Exam 4: GFR and RBF Flashcards
What are the renal functions?
Excretion of metabolic waste products Regulation of acid-base balance Control of arterial pressure Secretion, metabolism, and excretion of hormones Excretion of foreign chemicals Gluconeogenesis
What are the metabolic waste products from the kidney?
Urea
Creatinine
How does the regulation of acid-base balance occur?
Eliminate hydrogen ions, organic acids and bases
Conserve HCO3
How does control of arterial pressure occur?
Excrete or conserve Na and H2O and other electrolytes
What is renin?
Precursor for Angiotensin (I and II) for maintenance of arterial pressure
How much of the erythropoietin in the body is made in the kidney?
90%
What is phosphocreatine?
The main reservoir of rapidly available high energy phosphate bonds in muscle
What percentage of muscle creatine turns over on a daily basis to produce creatine?
1-2%
What is creatinine production proportional to?
Muscle mass
What may cause an increase in the amount of creatinine?
Acute muscle disease states
What may cause a decrease in the amount of creatinine?
Chronic muscle wasting
What is creatinine freely filtered by?
The glomerulus
Is creatinine secreted by renal tubules in the dog or horse?
No
Because creatinine is strictly filtered, what does it provide?
Glomerular filtration rate
What is age related to over a lifetime in terms of creatinine?
An increase in serum creatinine
What is one of the biggest causes of an increase in urea?
Our diet
What is the turnover of urea like?
Normal with wear and tear
What leads to greater urea production?
Excessive protein intake
Urea is a very potent osmotic particle. What does this mean?
Water will follow it
What will happen in excessive urea production?
GFR will increase and urine volume will increase
What will the kidney do with urea in dehydration?
It will actively reabsorb urea as a mechanism of retaining water or reducing water excretion
What is the most tightly controlled thing in the body?
H+
What are the 3 mechanisms of control of H+?
Buffers
Respiration
Kidney
What is the control of hydrogen with buffers like?
Instant acting- seconds
What is the control of hydrogen with respiration like?
Fast acting- minutes
What is the control of hydrogen like with the kidney?
Slow, but quantitatively the most important
When does urine formation begin?
When a large amount of fluid that is virtually free of protein is filtered from the glomerular capillaries into Bowman’s capsule
What happens to filtered fluid as it leaves Bowman’s capsule and passes through the tubules?
It is modified by reabsorption of water and specific solutes back into the blood or bu secretion of other substance from the peritubular capillaries into the tubules
What is an example of something that is freely filtered by the glomerular capillaries but it is neither reabsorbed or secreted?
Creatinine
What is the excretion rate like for substances that are freely filtered by the glomerular capillaries but are not reabsorbed or secreted?
It is equal to the rate at which it was filtered and provides a good estimate of GFR
What are examples of things that are freely filtered by the glomerular capillaries but they are partly reabsorbed from the tubules back into the blood?
Na
Cl
Ca
K
What is the excretion rate like for substances that are freely filtered by the glomerular capillaries but are partly reabsorbed from the tubules back into the blood?
It is less than t filtration rate at the glomerular capillaries
What are examples of things that are freely filtered at the glomerular capillaries but are not excreted into the urine because all the filtered substance is reabsorbed from the tubules back into the blood?
Glucose and amino acids
What are examples of things that are freely filtered at the glomerular capillaries and not reabsorbed, but additional quantities are secreted from the peritubular capillary blood into the renal tubules?
H
K
What is found in the cortex?
Glomeruli
Proximal and distal convoluted tubules
What is found in the medulla?
Loop of Henle Collecting ducts Renal papillae Pelvis Ureter
What percentage of cardiac output goes to the kidneys at rest?
20-25%
What percentage of renal plasma flow is filtered?
20%
What percentage of GFR comes out as urine?
1%, the rest is reabsorbed
What percentage of the body’s O2 consumption does the kidney use at rest?
10%
What percentage of the renal blood flow does the renal medulla receive?
1-2%
Describe the proximal convoluted tubule
High solute and water reabsorption
Describe the Loop of Henle
High water resorption
Electrolyte resorption
Distal end passe by original glomerulus
Describe the distal tubule
H2O and Na reabsorption
Describe the collecting duct
Final and variable H2O and Na reabsorption and K excretion
What is the physiological unit of the kidney?
Nephron
What are the glomerular capillaries like relative to normal capillaries?
They are under a high hydrostatic pressure
What does the high hydrostatic pressure with the increased permability of the glomerular capillary bed allow?
Allows 20% of the fluid and smaller solutes out of the capillary and into Bowman’s capsule
What percentage of the filtrate is reabsorbed at the proximal convoluted tubule? Loop of Henle?
65%
25%
What are the 2 types of nephrons?
Cortical and juxtamedullary
Which of the 2 types of nephrons is the most abundant?
Cortical nephrons
Describe cortical nephrons
Loop of Henle is short
Tubule nor vasculature penetrate very far into medulla
Extensive peritubular capillary network
Describe juxtamedullary nephron
Long loop of henle Tubule and vasculature penetrate deeply into medulla Inner medulla hypertonic Counter-current flow Vasa recta
What makes the kidney unique?
It has 2 capillary beds in series being fed from the same arterial input
What does each capillary bed have?
Its own control valve that work independently controlling the path of least resistance
What does the path of least resistance determine?
Whether fluid is filtered to become urine or not
What does the low O2 tension and total RBF make the renal medulla very susceptible to?
Hypoxia
What is most of the oxygen consumed by the kidneys related to?
The high rate of active sodium reabsorption by the renal tubules
What is a glomerulus?
A balled up network of brainching and anatomosing capillaries where the plasma is filtered in the first stage of urine formation
What is the glomerulus surrounded by?
Bowman’s capsule
What does the glomerulus receive its blood supply from?
An afferent arteriole of the renal circulation
What does the glomerulus drain into?
An efferent arteriole rather than a venule
What is the basic filtration unit of the kidney?
A glomerulus and its surrounding Bowman’s capsule
What is the glomerular filtration rate?
The rate at which blood is filtered through all of the glomeruli and this the measure of the overall renal function
What are the 3 major layers of the glomerular capillary membrane and make up the filtration barrier?
The endothelium
A basement membrane
A layer of epithelial cells (podocytes)
What are the endothelial cells of the filtration barrier like?
Fenestrated
What is the basement membrane of the filtration barrier like?
Collagen meshwork
What are the podocytes of the filtration barrier like?
Finger-like extensions that surround capillaries with slits between fingers
What does solute filtration depend on?
Molecular size
Ionic change
What do small molecules do to filterability?
Increase it
What do large molecules do to filterability?
Decrease it
What do cations do to filterability?
Increase it
What do anions do to filterability?
Decrease it
What is an example of something that impact how charge and size change filterability?
Dextrans
What are dextrans?
Polysaccharides that have been used to provide colloidal support in patients with low albumin levels
What is the overall charge of the filtration barrier?
Negative
What is the charge of albumin? What happens to it because of it?
Negative
It is mostly repelled. Some gets across, but is actively reabsorbed by the tubular epithelial cells
How do you get the capillary filtration coefficient?
Hydraulic conductivity x surface area
Is the capillary filtration coefficient under physiologic control?
No
When is the capillary filtration coefficient affected?
In some disease states
Is capillary hydrostatic pressure under physiologic control?
Yes
What is the capillary hydrostatic pressure the primary means of?
Physiologic regulation of GFR
Is capillary colloidal pressure under physiologic control?
No
Is Bowman’s capsule under physiologic control?
No
Is Bowman’s capsule colloidal pressure under physiologic control?
No
Why is the colloidal pressure negligible in Bowman’s capsule?
Protein should not be filtered
What are factors that influence glomerular hydrostatic pressure?
Arterial pressure
Afferent arteriolar resistance
Efferent arteriolar resistance
What does increased resistance of afferent arterioles do?
Makes the path of least resistance to bypass the kidney, decreases renal blood flow, and decreases glomerular hydrostatic pressure decreasing GFR
What does increased resistance of the efferent arterioles do?
Still can reduce overall renal blood flow, but increases glomerular hydrostatic pressure making the path of least resistance across the glomerulus into bowman’s capsule thereby increasing GFR
What does efferent constriction do?
Decreases RBF
Increases GFR
What does efferent dilation do?
Increase RBF
Decrease GFR
What does afferent constriction do?
Decrease RBF
Decrease GFR
What does afferent dilation do?
Increase in RBF
Increase in GFR even without efferent constriction
What is the goal of RBF control?
Maintain constant GFR over a wide range of flow and pressure ranges for waste excretion without excess loss of fluid
What could an increase in blood pressure by 25% do?
Increase urine output
What is a transient increase of blood pressure caused by?
Exercise
Fear
Excitement
What is a persistent increase of blood pressure caused by?
Hypertension in humans
Occasionally occurs in cats and dogs
Rare in horses
What would an increase in cardiac output do?
Increase urine output
What is necessary in hypovolemic states?
Minimize urine production yet maintain waste excretion
What may be necessary in severe hypovolemic states?
Conserve fluid at the expense of waste excretion
What does a strong sympathetic tone do?
Decreases RBF and GFR
What is endothelin?
Potent vasoconstrictor released by damaged vascular endothelial cells
What is the path of renal blood flow control?
Increased arterial pressure Increased stretch of blood vessel Increase smooth muscle Ca permeability Increased intracellular Ca Increased smooth muscle contraction and vascular resistance
What does angiotensin II formation occur during?
Hypovolemic “low flow” states and lowered arterial pressure
What does angiotensin II do?
Constricts efferent arteriole increasing glomerular hydrostatic pressure preserving GFR while reducing renal blood flow
What does the juxtaglomerular complex do?
Senses and regulates Na delivered to distal tubule
What does the macula densa sense?
Na
What is too much Na interpreted as?
Too high GFR, so it needs to decrease
What is too little Na interpreted as?
Too little GFR, so it needs to increase
What are the 2 effects that macula densa has with a decrease in Na?
Afferent arteriolar vasodilation decreasing resistance to blood flow into the glomerulus
Efferent arteriolar vasoconstriction increasing resistance to blood flow out of the glomerulus
What does the macula densa signal juxtaglomerular cells to do?
Release renin
Describe how protein impacts GFR
Increase in amino acids
Increase in proximal tubular amino acid reabsorption
Increase in proximal tubular NaCl reabsortion
Decrease in macula densa NaCl
Decrease in afferent arteriolar resistance
Increase in GFR
What is the most common form of acute renal failure in the horse?
Acute tubular nephrosis
