Renal physiology: Function, structure and ultrafiltration (7.5) Flashcards
Describe the key kidney functions
Regulation of fluid and electrolyte balance: Maintains ECF and plasma volume through control of H2O and NaCl secretion/resorption
Maintenance of plasma osmolarity: Through regulation of aquaporins
Excretion of waste products: Bilirubin, creatinine, urea
Endocrine: Generates erthypoeitin, calcitriol, renin and kalikinen (?)
Metabolism: Converts Vitamin D to its active form; generates ammonia (for acid base regulation); role in Ca2+ balance
Describe the gross structure of nephrons and relate the tubular histological features to their functions
Nephrons: Formed from a glomerulus (capillary tuft), afferent and efferent arterioles, Bowman’s capsule and a renal tubule
Proximal convoluted tubule: Main site of reabsorption. Surface area is increased for this function with villi on cuboidal epithelium. Uses Na+/K+/Cl- cotransporter to establish an electrochemical gradient which allows for subsequent resorption of other molecules via the sodium gradient.
Loop of Henle: Creates a high osmolarity, allowing for subsequent dilution and concentration of urine.
Thin descending and thick ascending limbs have different permeabilities.
Distal convoluted tubule: Under hormonal control. Less villi seen on epithelium. Loops between the afferent and efferent arterioles at the vascular pole of the kidney to form the juxtaglomerular apparatus.
Collecting duct: End of the tubule. Under hormonal control. 2 cell types present:
- Principle: Responsible for the excretion of potassium
- Intercalated:Response for maintaining acid-base balance
Compare and contrast cortical and juxtamedullary nephrons in terms of anatomical structure, post-glomerular blood supply and function
Nephron = The functional unit of the kidney
Cortical nephrons:
- Most numerous (80 %)
- The glomerulus is located within the renal cortex
- Post-glomerular blood supply = Peritubular capillaries
- Short loop of Henle
- Function: Filtration
Juxtamedullary nephrons:
- Minority nephron (20 %)
- The glomerulus is located at near the medulla-cortex transition zone, may be seen across the whole length of the medulla
- Post-glomerular blood supply = vasa recta
- Long loop of Henle
- Function: Concentration/dilution of urine
Explain the importance of the kidney microvasculature and innervation for physiological function
Importance of kidney microvasculature:
- Kidneys have 2 capillary beds (glomerulus and peritubular/vasa recta)
- Blood enters and exits the glomerulus via arterioles
- The majority of blood flow is directed to the cortex hence the medulla is sensitive to reductions in renal blood flow (may cause ischaemia and lead to necrosis of tubules)
Blood supply to the kidney is essential to allow for filtration of the blood plasma, allowing removal of waste products. The glomerulus allows for filtration whilst the peritubular arteries allow for ‘correction’, through the exchange of molecules.
Regulation of blood flow is essential to maintain homeostasis.
(Aorta → renal arteries → segmental arteries → lobar arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arteriole → glomerulus → efferent arteriole →
Importance of kidney innervation:
- Innervated by the sympathetic nervous system: Allows for changes to renal blood flow and GFR to maintain homeostasis through changes in vascular smooth muscle tone. Acts upon tubular cells to increase transporter activity and granular cellsto prompt renin release.
- Afferent sensory nerve fibres in the renal pelvis are stimulated by stretch
- Renorenal reflex: Allows the one kidney to compensate for loss of function in the other
Describe the structure and physiological function of the juxtaglomerular apparatus (JGA)
Structure: A portion of the renal nephron at which the DCT (distal convoluted tubule) loops between the afferent and efferent arterioles at the renal vascular pole.
3 cell types seen at the JGA
Macula Densa cells: Located in the DCT. Act to sense the ‘flow’ of sodium passing in the tubule. Act as chemoreceptors.
Granular cells: Located around the afferent arteriole. Secrete Renin. Act as baroreceptors.
Mesangial cells: Contractile cells with phagocytic properies. Located around the glomerulus, can cause changes in blood flow.
Physiological function: Acts to detect sodium flow and modify sodium excretion occuring from the tubule in relation to this.
If sodium flow is too high/low then the macula densa prompts granular cells to secrete renin. This causes constriction of the afferent and efferent arterioles, to modify the ‘flow’ of sodium and ultimately the excretion of sodium within the urine.
Describe the structure of the glomerular filtration barrier and its histological features
Formed from 3 layers
Glomerular capillary epithelium: Fenestrated. Thin, single cell
Basement membrane: Has negatively charged glycoproteins associated with it, repels plasma proteins (allows for the maintenance of blood colloid pressure)
Podocytes:‘Outer most’ layer relative the glomerular capillary, in contact with the capsular space. Pedicles with filtration slits between. Contractile mesangial cells are seen at the filtration slits, allowing for adjustment of diameter.
Define glomerular filtration rate (GFR) and describe filtration forces underlying the process and effects of influential factors
GFR: The flow rate of filtered fluid through the kidneys (ml/min)
GFR = Net filtration pressure x Kf
Filtration forces:
Net filtration pressure = Glomerular blood hydrostatic pressure (55 mmHg) - capsular hydrostatic pressure ( 15 mmHg) - blood oncotic colloid pressure (35 mmHg)
Influential factors:
Example: Urine retention would increase capsular hydrostatic pressure hence net filtration pressure, and consequently GFR, would be reduced
Explain the physiological importance of renal blood flow autoregulation in the kidney and describe the two regulatory processes that perform this function
Autoregulation ensures that renal perfusion/blood flow remains constant over a range of mean arterial blood pressures (80 - 180 mmHg) aiding the maintenance of homeostasis.
Autoregulation - Regulatory processes:
- Myogenic reflex: Changes in vascular smooth muscle in response to the pressure exerted on the vessel wall by vessel contents e.g. if the pressure on the vessel increases then the smooth muscle will contract.
- Tubuloglomerular feedback: The macula densa cells of the JGA sense flow of sodium in the lumen of the tubule. If sodium reabsorption into the capillaries is high then the flow of sodium within the region of the tubule passing the macula densa will be reduced. Filtration at the corresponding glomerulus will be increased, as enough sodium has already been reabsorbed, through vasodilation of the afferent arteriole.
If the macula dense sense high flow of sodium within the tubule, indicating that reabsorption of sodium into the capillary is low, then glomerular filtration is decreased, through vasoconstriction of the afferent arteriole. The decrease in GFR allows for increased Na+ reabsorption (the slower the passage of fluid the greater the time available for reabsorption)
Describe the influence of other factors on renal blood flow
Renal blood flow may receive autonomic and hormonal input in emergency situations e.g. stress, cold haemorrhage
Sympathetic input: Causes contraction of arteriole smooth muscle, increasing GFR. Can effect both afferent and efferent arteriole.
Achieved through the release of NA, acting on alpha 1 receptors to cause contraction of smooth muscle.
Hormonal input
NO, prostaglandins, BNP, ANP, ADH, angiotensin II, adenosine
Describe renal clearance and examples of substances used to determine GFR
Renal clearance: The volume of plasma that is cleared of a substance by the kidneys per unit of time
Clearance = urinary excretion rate / plasma concentration
Substances used to determine GFR: Creatinine, cystatin C, inohexol, inulin
Explain how renal plasma flow (RPF) is determined
Using para-amino-hippurric acid (PAH) marker (actively secreted from the blood into the tubules)
Explain how renal blood flow (RBF) is determined
RBF = RPF/(1-haematocrit)
