Lecture 52 – Tubular function and hormonal control

Question 1

Electrolyte balance across tubular epithelium

Answer 1

o Na+ is reabsorbed by active transport
o Electrochemical gradient drives anion reabsorption
o Water moves by osmosis, following solute reabsorption
o Concentrations of other solutes increase as fluid volume in lumen decreases. Permeable solutes are reabsorbed by diffusion

Question 2

Nephron blood supply

Answer 2

Each nephron has two arterioles and two sets of capillaries associated with it

Question 3

Na+ reabsorbed

Answer 3

o Na+ enters cell through membrane proteins, moving down its electrochemical gradient
o Na+ is pumped out the basolateral side of cell by the Na+-K+-ATPase

Question 4

Glucose and Na+ reabsorbed

Answer 4

o Na+ moving down its electrochemical gradient using the SFLT protein pulls glucose into the cell against its concentration gradient
o Glucose diffuses out of the basolateral side of the cell using the GLUT protein
o Na+ is pumped out by Na+-K+-ATPase

Question 5

Filtration rate of glucose vs plasma glucose concentration

Answer 5

Linear proportional positive relationship

Question 6

Sodium reabsorption

Answer 6

o 80% of oxygen consumed by the kidney
o Tied to reabsorption
 Water, chloride
 Glucose, amino acids, urea
o Tied to secretion of
 Potassium
 Hydrogen ions

Question 7

Sodium reabsorption table

Answer 7

refer to notes

Question 8

Renin-angiotensin system

Answer 8

o Renin release from granular cells is stimulated by EC volume depletion
 Fall in pressure at preglomerular arteriole
 Reduction in sodium chloride delivery to macula densa
 Sympathetic nerve activation

Question 9

Sodium balance

Answer 9

o Input
 Diet (food and drink) – 150 mmol
 Intravenous (normal saline) – 0 mmol
o Output
 Urine – 140 mmol
 Faeces – 8 mmol
 Skin – 2mmol

Question 10

water balance

Answer 10

o Input
 Diet (food and drink) – 2200 mL
 Metabolism 300mL
 Intravenous (5% dextrose) 0 mL
o Output
 Urine 1500 mL
 Lungs 500 mL
 Skin 400 mL
 Faeces 100mL

Question 11

Detecting changes in osmolarity

Answer 11

o osmoreceptor cells
 Anterior hypothalamus
 Response to changes in cell size

Question 12

Vasopressin and osmolarity

Answer 12

Linear proportional relationship that hits a plateau

Question 13

Vasopressin and ECF volume

Answer 13

Negative linear relationship

Question 14

Water and the nephron

Answer 14

o Proximal tubule
 Bulk absorption
o Descending limb of loop of Henle
 Dilution of filtrate
o Collecting ducts
 Fine-tuning according to needs

Question 15

Urine osmolality

Answer 15

o Ranges from 3-1200 mOsM/kgH2O
o Must excrete 600 mOsM of solute per day
o Must excrete at least 500mL of water per day

Question 16

Concentration of urine along the nephron

Answer 16

o Isosmotic fluid leaving the proximal tubule becomes progressively more concentrated in. the descending limb
o Removal of solute in the thick ascending limb creates hypoosmotic fluid
o Hormones control distal nephron permeability to water and solutes
o Urine osmolarity depends on reabsorption in the collecting duct

Question 17

Vasopressin binds to membrane receptor

Answer 17

o Receptor activates cAMP second messenger system
o Cell inserts AQP2 water pores into apical membrane
o Water is absorbed by osmosis into the blood

Question 18

Concentrated urine

Answer 18

With maximal vasopressin, the collecting duct is freely permeable to water. Water leaves by osmosis and is carried away by the base recta capillaries

Question 19

Dilute urine

Answer 19

In the absence of vasopressin, the collecting duct is impermeable to water and the urine is dilute.

Question 20

Movement of water across collecting duct lumen to medullary interstitial fluid to vasa recta (blood)

Answer 20

o Vasopressin binds to membrane receptor
o Receptor activates cAMP second messenger system
o Cell inserts AQP2 water pores into apical membrane
o Water is absorbed by osmosis into the blood

Question 21

Is reabsorption of secretion a bigger tubular activity?

Answer 21

reabsorption

Question 22

Where is reabsorption greatest along the nephron?

Answer 22

proximal tube

Question 23

What is the most important solute to be reabsorbed?

Answer 23

sodium

Question 24

Which solutes are actively reabsorbed?

Answer 24

Sodium, amino acids, glucose, lactate

Question 25

Glucose appears in the diabetic’s urine because diabetes?

Answer 25

o Because there is an upper limit of glucose that the tubules can reabsorb
o Tm = maximal absorption of substrate

Question 26

In healthy individuals’ glucose is typically filtered…

Answer 26

o In equimolar amounts found in plasma
o Glucose is not found in healthy individuals excreted urine
o Glucose and amino acid reabsorption is driven by secondary active transport
o Glucose transporters are saturable
o Glucose and amino acids are typically reabsorbed in the PACT

Question 27

Sodium reabsorption

Answer 27

o In the proximal convoluted tubule, H+ secretion occurs in exchange for Na+ reabsorption.
o H+/proton/hydrogen ion/hydrogen cation secretion in the proximal nephron is functionally due to the need to reabsorb HCO3-/bicarbonate.
o In the distal nephron H+ secretion is intended to regulate acid/base (pH) balance.

Question 28

Renin-angiotensin-aldosterone system

Answer 28

o The renin-angiotensin-aldosterone system (RAAS) is an endocrine/hormone regulatory system that is essential for the long-term regulation of blood pressure and fluid balance.
o Renin is an endocrine hormone released by the kidneys. Renin acts as an enzyme that catalyses the conversion of angiotensinogen to angiotensin 1.
o Angiotensinogen is produced and secreted from the liver. Angiotensin 1 (Ang 1) is then cleaved to form Ang/angiotensin II by the hormone angiotensin converting enzyme (ACE).
o ACE is synthesised in endothelial cells that line blood vessels. The high density of capillaries in the lungs means that the lungs are a major site of ANG II production.
o The name “angiotensin” indicates that ANG II is a potent vasoconstrictor. ANG II also stimulates aldosterone release from the adrenal cortex.
o Aldosterone/aldosterone promotes sodium reabsorption (in exchange for potassium secretion).

Question 29

Juxtaglomerular apparatus

Answer 29

o Macula densa cells detect Na+/sodium/Na content in ascending limb of the loop of Henle.
o During tubular glomerular feedback macula densa cell signal to smooth muscle cells on the afferent arteriole to constrict - restricting blood flow to the glomerular capillaries.
o However, if Na+/sodium/Na content in the ascending limb of the loop of Henle is low - suggesting excess water loss - the macula densa cells signal the release of renin from the granular cells.
o High water loss suggests low blood volume and a fall in mean arterial pressure. The pressor effects of Ang II/angiotensin II increases resistance, thus increasing MAP.

Question 30

Renal sodium excretion can be altered by what?

Answer 30

o Renin-angiotensin system
o Sympathetic nervous system
o Atrial natriuretic peptide

Question 31

The body detects sodium balance through

Answer 31

o Carotid baroreceptors
o Renal arterial pressure receptors
o Cardia atrial baroreceptors

Question 32

Infusion of 1L of normal saline will cause a change in blood volume of:

Answer 32

0.25L

Question 33

Tubular fluid leaving the loop of Henle is normally

Answer 33

dilute

Question 34

The medullary concentration gradient is mostly the result of

Answer 34

sodium and urea

Question 35

What things are needed to produce concentrated urine?

Answer 35

o Na/K/2Cl co-transporter
o Aquaporin 2
o cAMP

Question 36

a negative water balance will be most obvious as

Answer 36

o through changes in osmolarity
o using osmoreceptor cells

Question 37

a negative water balance will normally result in

Answer 37

o thirst and concentrated urine
o i.e. waking up in morning after not having water overnight

Question 38

what things are needed to produce concentrated urine

Answer 38

vasopressin (ADH)