Chapter 26: The Urinary System Part 2 Flashcards
3 processes are involved in urine formation and adjustment of blood composition:
- Glomerular Filtration
- Tubular Reabsorption
- Tubular Secretion
Glomerular Filtration
is a passive and nonselective process (no metabolic energy required)
• Hydrostatic pressure forces fluids and solutes through filtration membrane into glomerular capsule
• No reabsorption into capillaries of glomerulus occur
Tubular Reabsorption
selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts
Tubular Secretion
- reabsorption in reverse
- primarily occurs in the distal convoluted tubule when active transport moves substances like creatine and penicillin, from the blood into this tubule.
- disposes of unwanted solutes, eliminates solutes that were reabsorbed, rids the body of excess K+, and controls blood pH.
- Tubular secretion is most active in the proximal convoluted tubule, but occurs in the collecting ducts and distal convoluted tubules, as well.
Glomerular Blood Hydrostatic Pressure (GBHP) or Hydrostatic Pressure in Glomerular Capillaries (HPgc)
- It is the pressure of blood in the glomerular capillaries
- Chief force pushing water & solutes out of blood
- Glomerular Blood Pressure: 55 mmHg
Capsular Hydrostatic Pressure (CHP) or Hydrostatic pressure in capsular space (HPcs)
- Opposing force to GBHP by fluid already in the capsular space that tends to push water and solutes out of the filtrate and back into plasma
- results from the resistance to flow along the nephron and conducting system
- Filtrate Pressure in Capsule: 15 mm Hg
Blood Colloid Osmotic Pressure (BCOP) or Colloid osmotic pressure in capillaries (OPgc)
- pressure exerted by the proteins in the plasma which tends to retain fluid and also oppose filtration
- pulls fluid into capillaries from interstitial spaces
- “Pull” of Proteins in Blood: 30 mm Hg
Explain what is meant from NFP
Net Filtration Pressure (NFP): sum of forces
• Pressure responsible for filtrate formation
• Main controllable factor determining Glomerular Filtration Rate (GFR)
How is Net Filtration Pressure calculated?
NFP = GBHP (outward pressures) - (CHP + BCOP) (inward pressures)
= 55 - (15 + 30) -> NFP = 10 mm Hg
What forces determine filtration pressure?
- Glomerular (blood) hydrostatic pressure GHP or GBHP
- Capsular Hydrostatic Pressure (CHP)
- (Blood) Colloid Osmotic Pressure (BCOP)
Glomerular Filtration Rate (GFR) is influenced by
- Net filtration pressure
- Total surface area for filtration
- filtration membrane permeability
What is the relationship between glomerular filtration rate (GFR) and NFP?
- Primary factor that influences GFR (primary pressure is glomerular blood hydrostatic pressure (GBHP)
- Increase NFP = Increase GFR (directly proportional)
How does total surface area for filtration influence the rate of glomerular filtration rate?
Glomerular mesangial cells control by contracting
How does filtration membrane permeability influence glomerular filtration rate?
Much more permeable than other capillaries
How is GFR regulated?
- intrinsic controls (renal autoregulation)
2. extrinsic controls
Why is it important to maintain a constant GFR?
Constant GFR is important as it allows kidneys to make filtrate and maintain extracellular homeostasis
Increased GFR causes
increased urine output, which lowers blood pressure, and vice versa
Intrinsic Controls: Renal Autoregulation of GFR
-Main goal is to maintain GFR in the kidney
-Maintains nearly constant GFR when MAP is in range of 80–180 mm Hg
• Autoregulation stops if out of that range
2 Types of Renal Autoregulation:
- Myogenic Mechanism
2. Tubuloglomerular Feedback Mechanism
Myogenic Mechanism
Local smooth muscle (walls of afferent arteriole) contracts when stretched
– Increased MAP or Decreased MAP
Both help maintain normal GFR despite normal fluctuations in systemic blood pressure (MAP)
What happens when MAP is increased during the myogenic mechanism?
constriction of afferent arterioles:
- Reduces blood flow into glomerulus -> decreases GBHP
- Protects glomeruli from damaging high BP
What happens when MAP is decreased during the myogenic mechanism?
dilation of afferent arterioles:
-Increases blood flow into glomerulus -> increases GBHP
Tubuloglomerular Feedback Mechanism
Flow-dependent mechanism directed by macula densa cells of JGA
- Respond to: - filtrate’s NaCl concentration (filtrate’s osmolarity) &/or - flow of filtrate in renal tubules
Extrinsic Controls
– Purpose of extrinsic controls is to regulate GFR to maintain systemic blood pressure
– Extrinsic controls will override renal intrinsic controls if blood volume needs to be increase