormones/enzymes

removal of metabolic water includes

• urea from catabolism of protein

• uric acid from nucleic acids

• creatine from muscle creatine

• end products of hemoglobin

• rest from blood and excretion in the urine

removal of foreign chemicals includes

• drugs

• pesticides

• food additives

• metabolites from blood and excretion in the urine

gluconeogenesis

synthesis of glucose from amino acids and other precursors

production of hormones/enzymes includes

• erythropoietin, which controls erythrocyte production

• renin

• dihydroxyvitamin D, influence calcium balance

renin

enzyme that controls the formation of angiotensin and influences blood pressure and sodium balance

3 layers of filtration barrier between glomerulus and bowman’s capsule

1. endothelium

2. basal lamina

3. single celled epithelial lining of bowman’s capsule

endothelium

single celled capillary

basal lamina

non cellular layer of protein-aceous layer of basement membrane

1. Capillary Endothelium

2. Basement Membrane

3. Visceral Epithelium of Bowman’s Capsule

three layers that make up the filtration barrier between the glomerulus and bowman’s capsule.

Visceral Epithelium of Bowman’s Capsule

• The outermost layer made of specialized cells called podocytes.

• These cells have foot-processes (pedicels) that interdigitate to form filtration slits, which serve as the final barrier for macromolecules.

1. Bowman’s Capsule/Glomerular Capsule

2. Proximal Convoluted Tubule

3. Nephron Loop (Loop of Henle)

4. Distal Convoluted Tubule

5. Collecting Duct

segments of the nephron in the order in which a filtered solute encounters them

Collecting Duct

• The final segment (shared by multiple nephrons) where the ultimate concentration of urine is determined under hormonal control.

Distal Convoluted Tubule

• The segment where further selective secretion and reabsorption occur.

Nephron Loop (Loop of Henle)

• Consists of the descending limb and the ascending limb; it is critical for concentrating urine.

Proximal Convoluted Tubule

• The primary site for the reabsorption of water, ions, and organic nutrients (like glucose).

Bowman’s Capsule (Glomerular Capsule)

• The starting point where the initial filtrate is collected.

relationship between a nephron and its associated collecting duct

• nephron is the individual functional unit of the kidney, the collecting duct acts as a shared drainage canal for multiple nephrons

  • connection

  • structural transition

  • hormonal control

connection

• Several individual nephrons (specifically their distal convoluted tubules) drain their processed fluid into a single, common collecting duct.

• nephron/collecting duct

Structural Transition

• The collecting duct is technically the segment where the fluid leaves the individual nephron and begins its final path toward the renal pelvis.

• nephron/collecting duct

Hormonal Control

• While the nephron does the bulk of the "heavy lifting" for reabsorption, the collecting duct is the primary site where Vasopressin (ADH) and Aldosterone make the final, fine-tuning adjustments to water and sodium balance.

• nephron/collecting duct

nephrons

• are the individual "workers" producing the filtrate

collecting duct

• is the "manager" that determines the final concentration of the urine before it exits the kidney

Cortical Nephrons

• Abundance: Make up about 85% of all nephrons.

• Location: Their renal corpuscles are located in the outer cortex.

• Loop of Henle: They have short loops that only dip slightly into the outer medulla.

• Blood Supply: Their loops are surrounded by peritubular capillaries, which primarily handle reabsorption and secretion.

Juxtamedullary Nephrons

• Abundance: Make up the remaining 15%.

• Location: Their renal corpuscles sit at the border (the junction) of the cortex and medulla.

• Loop of Henle: They have long loops that plunge deep into the inner medulla.

• Blood Supply: They are associated with specialized long, looping capillaries called the vasa recta.

• Primary Function: These are the "heavy hitters" for creating the osmotic gradient in the medulla, which is what allows you to produce concentrated urine

nephron

• the functional unit of the kidney

• is uniquely associated with two capillary beds in series, connected by an arteriole. This "portal-like" arrangement allows for the high-pressure filtration and low-pressure reabsorption necessary for renal function

Afferent Arteriole

• Description: A short, high-resistance vessel that branches off the interlobular artery.

• Function: It delivers blood directly into the glomerulus. Its diameter can be adjusted (vasoconstriction or vasodilation) to regulate the Glomerular Filtration Rate (GFR).

Glomerulus (Glomerular Capillaries)

• Description: A compact, tuft-like network of capillaries fed by the afferent arteriole and drained by the efferent arteriole.

• Function: This is the filtration site. Unlike most capillary beds, it is a high-pressure system that forces fluid and low-molecular-weight solutes out of the blood and into Bowman’s capsule.

Efferent Arteriole

• Description: The vessel that carries blood away from the glomerulus.

• Function: It provides a second point of resistance. By constricting, it can "back up" blood in the glomerulus to increase filtration pressure. It then leads into the second capillary network (peritubular or vasa recta).

Peritubular Capillaries

• Description: A low-pressure capillary network that closely surrounds the convoluted tubules (proximal and distal) in the renal cortex.

• Function: These are specialized for reabsorption and secretion. They take up the water and solutes reabsorbed by the tubule cells and return them to the general circulation.

Vasa Recta

• Description: Long, "straight" loop-shaped capillaries that extend deep into the renal medulla, specifically associated with the juxtamedullary nephrons.

• Function: They parallel the Loops of Henle. Their unique hairpin shape allows them to provide nutrients to the medulla without washing out the high osmotic gradient necessary for concentrating urine (a process known as countercurrent exchange).

1. Glomerular Filtration

2. Tubular Reabsorption

3. Tubular Secretion

formation of urine and the regulation of blood composition are the results of three distinct processes occurring within the nephron, which are…?

Tubular Secretion

• Definition: The movement of substances from the peritubular capillaries into the tubular lumen.

• Process: This is an "additive" process that allows the kidney to remove substances from the blood that were not filtered at the glomerulus. It often involves active transport of hydrogen ions (H+), potassium (K+), and foreign chemicals (like drugs).

• Result: This provides a second chance for the body to eliminate waste products and finely tune acid-base or electrolyte balance.

Tubular Reabsorption

• Definition: The movement of substances from the tubular lumen across the epithelium and into the peritubular capillaries.

• Process: This is how the body recovers "useful" substances (like 99% of filtered water, nearly all glucose, and most sodium) that were filtered at the glomerulus. It can occur via active transport or passive diffusion.

• Result: Essential nutrients and water are returned to the general circulation.

Glomerular Filtration

• Definition: The bulk flow of essentially protein-free plasma from the glomerular capillaries into Bowman’s capsule.

• Process: High blood pressure in the glomerulus forces water and low-molecular-weight solutes (like glucose, amino acids, and ions) through a three-layer filtration barrier.

• Result: The formation of filtrate, which contains everything found in plasma except for blood cells and large proteins.

Amount Excreted = Amount Filtered + Amount Secreted - Amount Reabsorbed

Urinary Excretion Formula

1. Filtration: The Starting Point

2. Reabsorption: Reclaiming the Essentials

3. Secretion: Fine-Tuning and Waste Removal

The integration of these three processes allows the kidneys to precisely "choose" what stays in the body and what leaves, which are…?

Filtration

• The Starting Point

• The process begins with glomerular filtration, a non-selective bulk flow of plasma (minus proteins) into the tubule.

• This creates a "rough draft" of urine that contains both waste products and essential nutrients like glucose, water, and ions.

Reabsorption

• Reclaiming the Essentials

• As the filtrate moves through the tubule, tubular reabsorption pulls the high-value substances (glucose, amino acids, most water, and sodium) back into the blood.

  • volume control

  • composition control

Volume Control

• By adjusting the amount of water and sodium reabsorbed (regulated by hormones like ADH and Aldosterone), the kidney determines the final volume of urine.

• Reabsorption: Reclaiming the Essentials

Composition Control

• Essential nutrients are almost 100% reabsorbed, so they are normally absent from the final urine.

• Reabsorption: Reclaiming the Essentials

Secretion

• Fine-Tuning and Waste Removal

• Tubular secretion

  • ion balance

  • toxin removal

Tubular secretion

• provides a second chance to add substances to the urine that were not filtered or need to be removed in higher quantities.

  • Ion Balance

  • Toxin Removal

ion balance

• Secretion of Potassium (K+) and Hydrogen ions (H+) is the primary way the body regulates blood pH and electrolyte balance.

• Secretion: Fine-Tuning and Waste Removal

Toxin Removal

• Many drugs and metabolic waste products are actively secreted into the tubule to ensure rapid clearance from the blood.

• Secretion: Fine-Tuning and Waste Removal

final urine

• is a highly concentrated or dilute liquid containing only the substances the body does not need at that moment.

  • For example, if you are dehydrated, the kidney will increase water reabsorption, resulting in a low volume of highly concentrated urine.

• Integrated Result

Glomerular Filtration Rate (GFR)

• is defined as the total volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit of time (typically measured in milliliters per minute, mL/min).

• determined by the net filtration pressure, which is the sum of three distinct Starling forces. These forces either push fluid out of the blood or pull it back in.

• starling forces of the glomerulus

  • glomerular capillary blood pressure (Pgc)

  • fluid pressure in bowman's space (Pbs)

  • osmotic force due to plasma proteins (pi,gc)

osmotic force due to plasma proteins (pi,gc)

• description = "pull" created by proteins (like albumin) that remain in the blood because they are too large to be filtered

• Direction = into the capillary

• effect on filtration = opposes filtration

fluid pressure in bowman's space (Pbs)

• Description = hydrostatic pressure exerted by the fluid already present in the tubule

• Direction = into the capillary

• Effect on filtration = opposes filtration

glomerular capillary blood pressure (Pgc)

• Description = blood pressure within the glomerular capillaries, this is the dominant force

• Direction = out of the capillary

• Effect on Filtration = favors filtration