UA unit 1

Urinary system consists of

2 kidneys

o 2 ureters

o Bladder

o Urethra

Blood Order

Blood enters each kidney through renal artery → leaves through renal vein

Urine Order

Urine forms within kidney  drains into renal pelvis  funneled into ureter 

travels to bladder for temporary storage  eliminated through urethra

Kidney core functions

Removes what the body doesn't want

• Reabsorbs what the body does want

• Synthesizes

Synthesis can be affected by

Hormones: kidneys are involved with aldosterone, antidiuretic hormone (ADH), renin, erythropoietin, & vitamin D

o Renin (produced by kidneys) initiates the renin-angiotensin-aldosterone

system (RAAS) cascade

o Aldosterone and ADH act on distal nephron to assist with sodium & water

reabsorption

o Erythropoietin stimulates RBC production in bone marrow

o Kidneys perform final activation step of vitamin D, which body needs for

calcium regulation

Renal cortex

outer layer of kidney, houses blood-filtering components of

nephron, including the glomeruli and convoluted tubules

o Plays central role in filtering blood, removing waste, & balancing body fluids

antidiuretic hormone (ADH)

Antidiuretic hormone (ADH), also called vasopressin, is a chemical made by the brain that helps your body control the amount of water in your blood and urine

Renal medulla

inner part of the kidney, made of cone-shaped renal pyramids

o Contains tubules that control salt & water levels of urine

Vit D

Renin

(produced by kidneys) initiates the renin-angiotensin-aldosterone

system (RAAS) cascade

• Renin Release: The kidneys release an enzyme called renin into your blood.

• Angiotensin I: Renin meets a protein made by your liver (angiotensinogen) and turns it into Angiotensin I (which is inactive).

• Angiotensin II: An enzyme in your lungs called ACE (angiotensin-converting enzyme) changes Angiotensin I into Angiotensin II, which is active and powerful.

Renal pelvis

large, funnel-shaped space at center of kidney that acts as a

collecting basin for urine

o Gathers fluid from smaller, cup-like calyces & channels it into ureter to exit

kidney

Renal interstitium

fluid-filled space and tissue matrix located between the tubular

and vascular structures of the medulla; helps maintain proper osmotic environment

Nephron

Functional unit of the kidney that forms urine

• Each kidney contains approximately 1 to 1.5 million nephrons

• 2 types of nephrons:

o Cortical nephrons  make up ~85% of all nephrons & are located

primarily in renal cortex; mainly responsible for waste removal & nutrient

reabsorption

o Juxtamedullary nephrons  have longer loops of Henle that extend

deep into medulla; primary function is concentrating urine

Urine Formation

3 processes: glomerular filtration, tubular reabsorption, & tubular secretion

Urinary Filtrate Flow

1. Bowman capsule (contains glomerulus)

2. Proximal convoluted tubule (PCT)

3. Descending loop of Henle (D-Henle)

4. Ascending loop of Henle (A-Henle)

5. Distal convoluted tubule (DCT)

6. Collecting duct

7. Renal calyces

8. Ureter

9. Bladder

10. Urethra

Renal Blood Flow

1.Renal artery

2. Afferent arteriole (AA)

3. Glomerulus

4. Efferent arteriole (EA)

5. Peritubular capillaries

6. Vasa recta

7. Renal vein

Renal artery supplies blood

(~25% from heart) to the kidney

o Total renal blood flow = ~1200 mL/min

Total renal plasma flow

= ~600 to 700 mL/min

Efferent arteriole

is smaller than afferent arteriole

Difference in size creates hydrostatic pressure differential needed for glomerular filtration

Peritubular capillaries surround PCT & DCT

 immediate reabsorption of essential substances & final adjustment of urinary composition

Vasa recta

run alongside the loops of Henle in juxtamedullary nephrons

Major exchange of water & salts between blood and the medullary

interstitium takes place  maintains osmotic gradient that the medulla

needs for urine concentration

Glomerulus

Coil of ~8 capillary lobes inside Bowman capsule  acts as a nonselective

filter for plasma substances with a molecular weight <70,000

2 mechanisms: size-based filtration & “shield of negativity”

Plasma filtrate must cross 3 cellular layers to become filtrate:

Capillary wall (fenestrated endothelium): contain pores (fenestrations) that

increase permeability but block the passage of large molecules & blood cells

• Basement membrane (basal lamina): provides further restriction of large

molecules

• Visceral epithelium of Bowman capsule (podocytes): foot processes of cells

form thin filtration slits that enhance restriction

Hydrostatic pressure

Drives filtration across glomerular barrier by opposing fluid

pressure (Bowman capsule) & oncotic pressure of unfiltered plasma proteins

(glomerular capillaries)

Juxtaglomerular apparatus

Keeps glomerular blood pressure relatively constant by adjusting size of afferent & efferent arterioles:

• Falling blood pressure: AA dilates & EA constricts  prevents drop in renal blood

flow & buildup of toxic waste products

• Rising blood pressure: AA constricts  prevents overfiltration or damage to

glomerulus

Active transport

Glucose, amino acids, salts = PCT

Chloride= A-Henle

Sodium = PCT & DCT

Passive transport

Water= PCT, D-Henle, collecting duct

Urea= PCT & A-Henle

Sodium= A-Henle

Renal threshold

Amount of a substance measured in plasma that can be present before substance is excreted in urine

o Affects active transport

o Active transport has a maximal reabsorptive capacity called the tubular

maximum (Tm) — tubules can only reabsorb so much of a substance per unit time

▪ When the plasma concentration of a normally-reabsorbed substance becomes abnormally high, the filtrate concentration exceeds Tm & substance begins appearing in urine; plasma concentration at which this occurs is called the renal threshold

▪ ↑ plasma conc. of substance  threshold exceeded  ↑ urine conc.

of substance

Countercurrent mechanism

selective process where water is reabsorbed in D-Henle & salt is actively reabsorbed in the A-Henle to maintain medullary/osmotic gradient

o High osmotic gradient located in renal medulla (where loops of Henle

reside)

o Selective  A-Henle is impermeable to water (water cannot be reabsorbed here; only Na+ & Cl- are reabsorbed)

▪ Prevents dilution of medullary interstitium

Aldesterone

Hormone that regulates Na+ reabsorption in DCT

o Controlled by body’s Na+ concentration

o Produced in adrenal cortex

ADH

hormone that regulates H2O reabsorption in late DCT & collecting duct

o Controlled by body’s H2O concentration

o Produced in hypothalamus, released by pituitary gland

o Determines whether walls of DCT & collecting duct are permeable to H2O

o ↑ Body Hydration = ↓ ADH = ↑ Urine Volume

o ↓ Body Hydration = ↑ ADH = ↓ Urine Volume

Tubular secretion

reverse of reabsorption

o Moves substances from blood in peritubular capillaries into tubular filtrate

o 2 major functions:

▪ Eliminating waste products that were not filtered by glomerulus

• Ex. Medications that are bound to plasma proteins develop strong affinity for tubular cells & dissociate from carrier proteins  transported into filtrate (major site =

PCT)

Regulating body's acid-base balance through secretion of hydrogen

ions (H+)

Acid-base balance

7.4 blood ph

All 3 processes  occur simultaneously at rates determined by body's acid-base balance

• Disruption of secretory functions can result in metabolic acidosis or renal tubular

acidosis

Bicarbonate

(HCO3-) reabsorption: HCO3- is filtered by glomerulus & reabsorbed into blood (mostly in PCT) to eliminate excess acid

o Secretion of H+ by renal tubular cells into filtrate prevents filtered HCO3-

from being excreted & instead returns HCO3- to plasma

o Achieves ~100% reabsorption of filtered HCO3-

Because hydrogen ions are small, they are readily filtered & reabsorbed — so the actual excretion of excess H+ ions depends on tubular secretion, through 2 primary

mechanisms:Ammonium formation; phosphate buffering:

Phosphate buffering:

a secreted H+ ion combines with a filtered phosphate ion

(rather than a HCO3- ion) & is excreted rather than reabsorbed

Ammonium formation:

o

PCT  ammonia is produced from breakdown of the amino acid glutamine;

ammonia reacts with H+ to form ammonium ion (NH4+), which is excreted

o DCT & collecting duct  can also produce NH4+ ions if additional H+ ion

elimination is needed