Urinary System Physiology Flashcards
What do kidneys regulate?
- Plasma ionic composition (keep ions that we need, pee ions that we don’t need).
- Plasma volume and pressure (role in blood pressure).
- Plasma osmolarity (keep overall concentration of solute constant) and pH (concentration of hydrogen ions).
- Removal of metabolic wastes.
- Number of red blood cells.
- Vitamin D production.
Each function is vital! - Concentration of calcium.
Sections of the Kidneys
Renal cortex: outer regions.
Medulla: inner regions.
Pyramids: conical sections of the medulla separated by renal columns.
Order in Which Urine is Collected
Papillae → minor calyces → major calyces → ureter
The Nephron
Functional unit of kidneys.
Filter the blood and form urine.
Composed of renal corpuscle and renal tubule.
Renal Corpuscle
- Glomerulus: high pressure capillary bed
2. Bowman’s capsule: proximal end that surrounds glomerulus.
Renal Tubule Composition
- Proximal tubule
- Loop of Henle
- Distal convoluted tubule
- Collecting duct
Juxtaglomerular Apparatus
Helps regulating blood pressure.
- Macula densa: cells of distal tubule.
- Granular cells (juxtaglomerular cells): secretion of renin (regulation of blood pressure).
Which parts of the nephron are in the cortex?
Renal corpuscles, proximal and distal convoluted tubule.
Which parts of the nephron extend into the medulla?
Loop of Henle (has descending and ascending limb). Collecting ducts (bring the forming urine to papillae).
Vascular System of the Nephron
Efferent arteriole form a 2nd capillary bed around renal tubule, supply blood to glomerulus.
Regulation of Composition of Plasma
Exchange of solutes and fluids between plasma and filtrate. Achieved through 3 functions: 1. Filtration 2. Reabsorption 3. Secretion
Bowman’s Capsule
Captures and directs filtrate to proximal tubule.
Parietal layer: simple squamous epithelium.
Visceral layer: cells cover the glomerular capillaries. Form a sieve.
How much of plasma enters Bowman’s capsule?
10-20%
Filtration Based on Size
Glomerular capillaries are fenestrated (extra spaces that favor movement of fluid). Blood cells and large proteins cannot pass.
Glomerular Filtration
Creation of a filtrate with a composition very similar to plasma (no protein).
What is Glomerular Filtration Rate (GFR) Influenced by?
- Hydrostatic pressure: that fluid exerts on the surface of capillaries.
- Osmotic pressure: overall concentration of fluid/solute. (absence of proteins in filtrate results in pressure near zero).
What are the forces favoring filtration?
Glomerular capillary hydrostatic pressure (60 mmHg)*
Bowman’s capsule osmotic pressure (0 mmHg)
What are the forces opposing filtration?
Bowman’s capsule hydrostatic pressure (15 mmHg)
Glomerular capillary osmotic pressure (30 mmHg)
Glomerular Filtration Rate (GFR)
Volume of plasma filtered per unit of time.
125 mL/min. Entire plasma filtrates through in 20 min.
180L of filtrate a day. Massive reabsorption.
1,5L of urine a day.
Renal Plasma Flow
Volume of plasma flowing through kidneys.
625 mL/min.
How much of the plasma flowing through the kidneys will pass inside the renal tubule?
20% → 125 mL/min / 625 mL/min = 0.20
Mean Arterial Rate vs. Glomerular Filtration Rate
MAP: General blood pressure.
If MAP increases, GFR will increase as well BUT we have a mechanism to regulate GFR (kept constant between 80 mmHg - 180 mmHg.
GFR Regulation - Myogenic Regulation
Intrinsic control.
Increased MAP stretch smooth muscle in afferent arteriole. Contraction results in vasoconstriction. Increased resistance, decreased blood flow. Decreased pressure in glomerulus.
GFR Regulation - Tubuloglomerular Feedback
Intrinsic control.
Macula densa sense a change in GFR. Secretion of paracrine signals. Smooth muscle contraction/relaxation in afferent arteriole.
Increased GFR: vasoconstriction
Decreased GFR: vasodilatation
GFR Regulation - Extrinsic control
MAP < 80 mmHg
Sympathetic activation through baroreceptor reflex.
Vasoconstriction in afferent and efferent arterioles.
Increased resistance (helps increase MAP)
Decreased blood flow
Decreased GFR
Tubular Reabsorption
Massive, mostly in proximal* and distal convoluted tubules. Substances needs to cross tubule epithelium and capillary endothelium.
Active transport across either apical or basolateral membrane in conjunction with facilitated diffusion across the other.
Water Reabsorption
Follows active reabsorption of solute.
Tubular Reabsorption - Proximal Tubule
70% of water, Na+, K+. 100% glucose, amino acids, other organic substances. Recovery of bicarbonate ions for blood’s pH homeostasis.
Mass absorber.
Tubular Secretion - Secreted Substances
Hydrogen ions Potassium ions Urea Drugs Ammonia Creatinine
Tubular Secretion - Aquaporin
Water channels that facilitates water reabsorption.
Tubular Reabsorption - Loop of Henle
Recover Na+ and water.
Concentration of solute goes from isotonic 300 (descending) to 1200 hypertonic (bottom of loop) to 100 hypotonic (ascending).
Descending: aquaporin channels, unrestricted movement of water.
Ascending: completely impermeable to water.
Tubular Reabsorption - Collecting Ducts
Fine tuning of water balance and blood pressure.
Water can only pass through aquaporins in response to hormonal signals.
Concentration of solute in tubular fluid depends on how much H2O we need, the more H2O is reabsorbed, the higher the concentration/osmolarity (low volume of urine).
Concentration in medulla increases with depth.
Excretion
Elimination of solute and water from the body.
Amount = amount filtered + amount secreted - amount reabsorbed.
Excretion Rate
Depends on:
- Filtered load (GFR x plasma concentration)
- Rate of secretion
- Rate of reabsorption
Excretion rate > filtered load: Secretion
Excretion rate < filtered load: Reabsorbed
But we can only determine net effect (cannot exclude one or the other)
Acid Base Balance - Acidosis
pH < 7.35 Depression of CNS Cardiac arrythmias Potassium retention Fatal: pH < 6.8 Coma Respiratory failure
Normal pH Range
7.38 - 7.42
maintained by lungs and kidneys
Acid Base Balance - Alkalosis
pH > 7.45 Increase CNS excitability Potassium depletion Fatal: pH > 8 Muscle seizures and convulsions Spasms of respiratory muscles
Bicarbonate Buffer System - Acid Base Disturbance
Major buffer system in plasma, minimizes changes in pH.
CO2 + H2O ⇆ H2CO3 ⇆ H+ + HCO3-
CO2 increases - H+ increases - pH decreases.
In order to maintain 7.4 pH: [HCO3-] / [CO2] = 20:1
- Immediate action
- Temporarily limit changes
- Cannot reverse changes in pH
Respiratory Compensation - Acid Base Disturbance
Within minutes.
True homeostatic mechanism.
Increase ventilation → Decrease CO2 → Increase pH
Decrease ventilation → Increase CO2 → Decrease pH
Renal Compensation - Acid Base Disturbance
If pH increases → Decrease in H+ secretion, decrease in HCO3- reabsorption (in order to maintain pH).
If pH decreases → Increase in H+ secretion, increase in HCO3- reabsorption, Increase HCO3- production (in order to maintain pH).
