Urinary System Physiology

Question 1

What do kidneys regulate?

Answer 1

• 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.

Question 2

Sections of the Kidneys

Answer 2

Renal cortex: outer regions.
Medulla: inner regions.
Pyramids: conical sections of the medulla separated by renal columns.

Question 3

Order in Which Urine is Collected

Answer 3

Papillae → minor calyces → major calyces → ureter

Question 4

The Nephron

Answer 4

Functional unit of kidneys.
Filter the blood and form urine.
Composed of renal corpuscle and renal tubule.

Question 5

Renal Corpuscle

Answer 5

1. Glomerulus: high pressure capillary bed

2. Bowman’s capsule: proximal end that surrounds glomerulus.

Question 6

Renal Tubule Composition

Answer 6

1. Proximal tubule
2. Loop of Henle
3. Distal convoluted tubule
4. Collecting duct

Question 7

Juxtaglomerular Apparatus

Answer 7

Helps regulating blood pressure.

1. Macula densa: cells of distal tubule.
2. Granular cells (juxtaglomerular cells): secretion of renin (regulation of blood pressure).

Question 8

Which parts of the nephron are in the cortex?

Answer 8

Renal corpuscles, proximal and distal convoluted tubule.

Question 9

Which parts of the nephron extend into the medulla?

Answer 9

Loop of Henle (has descending and ascending limb).
Collecting ducts (bring the forming urine to papillae).

Question 10

Vascular System of the Nephron

Answer 10

Efferent arteriole form a 2nd capillary bed around renal tubule, supply blood to glomerulus.

Question 11

Regulation of Composition of Plasma

Answer 11

Exchange of solutes and fluids between plasma and filtrate.
Achieved through 3 functions: 
1. Filtration
2. Reabsorption
3. Secretion

Question 12

Bowman’s Capsule

Answer 12

Captures and directs filtrate to proximal tubule.
Parietal layer: simple squamous epithelium.
Visceral layer: cells cover the glomerular capillaries. Form a sieve.

Question 13

How much of plasma enters Bowman’s capsule?

Answer 13

10-20%

Question 14

Filtration Based on Size

Answer 14

Glomerular capillaries are fenestrated (extra spaces that favor movement of fluid). Blood cells and large proteins cannot pass.

Question 15

Glomerular Filtration

Answer 15

Creation of a filtrate with a composition very similar to plasma (no protein).

Question 16

What is Glomerular Filtration Rate (GFR) Influenced by?

Answer 16

1. Hydrostatic pressure: that fluid exerts on the surface of capillaries.
2. Osmotic pressure: overall concentration of fluid/solute. (absence of proteins in filtrate results in pressure near zero).

Question 17

What are the forces favoring filtration?

Answer 17

Glomerular capillary hydrostatic pressure (60 mmHg)*

Bowman’s capsule osmotic pressure (0 mmHg)

Question 18

What are the forces opposing filtration?

Answer 18

Bowman’s capsule hydrostatic pressure (15 mmHg)

Glomerular capillary osmotic pressure (30 mmHg)

Question 19

Glomerular Filtration Rate (GFR)

Answer 19

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.

Question 20

Renal Plasma Flow

Answer 20

Volume of plasma flowing through kidneys.

625 mL/min.

Question 21

How much of the plasma flowing through the kidneys will pass inside the renal tubule?

Answer 21

20% → 125 mL/min / 625 mL/min = 0.20

Question 22

Mean Arterial Rate vs. Glomerular Filtration Rate

Answer 22

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.

Question 23

GFR Regulation - Myogenic Regulation

Answer 23

Intrinsic control.
Increased MAP stretch smooth muscle in afferent arteriole. Contraction results in vasoconstriction. Increased resistance, decreased blood flow. Decreased pressure in glomerulus.

Question 24

GFR Regulation - Tubuloglomerular Feedback

Answer 24

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

Question 25

GFR Regulation - Extrinsic control

Answer 25

MAP < 80 mmHg Sympathetic activation through baroreceptor reflex. Vasoconstriction in afferent and efferent arterioles. Increased resistance (helps increase MAP) Decreased blood flow Decreased GFR

Question 26

Tubular Reabsorption

Answer 26

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.

Question 27

Water Reabsorption

Answer 27

Follows active reabsorption of solute.

Question 28

Tubular Reabsorption - Proximal Tubule

Answer 28

70% of water, Na+, K+. 100% glucose, amino acids, other organic substances. Recovery of bicarbonate ions for blood's pH homeostasis. Mass absorber.

Question 29

Tubular Secretion - Secreted Substances

Answer 29

``` Hydrogen ions Potassium ions Urea Drugs Ammonia Creatinine ```

Question 30

Tubular Secretion - Aquaporin

Answer 30

Water channels that facilitates water reabsorption.

Question 31

Tubular Reabsorption - Loop of Henle

Answer 31

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.

Question 32

Tubular Reabsorption - Collecting Ducts

Answer 32

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.

Question 33

Excretion

Answer 33

Elimination of solute and water from the body. | Amount = amount filtered + amount secreted - amount reabsorbed.

Question 34

Excretion Rate

Answer 34

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)

Question 35

Acid Base Balance - Acidosis

Answer 35

``` pH < 7.35 Depression of CNS Cardiac arrythmias Potassium retention Fatal: pH < 6.8 Coma Respiratory failure ```

Question 36

Normal pH Range

Answer 36

7.38 - 7.42 | maintained by lungs and kidneys

Question 37

Acid Base Balance - Alkalosis

Answer 37

``` pH > 7.45 Increase CNS excitability Potassium depletion Fatal: pH > 8 Muscle seizures and convulsions Spasms of respiratory muscles ```

Question 38

Bicarbonate Buffer System - Acid Base Disturbance

Answer 38

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

Question 39

Respiratory Compensation - Acid Base Disturbance

Answer 39

Within minutes. True homeostatic mechanism. Increase ventilation → Decrease CO2 → Increase pH Decrease ventilation → Increase CO2 → Decrease pH

Question 40

Renal Compensation - Acid Base Disturbance

Answer 40

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).