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Renal physiology 1
Medicine (A100)
University of Dundee
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Renal physiology
Body fluid compartments – look through slides
Osmolarity is the concentration of osmotically active particles present in a solution the units being osmol/l or mosmol/l in the human body as it exists as weak salt solutions. Osmolarity can be calculated if the molecular concentration of the solution AND the number of osmotically active particles present. Osmolality (osmol/kg) and osmolarity are interchangeable in weak salt solutions i. body fluids. Tonicity is the effect a solution has on cell volume. Isotonic solution – doesn’t change the cell volumes Hypertonic solution – decrease in cell volume Hypotonic solution – increase in cell volume may lead to lysis Tonicity incorporates the osmolarity of the solution and the ability of the solute to cross the semi-permeable membrane. E. in the case of 300osmol/l of sucrose and urea, the urea will easily enter the cell and water will follow. This makes it a hypotonic solution, sucrose does not have this effect and is isotonic.
Water makes up a large proportion of total body weight, it is slightly less in women due to a greater presence of fat in their bodies. Water can be split into several compartments: ECF (plasma, interstitial fluid and negligible lymph and negligible transcellular fluid); ICF. Tracers are used to measure fluid compartments. Specific tracers are used to measure specific fluid compartments.
Water loss and gain is controlled by homeostatic mechanisms. Losses can be classed as sensible and insensible losses. Insensible losses include through lungs and skin and these are not under physiological control. Urine production can be decreased by external factors but never turned off.
Ionic composition of body fluid varies: sodium and chloride exist mainly in the ECF whereas potassium and carbonate exist mainly in the ICF. The osmolarity of the ECF is the same as the ICF. Any change in the water content or solute content of the compartments will cause a change in osmolarity and incur an osmotic gradient leading to a fluid shift between the compartments.
Kidney structure and function
The kidney has 5 main fucntions: Water balance Salt balance Acid-base balance Excretion of waste products Excretion of exogenous foreign compounds Conversion of vitamin D to its active form calcitriol (promotes absorption of Ca2+ in GI tract) Also: Maintenance of plasma volume Maintenance of plasma osmolarity Secretion of renin Secretion of erythropoietin
The nephron This is the functional unit of the kidney and each kidney contains around 1 million. Its main functional mechanisms are filtration, reabsorption and secretion. The outer cortex has a granulated appearance and the inner medulla has a striated appearance. There are two types of nephron: cortical (80% of nephrons) and juxtamedullary (20% of nephrons). Juxtamedullary nephrons have longer loop of Henles; lack peritubular capillaries (instead have a single “vasa recta” artery); produce more concentrated urine. Cortical shown below.
The blood flow into the nephron follows a pathway: Artery – afferent arteriole (large) – glomerulus (more porous than systemic capillaries) – efferent arteriole (small) – peritubular capillaries (supply kidney tissue) – venule – vein.
The tubular component is structured similarly. The early part that surrounds the glomerulus is called bowmans capsule and this is where initial filtration occurs. Only the fluid entering the collecting duct is called urine as it is no longer modified. Bowmans capsule – proximal convoluted tubule – loop of Henle (ascending and descending limb) – distal convoluted tubule – collecting duct – renal pelvis.
The two arterioles contributing to the glomerulus exist in a fork like position and the distal convoluted tubule passes through the space in between. Here, you can find the granular cells that secrete renin and the macula densa that senses salt in the distal tubule.
GFR = Kf x net filtration pressure
(Kf is the filtration coefficient that determines how leaky the glomerulus is)
Regulation of GFR
Extrinsic controls Intrinsic controls
Sympathetic control via baroreceptor
reflex
Myogenic mechanism – stretch of
afferent arteriole due to increase in
BP causes contraction of that
arteriole
Tubuloglomerular feedback
mechanism – macula densa senses
NaCl increase and causes constriction
of afferent arteriole
Extrinsic controls will always override the intrinsic controls.
Read slides from 18 onwards (powerpoint number 3)
Renal physiology 1
Module: Medicine (A100)
University: University of Dundee
