Q21

What factors affect filtration across the glomerular capillary bed?

  • Permeability and area of the glomerular capillary bed.
  • Hydrostatic pressures in the capillary and the tubule
  • Oncotic pressure in the plasma and the filtrate.

How can GFR be measured?

  • UxV / Px or concepts.

Q22

What is the renal response to respiratory acidosis?

  • Increased H+ secretion and HC03 – absorption.

What buffering systems are there for H+ in renal tubular fluid?

  • At least HC03 – and one of HPO/- or NH3 with explanation of buffering mechanism.

Q23

How does the renin-angiotensin system respond to hypotension?

  • With a drop in BP, renin is released from the JG cells and act on a renin substrate to form angiotensin I, which is converted to angiotensin II in the lung.
  • Angiotensin II causes vasoconstriction and decrease the excretion of both salt and water (long term effect).

What are the other effects of the renin-angiotensin system?

  • Salt and water retention
  • Stimulate aldosterone secretion
  • Faciliate the release of noradrenaline
  • Downgrade the baro-receptors
  • Increase the secretion of vasopressin

Q24

Describe the physiological characteristics of renal blood flow.

  • Renal blood flow is 25% of the cardiac output.
  • The glomerular capillary pressure is 40% of systemic arterial pressure.
  • The peritubular capillary network and renal veins are low pressure systems.
  • The renal cortex gets higher blood flow, but has low oxygen extraction (filtration).
  • Renal medulla gets less blood flow, but high oxygen extraction (osmolality) and sensitive to hypoxia

Pass criteria:

  • 3 out of 5

What are the factors that affect renal blood flow?

  • Decreased MAP – decreased baroreceptor firing – renal vasoconstriction – decreased RBF.
  • Exercise decreased RBF.
  • Pg ­ increased Rcbf decreased Rmbf.
  • Proteins ­increased RBF ­increased GCP.
  • Dopamine and ACh – vasodilatation – ­ increased RBF.
  • NA – vasoconstriction 1 > 2 constricts afferent arterioles and interlobular arteries decreased RBF.
  • Posture – lying to standing decreased RBF.
  • AgII constricts efferent arteriole increased perfusion pressure.

How can renal blood flow be calculated?

  • By determining clearance of PAH, its extraction ratio and the haematocrit

Q25

How does the kidney acidify the urine?

  • Secretion of hydrogen ions.
  • Binding of the hydrogen ions with buffers.
  • Secretion/absorption of bicarbonate ions.

Pass criteria

  • 2 out of 3

Is there a difference between the proximal and distal tubules?

  • PCT/DCT/CD secrete H+.
  • PCT via Na+/H+ exchange.
  • Na+/K+ATPase – Na+ from cell to interstitium.
  • DCT/CD H+ secretion ATP driven proton pump.

What factors increase acid secretion?

  • Factors which increase acid secretion
    • increased PCO2 ­ increased PaCO2 ­ increased aldosterone
    • decreased K+ increased CA concentration
    • increased K+, ­ decreased H+ secretion

Q26

What determines renal blood flow?

  • Systemic blood pressure
  • Renal vascular resistance, which is in turn influenced by:
    • Catecholamines (nerves & systemic)
    • Angiotensin II (JG cells -> renin)
    • Prostaglandins
  • Control systems:
    • Renal autoregulation (myogenic- stretch response, vasodilator metabolites, ? NO, ?prostaglandins)
    • JG apparatus
    • Renal sympathetic nerves

What are the consequences of a sustained reduction of renal blood flow?

  • Renal blood flow maintained MBP >70
  • Medulla is vulnerable to hypoxia (high MR)
  • ATN
  • Uraemia

Q27

What is THIRST, and what causes it?

  • An appetite, under hypothalamic control
  • Increased plasma osmolality
    • osmoreceptors in anterior hypothalamus
  • Hypovolaemia
    • Renin-angiotensin system
    • Baroreceptors in heart and blood vessels
  • Prandial
    • Learned or habit response
    • Osmolality & GI hormone effects
  • Psychogenic
  • Dry pharyngeal mucous membranes

What are the actions of vasopressin (ADH), and what influences secretion of this hormone?

ACTIONS:

  • Retention of water by kidney (collecting duct permeability), thus decreasing blood osmolality
  • V2 receptors -> insertion of aquaporin-2 (water channel proteins stored in endosomes) into cell membranes
  • Decreased cardiac output (via area postrema)
  • Vasoconstriction via V1 receptors
  • Glycogenolysis
  • ACTH secretion from ant pituitary

SECRETION INFLUENCED BY:

  • Osmolality
  • ECF volume (low pressure receptors in great veins, atria and pulmonary vessels, high pressure receptors in carotid sinuses and aortic arch)
  • Increased secretion with high osmolality &/or low ECF Volume, and visa versa
  • Pain, nausea, surgical stress and some emotions increase secretion
  • Alcohol decreases secretion

Q28

What happens to potassium as it passes through a nephron?

  • Freely filtered
  • 67% reabsorbed prox tub (with Na + H2O)
  • 20% reabsorbed asc limb (with Na and Cl)
  • Distal tubule reabsorbs or secretes (H/K/ATPase)
  • Reabsorbed in alpha intercalated cells
  • Secretion by principal cells
  • Diet, aldosterone, A/B, lumen ions, diuretics

How does potassium handling by the kidney change in response to changes in pH?

  • H and K are exchanged
  • Acidosis decreases K excretion
  • H makes K move into circulation, less for excretion
  • Alkalosis increases K excretion

How does aldosterone increase K secretion?

  • Increased Na entry into cells
  • Increased pumping out of Na by Na-K pump
  • Increased K uptake into principal cells
  • Increased K conc inc secretion driving force
  • Also inc luminal membrane K channels

Q29

What general mechanisms are involved in renal tubular reabsorption and secretion?

Mechanisms involved in re-absorption and secretion include endocytosis, passive diffusion and facilitated diffusion and active transport.

2 of Bold to Pass

How is Sodium reabsorped in the various parts of the nephron?

No sodium transport in Thin descending Loop of Henle.
In rest of system, sodium moves by co-transport, exchange or down concentration gradient.

Sodium pumped out of cell by Active Sodium-Cl-Potassium pump in basolateral membrane.
60% in PCT by Sodium-Hydrogen exchange.
30% in thick ascending Limb via Sodium —Potassium co-transport.
7% in DCT via Sodium-Chloride exchange

Site                                                        Apical Transporter                          Function

Proximal tubule                                                Na/        CT                                           Na uptake,          uptake

Na /P, CT                                             Na- uptake, Pt uptak

Na’         CT                                           Na’ uptake,         .. uptake

Na/lactate CT                                     Na uptake, lactate uptake

Na/H exchanger                               Na’ uptake. H extrusi

CI base exchanger                           Cl uptake
Thick ascending limb                       Na-K-2Cl CT                                        Na- uptake, H uptake, K uptake

Na/H exchanger                               Na’ uptake, H extrusio

K channels                                          K extrusion (recycling)

Distal convoluted tubule               CT                                                           Na uptake, Cl uptake

Collecting duct                                  Na.”. channel (ENaC)                      Na uptake
Pass: Bold to pass, demonstrating reasonable understanding of different processes


Q30

Describe the renal response to metabolic acidosis.
Prompts: (i) What prevents FT secretion stopping when urine pH falls to 4.5? (ii) Can you name any of the buffers that operate?

  • Renal mechanisms operate to compensate for metabolic acidosis and return the serum
  • pH towards normal
  • Anions that replace HCO3-are filtered at the glomerulus along with corresponding
  • cations (mainly Na-)
  • Renal tubule cells secrete 11-. into tubular fluid in exchange for Na’ and HCO3-
  • Buffering in the urine gives greater capacity to this system (otherwise limiting pH of
  • 4.5 would stop futher H secretion)
  • Buffering systems include: Bicarbonate, Phosphate, Ammonia

Pass: Compensatory mechanisms identified
Must know H- secreted into tubular fluid in exchange for Na_
Must know about buffering and give two buffers