Renal 3

Question 1

Describe ultrafiltration at the glomerulus, tubular reabsorption, and tubular secretion.

Answer 1

• Ultrafiltration: hydrostatic pressure higher than in other capillaries and constant thru entire length of glomerular capillaries. Filtration between afferent and efferent arterioles. Must go from Capillary lumen  endothelial cell layer  basement membrane  Podocyte/epithelial cell layer in Bowman’s capsule
• Tubular reabsorption: Can alter plasma volume/composition by slight changes.
• Tubular secretion: active transport into renal tubular lumen

Question 2

Explain why large, negatively charged molecules are less permeable to the glomerular capillary than smaller or neutral molecules.

Answer 2

The negatively charged molecules are repelled by the negatively charged endothelial cells and filtration slits.

Question 3

List the Starling forces and tell whether each would promote filtration or reabsorption at a glomerular capillary.

Answer 3

• Forces favoring filtration: Capillary hydrostatic pressure and oncotic pressure in Bowman’s Capsule (latter usually 0)
• Forces favoring reabsorption: Capillary oncotic pressure (which rises by efferent arteriole as ultrafiltrate leaves) and hydrostatic pressure of Bowman’s capsule

Question 4

Define term filtration fraction. How do changes in the filtration fraction affect the oncotic pressure at the efferent end of the glomerular capillaries?

Answer 4

• The percent of renal plasma flow that is filtered at the glomerulus. If the FF is increased then the oncotic pressure at efferent end of glomerulus is also increased and vice-versa.

FF=GFR/RPF

Question 5

Describe the different transport mechanisms that are used for reabsorption and secretion.

Answer 5

• Simple diffusion: passive and does not need energy. Uncharged solutes, charged solutes, small lipid soluble molecules. System not saturatble and no comptetitive/non-competitive inhibition. Water, Cl, and urea use this.
• Facilitated diffusion: carrier-mediated transport down electrochemical gradient. Carrier can be specific of transport a group of compounds. When it is a group there can be competition. This has a maximum transport rate.
• Primary Active transport: Uses a protein carrier but against the electrochemical gradient. Needs energy from ATP. Ex: Na-K ATPase pump
• Secondary Active transport: Carrier protein interacts with 2 or more solutes. One solute crosses membrane down its electrochemical gradient while other is against. Can be symport or antiport. Can be saturated and has transport maximum.

Question 6

Describe how sodium, glucose, and water are reabsorbed. Be able to trace path from tubule lumen to capillary describing transport process used.

Answer 6

• Na: 60-67% reabsorbed at the proximal tubule.
• (apical) into the cell from lumen of nephron: passive across gradient, often used as 2dary active transport (Na-H antiport, Na-gluclose/Na-AA cotransport). NaCl is also transported trancellularly and paracellularly
• (basolateral)from cell to interstitial fluid: primary active transport using Na-K ATPase pump
  
  • Glucose: 100% is reabsorbed b’c almost none is excreted
    
    • Apical: 2ndary active transport with Na.
    • Basolateral: Facilitated diffusion down concentration gradient
• Water: The reabsorption of solute creates a transtubular osmotic gradient that results in isosmotic reabsorption in proximal tubule.

Question 7

Describe how proteins and peptides are reabsorbed.

Answer 7

Transcytosis: endocytosis at one side of cell and exocytosis at the other end
- Proteins are reabsorbed at the proximal tubule and enzymes on the luminal surface partially degrade proteins. Then into AA’s. Carrier proteins then take the AA’s thru the basolateral membrane.

Question 8

Define “transport maximum” (other names tubular maximum, or maximum tubular transport capacity).

Answer 8

The point at which increases in concentration do not result in an increase in movement of a substance across a membrane.

Question 9

Explain why the threshold concentration for glucose is less than the transport maximum for glucose.

Answer 9

• We will hit a threshold because not all nephrons have the same transport maximum and not all glucose is always removed from each tubule.

Question 10

Explain why a person with a very high blood level of glucose will have a high urine volume (high urine excretion rate). What is osmotic diuresis?

Answer 10

high blood level of glucose= more glucose excreted (past transport max).
more glucose in urine which holds water and sodium in the urine)