REABSORPTION Flashcards
• The body cannot lose______ every minute
• Therefore, some substances in the urine need to be_____
120-125 mL of water
REABSORBED
- returns of most of the filtered water and many solutes to the bloodstream
• About_____% of filtered water reabsorbed
•___________ make the largest contribution
TUBULAR REABSORPTION
99%
Proximal convoluted tubule cells
- Movement of a substance across a cell membrane and against an osmotic gradient (always transcellular)
Active transport
• Needs a carrier protein to transport substance and requires energy (e.g., hydrolysis of ATP)
Active transport
- movement is due to differences in the concentration or electrical potentials (may be transcellular or paracellular)
• Does not need carrier protein and does not require energy
Passive transport
REABSORPTION OF SODIUM
• Sodium diffuses across the luminal membrane (also called the______) into the cell down an electrochemical gradient
• Sodium is transported across the basolateral membrane against an electrochemical gradient by the______
apical membrane
Nat-K+ ATPase pump.
• An example of secondary active transport
SGLT (sodium glucose linked transporter)
GLUT (glucose transporter)
•__________
• Carries glucose into the tubular cell cytoplasm
•_________
• Helps glucose diffuse out of the cell (facilitated diffusion)
SGLT (sodium glucose linked transporter)
GLUT (glucose transporter)
Reabsorption of proteins and water
PCT reabsorbs proteins via_______
• Digestion into amino acids in vesicles
pinocytosis
Reabsorption of proteins and water
• Water reabsorption
• Due to_______ and ______
• Dependent on______
aquaporins and tight junctions
membrane permeability
Reabsorption of proteins and water
• Water reabsorption
________: always high due to abundant aquaporins
_________: always low (low surface area and less permeable tight junctions)
• PCT/DLOH (descending loop of henle)
• ALOH (ascending loop of henle)
•_____: passive reabsorption via urea transporters
•______: transcellular passive diffusion
• Transported along with sodium due to lumen negative potential
Urea
Chloride
Active transport
•- Highest rate of reabsorption of a substance before it appears in urine; amount of solute reabsorbed per minute
Maximal reabsorptive capacity (Tm)
• Example: Glucose is at 350 mg/min. (once this is reached, all nephrons have reached maximal capacity to reabsorb glucose)
Maximal reabsorptive capacity
Active transport
• Example: Glucose is at 160-180 mg/dL
• Note: When the plasma glucose concentration exceeds the transport maximum, reabsorption stops, and the substance is excreted in urine.
Renal threshold
Active transport
_________- the plasma concentration of substances at which active transport of reabsorption stops
Renal threshold
• Highly metabolic (high number of mitochondria) and contains many brush borders
PCT
PCT
_______: sodium reabsorption and co-transport with glucose, amino acids, other solutes
_______: sodium and chloride reabsorption
• Early PCT
• Late PCT
• Early PCT:…
• Late PCT:…
sodium reabsorption and co-transport with glucose, amino acids, other solutes
sodium and chloride reabsorption
• Thin epithelial membranes with no brush borders and few mitochondria
• Simple diffusion of water and some solutes
• Thin Descending loop of henle
• Thick epithelial cells with high metabolic activity
• Thick ALOH
Thick ALOH
• Active reabsorption of (3) via______
sodium, potassium, and chloride
NKCC2 co-transporter
• Note: Loop diuretics (3) are inhibitors of NKCC2
furosemide
ethacrynic acid
bumetanide
_______is impermeable to water
ALoH
• As filtrate moves through the ascending limb, NaCl moves out making the filtrate more and more______
dilute
Significance: Keeping the renal medulla region of the kidney at high osmolarity so water can move passively out of the filtrate in the CT
Loop of henle
DCT
Early distal tubule
• First portion: forms the______
• Second portion: same reabsorptive characteristics of the______
macula densa
thick ALOH
DCT
Avid reabsorption of mostions
• Sodium-chloride co-transporter
• Note:_______ diuretics inhibit sodium-chloride co-transporter
Thiazide diuretics
• Virtually impermeable to water (requires ADH for water reabsorption) and urea
DCT
Tm of glucose = ~
• Once this limit is reached, any extra glucose is not reabsorbed and is lost in the urine (glycosuria).
350 mg/min
Glucose has a renal threshold of…
160-180 mg/dL
Tm vs renal threshold
Tm is about the reabsorption rate (mg/min)
renal threshold is about blood concentration (mg/dL)
• Permeable to water but NOT to solutes.
• Water moves out of the tubule into the hypertonic renal medulla (passive osmosis via aquaporins).
• No significant active transport occurs here.
Thin Descending Limb of Loop of Henle (DLOH): Passive Water Reabsorption
• Impermeable to water but actively reabsorbs Na⁺, K⁺, and Cl⁻ using the NKCC2 (Na⁺/K⁺/2Cl⁻ co-transporter).
• Creates a high osmolarity in the renal medulla, which helps water reabsorption in the collecting duct later.
• Loop diuretics (e.g., furosemide, bumetanide, ethacrynic acid) block NKCC2, preventing Na⁺ reabsorption and causing water loss (diuresis).
Thick Ascending Limb of Loop of Henle (ALOH): Active Na⁺ Reabsorption
The______ is similar to the thick ascending limb and is responsible for reabsorbing sodium, chloride, and other ions but is impermeable to water (unless ADH is present).
early DCT
Reabsorption in…
• Uses the Na⁺/Cl⁻ co-transporter to actively reabsorb sodium and chloride.
• Thiazide diuretics inhibit this transporter, leading to sodium and water excretion.
• The first portion of the DCT contains the macula densa, which senses sodium concentration and regulates the renin-angiotensin-aldosterone system (RAAS).
Early DCT
Late Distal Tubule & Cortical Collecting Tubule
Regulated by…
Aldosterone & ADH
Late DCT and cortical collecting tubules
Principal Cells: Aldosterone-Regulated Na⁺ Reabsorption
• Reabsorb Na⁺ via_______
• Secrete K⁺ via______(regulated by aldosterone).
• Aldosterone increases_____ reabsorption and _____ secretion, helping to regulate blood pressure.
• Potassium-sparing diuretics (spironolactone, amiloride) block aldosterone and ENaC, reducing sodium reabsorption.
epithelial sodium channels (ENaC).
ROMK channels
Na⁺ reabsorption & K⁺ secretion
Target of potassium-sparing diuretics
• Mineralocorticoid receptor antagonists, e. g. spironolactone
• Sodium channel blockers, e. g. amiloride
DCT and collecting tubules
• Permeability to water depends on ADH (binds only to aquaporin-2)
DCT
Final site for urine processing
Medullary collecting duct
Medullary collecting duct
• Permeability to water and urea is controlled by______
• Presence of aquaporins and urea transporters (urea goes back to the_____)
ADH
DLOH
Countercurrent mechanism
• Renal concentration begins in the loop of Henle where filtrate is exposed to high gradient of the renal medulla.
• Water is reabsorbed though osmosis in the_______
• Na and Cl are reabsorbed in the______
• **Dilution of the highly concentrated medulla is prevented by the impermeable walls of the______ to water.
• Therefore, most of the water delivered to this segment remains in the tubule, despite reabsorption of large amounts of solute.
• Thus, the tubular fluid becomes very dilute and results into diluted urine.
descending loop of Henle
ascending loop of Henle
ascending loop
Active transport
Substances
Glucose, aa, salts
Chloride
Sodium
Passive transport
Substances
Water
Urea
Sodium
CONTROL OF WATER LOSS
How concentrated or diluted your urine is, depends upon the body’s state of_____
hydration
•Dehydration =______ urine
•Drinking lots of water =_____ urine
hypertonic urine (more concentrated)
hypotonic urine (called water diuresis)
High blood osmolarity in a dehydrated person → stim._______ → release
_______→ ^_______ in cell membrane, ^ CD’s water permeability.
Pituitary gland
ADH
aquaporin channels
• The active transport of substances from the peritubular capillaries into the tubular lumen, opposite of reabsorption.
• Important for eliminating waste products, excess ions, and maintaining acid-base balance.
Tubular secretion
Passage of substances from the blood in the peritubular capillaries into the tubules
• Elimination of waste products not filtered by the glomerulus
• Regulation of acid-base balance in the body through the secretion of hydrogen
Tubular secretion
• Many foreign substances such as medications are not filtered by the glomerulus because they are bound to proteins, but they develop affinity to the tubules which causes them to dissociate from their protein carriers = SECRETED
Tubular secretion
Normal blood pH:____ (excess acids must be eliminated)
7.4
: responsible for buffering capacity
• Readily filtered and reabsorbed
Bicarbonate ions
________: secreted by RTE cells into the filtrate (sodium-hydrogen exchanger)
• Prevents the filtered bicarbonate from being excreted
• Hydrogen ions
The kidneys regulate acid-base balance by reabsorbing_____, excreting______, and using _________as buffers.
bicarbonate
hydrogen ions
phosphate and ammonium
Urinary Excretion
Urinary Excretion = Glomerular Filtration - Tubular Reabsorption + Tubular Secretion
• : Initial filtration of blood, removing wastes and excess substances.
• : Recovers essential nutrients, water, and ions.
• : Removes additional wastes and regulates acid-base balance.
Glomerular Filtration
Tubular Reabsorption
Tubular Secretion
