Renal 21 to 30
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.
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.
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
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
- 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
How does the kidney acidify the urine?
- Secretion of hydrogen ions.
- Binding of the hydrogen ions with buffers.
- Secretion/absorption of bicarbonate ions.
- 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
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)
- 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)
What is THIRST, and what causes it?
- An appetite, under hypothalamic control
- Increased plasma osmolality
- osmoreceptors in anterior hypothalamus
- Renin-angiotensin system
- Baroreceptors in heart and blood vessels
- Learned or habit response
- Osmolality & GI hormone effects
- Dry pharyngeal mucous membranes
What are the actions of vasopressin (ADH), and what influences secretion of this hormone?
- 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
- ACTH secretion from ant pituitary
SECRETION INFLUENCED BY:
- 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
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
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
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