3 - Renal Physiology I Flashcards
What are the functions of the kidney?
Every thirty minutes, the kidneys filter the entire blood supply in your body (blood flow about 20% of cardiac output)
- Excretion of metabolic waste products and foreign substances
- Regulation of body fluids and electrolytes balance, acid-base balance and arterial blood pressure
- Elaboration of endocrine hormones
- Gluconeogenesis
Describe the excretion of metabolic waste products and foreign substances
Urea (protein metabolism), Uric acid (nucleic acid), creatinine (muscle creatine), chemicals and pesticides, etc.
Describe the elaboration of endocrine hormones
Secretion, metabolism and excretion of hormones [erythropoietin, 1,25-dihydroxy vitamin D3 (vitamin D activation), renin]
Describe the function of gluconeogenesis
Most gluconeogenesis take place in the liver but a large proportion takes place in the kidneys especially during a long fast.
Describe the “system approach” to kidney function (how does it contribute to the body as a system?)
Kidney’s relationship to extrarenal environment (the rest of the body)…
Excretion of:
- Metabolic waste products
- Foreign substances
Adjustment of:
- Fluid volume
- Solute concentration
Elaboration of endocrine hormones
Gluconeogenesis
Describe the role of the body in signaling the kidney in this “system approach” to kidney function
- Blood pressure and volume
- Plasma composition
- Neuroendocrine
Describe the intrarenal processes of the “system approach” (does not interact with the body, is just the kidney’s own functions)
Basic kidney process
Local control:
- Renal auto-regulation
- Autocrine/paracrine systems
Describe the basic physiological anatomy of the kidney
The two kidneys lie on the posterior wall of the abdomen
The ureter carries the final urine from the kidney to the bladder
In a bisected kidney, from
top to the bottom, the two major regions visualized are the cortex (outer region) and medulla (inner region)
The medulla is divided in multiple cone-shaped masses of tissue called renal pyramids
The base of each pyramid terminates in the papilla, which projects into the space of the renal pelvis
Describe the basic tubular segments of the nephron
- Each kidney in the human contains about 1 million
of nephrons - The nephron is the functional
unit of the kidney - Each nephron contains a
glomerulus and long tubule - The glomerulus contains a
collection of capillary (glomerular
capillary), through which large
amount of fluid is filtered from the blood - The glomerulus is incased in the Bowman’s capsule
Describe the structure of the glomerulus
The glomerular capillary membrane is similar to that of other capillaries, except that it has three (instead of the usual two) major layers:
- Endothelium of the capillary
- A basement membrane
- A layer of epithelial cell (podocytes)
What do these layers of the glomerulus make up?
These layers make up the filtration barrier which filters fluid and solute with high filtration rate
Describe the filtration rate across the glomerular capillary
The high filtration rate across the glomerular capillary is supported by:
- The perforation of the endothelium (fenestrae)
- Meshwork of the basement membrane
- The long foot-like processes of the podocytes
Describe where the fluid goes after it is filtered in the glomerular capillaries
Fluid filtered from the glomerular capillaries flows into the Bowman’s capsule and, then, into the renal tubule
What happens in the renal tubule?
The tubule is where the filtered fluid is converted in urine
What are all the segments of the renal tubule?
- Proximal Tubule
- Loop of Henle
- Distal Tubule
- Connecting Tubule
- Collecting Tubule
- Collecting Duct
Describe the vasculature in kidney
- The renal artery enters the kidney and, then, branches progressively
- The venous system run parallel to the arteriolar vessels
- The vascular structures supplying the juxtamedullary nephron differ from those supply the cortical nephrons
What are the regional differences we see in nephron types?
There are two types of nephrons
- Cortical nephrons
- Juxtamedually nephrons
Describe cortical nephrons
Nephrons that have glomeruli located in the outer cortex and short Loop of Henle
Describe juxtamedullary nephrons
Nephrons that have glomeruli located deep in the cortex and long Loop of Henle (20-30%)
Only 20-30% of all nephrons are juxtamedullary, the rest are cortical
Describe the microcirculation of the nephron in the cortical nephrons
- The cortical nephrons are
surrounded by an extensive
network of peritubular capillaries - The renal circulation
is unique in that it has two capillary beds, the glomerular and peritubular capillaries - The glomerular and peritubular capillaries are arranged in series and separated by the efferent arteriole
** Arranged IN SERIES - important for regulation and flexibility **
Describe the microcirculation of the nephrons in juxtamedullary nephrons
- The juxtamedullary nephrons
are surrounded by specialized
peritubular capillaries (vasa recta) - The vasa recta extend downward into the medulla, lying side by side with the Loop of Henle
- This specialized network of capillaries in the medulla plays an essential role in the formation of concentrated urine
Describe the basic 4 steps for urine formation
1 - Glomerular filtration
2 - Reabsorption of stubstance from the renal tubules into the blood
3 - Secretion of substances from the blood into the renal tubules
4 - Urinary excretion
How can you calculate or determine the amount of excretion of urine?
EXCRETION = FILTRATION - REABSORPTION + SECRETION
** know this ***
What is the process of glomerular filtrate production?
- Urine formation begins with filtration of large amounts of fluid through the glomerular capillaries into Bowman’s capsule
- Filtrated fluid is essential protein-free and devoid of cellular elements
- The concentration of other constituents of the glomerular filtrate, including salts and organic molecules is similar to the concentration in the plasma
What are the exceptions in this process of glomerular filtrate production?
Exceptions to this generalization include a few molecular-weight substances, such as calcium and fatty acid that are bound to plasma proteins
How do you determine the glomerular filtration rate (GFR)?
The Glomerular Filtration Rate (GFR) is determined by the net filtration pressure and glomerular capillary filtration fraction coefficient
How do you determine the net filtration pressure?
The net filtration pressure is determined by the sum of the following across the glomerular:
- Hydrostatic Forces (Pressure)
- Colloid Osmotic Forces (Pressure)
What is hydrostatic pressure>
Hydrostatic pressure is the pressure that the fluid exerts on the walls of its container
In the capillary it is the force that is exerted by a fluid against the capillary wall
What is colloid osmotic pressure (oncotic pressure)
Osmotic pressure is the pressure required to prevent the solvent migration across a semipermeable membrane via osmosis
In the capillaries it is the pressure exerted by proteins in the plasma
Give a summary of the forces and pressures that cause filtration by the glomerular capillaries
Forces and pressures that FAVOR filtration
- Glomerular hydrostatic pressure
Forces and pressures that OPPOSE filtration
- Glomerular colloid osmotic pressure
- Bowman’s capsule pressure
What is the effect of vasoconstriction of afferent arterioles on GFR?
Vasoconstriction of afferent arteriole (AA) leads to a decreased GFR
What is the effect of vasoconstriction of the efferent arterioles on GFR?
Vasoconstriction of efferent arteriole (EA) leads to an increased GFR
What is the effect of vasodilation of efferent arterioles on GFR?
Vasodilation of efferent arteriole (EA) leads to a decreased GFR
What is the effect of vasodilation of afferent arterioles on GFR?
Vasodilation of afferent arteriole (AA) leads to an increased GFR
Describe how tubular re-absorption/secretion is highly selective
- The kidney controls the rate of tubular re-absorption or secretion of solutes independently of one another
- This capacity of the kidney is essential for precise control of the composition and volume of the body fluid
- The selectivity of tubular re-absorption/secretion is due to the luminal membrane and tight junctions permeability
How does the function depend on structure?
- Proximal tubule: tall cell with a lot of membrane allows for a lot of reabsorption and secretion ***
- Thick ascending loop of Henle: also very tall, also a lot of reabsorption and secretion ***
- Thin descending loop of Henle: no resorption of ions, just water
- Distal tubule and collecting duct: intercalated cells are important for the absorption and secretion of potassium
* Important concepts*
Describe reabsorption of filtered fluid and solutes from the tubular lumen into the blood
Two options…
- Transcellular pathway = Through the cells (diffusion or active transport)
- Paracellular pathway = Around the cells (diffusion)
Describe the transcellular (through the cells) pathway
Uses primary and secondary ACTIVE transport
- Metabolic substrates such as glucose, amino acid and fatty acid
- All if these various fuels are converted in energy ATP
- Electrochemical gradients such as low cell sodium and negative voltage drive a whole array of transport processes
What are the net reabsortive forces
- Hydrostatic forces
- Colloid osmotic forces
Calculate this by taking the sum of forces favoring reabsorption minus the sum of the forces opposing reabsorption
Give a summary of key concepts
- The kidneys have a very large blood flow relative to their mass that is regulated for functional reasons rather metabolic demand
- Glomerular filtration proceeds trough a 3-layered barrier that restrict filtration of large molecules such as proteins
- Glomerular Filtration Rate (GFR) is determined by the permeability of the filtration barrier and net re-absorptive forces
- Control of the resistances of the afferent and efferent arterioles permits independent control of GFR and renal blood flow
- The kidneys moves solutes across membranes by multiple transport mechanisms
What are the percentages of substrates reabsorbed from the urine back into the blood?
Sodium = 1-2%
Amino acids = 1%
Glucose = 1%
Potassium = 12%
How do you calculate fractional excretion?
Amount excreted/Amount filtered
Which substances have the same level of excretion (out of body) as initially secreted (into urine ) - NO reabsorption?
This means the fractional excretion is 1 in FEEF (fractional excretion = amount excreted/amount filtered =1)
Creatinine
Which substances have LESS excretion (out of body) than initially secreted (into urine ) - there is REABSORPTION?
FEEF
Which substances have NO excretion and all of the substrate is reabsorbed?
FEEF = 0
Examples: amino acids, oligopeptides, glucose
carboxylates
Which substances have MORE excretion (out of body) than was initially secreted (into the urine)?
FEEF >1
Examples: potassium, proton
organic acids and basis
urea, urate
