(26-28) Organ Systems: Kidney

L26-L28

One function of the kidney is the production of hormones/enzymes. List the other 4 functions, and then the 3 hormones/enzymes/enzymatic processes the kidneys produce.

1. Regulation of Water, Salts, Acid-Base balance

2. Removal of metabolic waste

3. Removal of foreign chemicals

4. Gluconeogenesis (formation of glucose)

5. Production of hormones / enzymes

• Erythropoietin (controls red blood cell production)

• Renin (controls blood pressure and sodium balance)

• Activates Vitamin D (regulates calcium)

Label the kidney (7 labels)

+

• renal vein

• renal artery

The capillaries surrounding the nephron are called the “peritubular capillaries”, while those surrounding the loop of Henle are specifically called the “vasa recta”.

Label all other parts of nephron (6 labels)

Identify nephron structures in cortex

glomerulus, Bowman’s capsule, proximal & distal convoluted tubules

Identify nephron structures in medulla

loop of Henle & collecting duct

identify 3 main functions/steps to nephron function

glomerular filtration * tubular secretion * tubular reabsorption

Describe the process of glomerular filtration & describe which substances can/(not)  pass through

• Podocytes surround blood vessels within the glomerulus

• Foot-like process create a sieve-like structure

• Allows free passage of small molecules from blood to kidney tubules

Define Glomerular Filtration Rate (GFR)

Glomerular Filtration Rate (GFR) - Volume of fluid filtered from glomeruli to bowman’s space per unit time

What causes the flow of fluid through the glomeruli? How is Glomerular Filtration Rate regulated?

• Fluid filtered through glomeruli by pressure of flow through blood vessels

• GFR regulated by adjusting blood pressure either side of glomerulus

What reduces GFR (by reducing pressure in glomerulus) and what is the effect on water & salt loss/excretion?

• Constricting afferent vessels

• Dilating efferent vessels

• Thus, Lower force pushing substances through

• Thus, water and salt loss reduced through excretion

What increases GFR (by increasing pressure in glomerulus) and what is the effect on water & salt loss/excretion?

• Constricting efferent vessels

• Dilating afferent vessels

• Thus, Greater force pushing substances through

• Thus, water and salt loss increased through excretion

What does it mean that capillaries in the glomerulus are “fenestrated”, and what is the purpose of this?

• "fenestrated" :- a unique type of capillary that has numerous small poresin their walls, which  water and small solute molecules to pass into Bowman's capsule, while blocking larger components (like blood cells)

What is the largest size a substance can be to pass into Bowman’s capsule (in nm), and describe the selectivity of this.

• Allows free passage of small molecules (<8nm) from blood to kidney tubules;

  • Removes based on size, regardless of if it should or should not remove from body

  • Causes useful substances to be lost in filtrate, along with intended toxic metabolites

  • Proteins (& drugs bound to proteins) cannot pass through, and are retained in the blood

which side of the tubule cell is the apical vs. the basolateral

Arrow 1 - apical surface

Arrow 2 - basolateral surface

Describe the two mechanisms by which Na+ is reabsorbed through the apical surface of the tubule cell, in the proximal convoluted tubule. (2 mechanisms)

• Co-transport (X) with other molecules. E.g. Glucose

  • Everything moves the same way

• Counter-transport of Hydrogen ions (Na+ in , H+ out)

  • Substances moving in diff. directions

Differntiate between co-transport & counter-transpprt

co: substances leave cell tg

counter: substances go opposite ways

Describe the mechanism by which Na+ is reabsorbed through the basolateral surface of the tubule cell, and other previously co-trasnported substances, in the proximal convoluted tubule.

• Na+ moved from cells to interstitial space and then blood via active transport

• Na+/K+ ATPase pump, requires ATP

• K+ returns to interstitial fluid via K+ specific ion channels

  • Down concentration gradient

• Co-transported molecules  (X) moved specific transporters (e.g Glucose-specific transporters via glucose transporters)

Describe the mechanisms by which Na+ is reabsorbed through the apical surface of the tubule cell, in the ascending loop of Henle. 

• Na+ moved from tubules into cells through cotransport  with K+ and Cl-

• Na-K-Cl cotransporter (NKCC)

• K+ moves back into tubule via ion channels

  • Down concentration gradient

Describe the mechanisms by which Na+ is reabsorbed through the basolateral surface of the tubule cell, in the ascending loop of Henle. 

• Na+/K+ ATPase moves Na+ out to interstitial space and K+ into cells

• K+ returns to interstitial fluid via K+ specific ion channels

  • Down concentration gradient

• Cl- returns to interstitial fluid via Cl- specific ion channels

Describe the mechanisms by which Na+ is reabsorbed through the apical & basolateral surfaces of the tubule cell, in the collectng duct. 

• Tube – Cell (Apical Surface)

  • Channels specific to Na+ & K+ each individually present

  • Na+ moves into cell and K+ moves out through ion channel

  • Down concentration gradient

 

• Cell – Body (Basolateral Surface):

  • Na+/K+ ATPase moves Na+ out to interstitial space and K+ into cells

What hormone acts on the collecting duct to regulate Na+ reabsorption? Describe the mechanism.

• Aldosterone : steroid hormone

  • Acts on nuclear receptors

• Regulates Na+ reabsorption and blood pressure

• Acts in collecting duct tubule cells

• Increases expression of Na+ & K+ ion channels and Na+/K+ ATPase

  • Since there are more channels/pumps, there is higher reabsorption

  • Since it takes time to synthesize more proteins, etc. (other cellular processes), this process take time

Identify the areas of the nephron involved in: filtration, Na+ reabsorption, Water reabsorption

Describe the mechanisms by which water is reabsorbed, in the descending loop of Henle? (Must hit all points in the answer, extra points for illustrating w/ diagram)

1. Only descending loop permeant (permeable) to water (i.e. water cannot exit ascending tubule)

2. Ascending ↑ loop releases salts, which become reabsorbed by interstitial fluid/blood – creating hypertonic solution (higher salt conc.) in interstitial fluid around loop

3. This hypertonic interstitial fluid causes water to diffuse out of descending loop

• Blood flow around loop of Henle is counter to flow through loop of Henle.

• Freshly filtered blood (low in salt) first encounters ascending loop of Henle (pumping out salt)

• Encourages salt absorption

• This salt-rich blood then circulates around descending loop

• Water from descending loop moves into blood

• Maintains concentration gradients

Describe the mechanisms by which water is reabsorbed, in the collecting duct? (Must hit all points in the answer)

• Collecting duct tubule cells contain lots of water channels (Aquaporins)

• Interstitial space full of salt from Na+ reabsorption

• Allows free flow of water out of tubule, through cell, and into interstitial space

What hormone acts on the collecting duct to regulate water reabsorption? Describe the mechanism.

• Vasopressin: hormone that controls water reabsorption

• Acts on receptors on collecting duct cells

• This causes aquaporin-rich vesicles inside cell to fuse with cell membrane

  • Thus delivering aquaporins to apical cell surface

• Increases number of channels water can use to cross into body

  • Thus more water absorption into blood

Identify the key distinctions between with aldosterone vs. vasopressin?

• In aldosterone, action takes hours as more transport proteins need to be synthesised

• With vasopressin, action takes minutes as aquaporins are already present in vesicles