Physiology: Kidney Structure & Function Flashcards
List some functions of the kidney (10 possible answers below)
- Water balance (~1.5L of urine excreted /day)
- Salt balance (~10g of salt excreted /day)
- Maintenance of plasma volume and osmolarity (due to H2O and salt balance + fluid compartment shift)
- Acid-base balance (excrete H+ and reabsorb HCO3-)
- Excrete metabolic waste products (e.g., urea, bilirubin, uric acid)
- Excrete exogenous foreign compounds (e.g., drugs, food additives, pesticides)
- Control arterial blood pressure (through granular cell secretion of renin)
- Stimulate rbc production (through secretion of erythropoietin)
- Convert vitamin D to active calcitrol (promoted Ca2+ absorption in the GI tract)
Why do the kidneys play such a vital role in the body’s homeostasis?
Balance of water and electrolytes in the body depends on balance between body input and outputs
The kidneys control excretion of water and electrolytes
What does the urinary system consist of?
The kidneys (produce urine) The structures that store and carry the urine from the kidneys to the outside of the body e.g., ureter, bladder, urethra
The kidney receives ~X% of cardiac output
20-25%
What is a nephron?
How many are found in each kidney?
The functional unit of the kidney i.e., the smallest unit which is able to produce urine
~1 million nephrons are found in each kidney
What are the 3 processes a nephron goes through to produce urine?
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
Describe the unique blood supply to a nephron
- The renal artery gives off afferent arterioles
- These supply the glomerulus (a tangle of capillaries within the Bowman’s capsule)
- The efferent arteriole carries blood exiting the glomerulus
- Peritubular capillaries follow the loop of Henle down into the medulla
- Deoxygenated blood drains to the renal venules, then back to the renal vein
What are the 5 parts of the tubular component of the nephron?
- Bowman’s capsule (where the initial tubular fluid is formed and drains)
- Proximal tubule
- Loop of Henle (descends into medulla then ascends back into cortex)
- Distal tubule
- Collecting duct (receives tubular fluid from many different nephrons and passes it down the collecting duct to the renal pelvis)
Name the two types of nephron and state what proportion of the nephrons they make up
Cortical nephrons (~80%) Juxtamedullary nephrons (~20%)
What are the 3 main differences between the cortical and juxtamedullary nephrons?
Juxtamedullary nephrons have…
- A longer Loop of Henle which descends further down into the renal medulla
- A single vasa recta that follows the Loop of Henle rather than peritubular capillaries
- Produce more concentrated urine
Describe the structure of the glomerulus
- A tangle of capillaries held within the Bowman’s capsule
- Supplied by an afferent arteriole and drained by an efferent arteriole
- Capillary wall made up of fenestrated endothelium cells
- Basement membrane separates endothelium from the podocytes making up the inner layer of the Bowman’s capsule
What is the function of the glomerulus?
As blood is delivered through the glomerulus, a portion is filtered and enters the lumen of the Bowman’s capsule to form the initial tubular fluid -> i.e., it is the site where filtration begins
What is the juxtaglomerular apparatus?
The region of the distal convoluted tubule that passes between the afferent and efferent arterioles of the nephron
(juxta-glomerular = next to the glomerulus)
Name two specialised cell types found in the juxtaglomerular apparatus
Macula densa cells
Granular (aka juxtaglomerular) cells
Where are the macula densa cells found in the juxtaglomerular apparatus and what do they do?
- Located in the wall of the distal convoluted tubule
- They detect the amount of NaCl (salt) in the tubular fluid passing through them, then send signals to the smooth muscle cells lining the afferent arteriole to cause vaso- constriction or dilation in order to reduce or increase blood flow to the glomerulus
An increase in the salt concentration of the tubular fluid causes the macula densa cells to signal the afferent arteriole to vasodilate/vasoconstrict?
Vasoconstrict (to reduce flow through the glomerulus, hence reducing GFR)
A decrease in the salt concentration of the tubular fluid causes the macula densa cells to signal the afferent arteriole to vasodilate/vasoconstrict?
Vasodilate (to increase flow through the glomerulus, hence increasing GFR)
Where are the granular cells of the juxtaglomerular apparatus located and what do they do?
- Located in the tunica media of the afferent arteriole
- They are modified vascular cells that produce and secrete renin to initiate RAAS for increasing BP
?% of the plasma that enters the glomerulus is filtered?
20%
The rest exits via the efferent arteriole
Plasma that leaves in the efferent arteriole is then carried into…?
Peritubular capillaries (cortical nephron) Vasa recta (juxtamedullary nephron)
How is the filtered plasma modified in the tubules? (2)
- Some substances are reabsorbed into the peritubular capillaries/vasa recta (tubular reabsorption)
- Some substances are secreted from the peritubular capillaries/vasa recta and added to the filtrate (tubular secretion)
What is urine?
The final modified filtrate of the blood, produced as a by-product of the kidneys’ regulation of ECF composition and volume
How does rate of excretion relate to rate of filtration, reabsorption, and secretion?
Rate of excretion = Rate of…
Filtration - reabsorption + secretion
What is meant by the rate of filtration of a substance?
The mass of the substance filtered into the Bowman’s capsule per unit time
What is the equation for calculating rate of filtration of a substance?
Rate of filtration = plasma conc. x GFR
As adult GFR is constant at ~125 mL/min, what does this tell us about the relationship between plasma conc. of a substance and filtration rate?
(Rate of filtration = plasma conc. x GFR)
As plasma conc. of the substance increases, rate of filtration increases
As plasma conc. of the substance decreases, rate of filtration decreases
What is meant by the rate of excretion of a substance?
The mass of the substance excreted into the urine per unit time
What is the equation for calculating rate of excretion of a substance?
Rate of excretion = urine conc. x urine production rate (Vu)
What is the average urine production rate?
~1.5L/day which is ~1mL/minute
Value varies depending on hydration status
Rate of filtration and rate of excretion are relatively easy/hard to measure and calculate?
Easy
Rates of reabsorption and secretion reflect the function of X, and this is obtained as the difference between Y and Z
X - tubular modification of filtrate
Y - filtration
Z - excretion
How can you tell if net reabsorption of a substance has occurred?
If rate of filtration > rate of excretion
How is rate of reabsorption of a substance calculated?
Rate of reabsorption = rate of filtration - rate of excretion
(these are the first 2 equations mentioned)
How can you tell if net secretion of a substance has occurred?
If rate of filtration < rate of excretion
How is rate of secretion of a substance calculated?
Rate of secretion = rate of excretion - rate of filtration
these are the first 2 equations mentioned
