Reabsorption and Secretion

Proximal Convoluted Tubule

The proximal convoluted tubuleThe first part of the nephron tubule where most reabsorption of water, ions, and nutrients occurs. drains the filtrateThe fluid that is filtered from the blood into the nephron and will eventually become urine. from the capsular spaceThe space between the glomerular capsule and the glomerulus where filtrate collects. of the renal corpuscleThe structure in the nephron that consists of the glomerulus and Bowman’s capsule, where filtratio.  This filtrate is rich in everything that we don’t want leaving the body via urineThe liquid waste excreted by the kidneys..  No, seriously.  The simple cuboidal cellsThe basic structural and functional units of life. of the PCT have many surface-area expanding villiFinger-like projections in the small intestine that increase surface area for absorption.. They reabsorb any ionsCharged atoms or molecules. like sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. and calcium cation. Energy moleculesGroups of atoms bonded together. such as glucoseA simple sugar that is the main source of energy for cells. and fatty acids are also absorbed. They reabsorb pretty much anything that we want to keep. 

Remember that glomerular filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre separates everything by size, not need.  Albumin is represented by my eggs here. It is the dividing line in glomerular filtration. This determines whether an item is kept in the glomerulusA network of capillaries in the nephron where blood filtration occurs. or excused into the filtrate in the capsular space.  This reminds me of my parents. 

My father is a pack rat, borderline hoarder. After my Mom passed, I realized that she was the force. She kept my father from tipping that balance to becoming a hoarder.  But, during my childhood, my Mom was always cleaning stuff out. She made piles to go to the Salvation Army. My father would always be going through that pile. He would say, “Oh, how can you throw this away?  This is awesome!  Of course, we want to keep this!”  I feel like that’s what the PCT is always saying to the glomerulus. It asks, “How come you threw away all this sodium?  We need sodium!  I’m taking this back.” The PCT does not do much secretionThe process of moving substances from the blood into the nephron tubule to be excreted in urine..  It’s all about reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p.  The only thing that the PCT might secrete is hydrogen cations(H⁺): Protons that influence pH levels in body fluids..  This would make the blood less acidicA solution with a pH below 7, having a higher concentration of H⁺ ions. and make the filtrate, and ultimately the urine

Descending Nephron Loop

As filtrate descends into the descending loopThe portion of the nephron loop that is permeable to water but not solutes, leading to water reabsor, the cells transition to cuboidal cells with no villi. As the descending loop dips into the medulla of the kidney, simple squamous cells make up the nephronThe functional unit of the kidney that filters blood and produces urine. loop.  The descending nephron loop reabsorbs as much waterThe universal solvent essential for life. as possible. It moves water from the filtrate to the blood in the peritubular capillary or a vasa rectaCapillaries surrounding the loop of Henle in juxtamedullary nephrons that help maintain the medullar. 

Juxtamedullary nephrons dip deepAway from the surface of the body. into the medullary pyramids. The environment becomes increasingly salty. This process extracts more and more water from the nephron.  These nephrons are active when you are dehydrated. They have loops and an extended distance over which they can reabsorb water. As the filtrate continues to the bottom of the loop, it becomes more and more concentrated with solutes.  It’s losing water!  The colloid osmotic pressureThe pressure exerted by proteins (mainly albumin) in the blood that pulls water into the capillaries of the filtrate is increasing.

Ascending Nephron Loop

Similar to the descending loop, the ascending loopThe portion of the nephron loop that actively transports sodium and chloride out, making the medulla has a thin segment. This segment is made of simple squamous cells. It also has a thick segment made of simple cuboidal cells.  When the filtrate enters the ascending loop, it is very concentrated with solutes. The water was sucked out of it by the descending loop.  The ascending loop is tasked with reclaiming sodium, which is intensely important for our blood colloidA mixture where small particles are dispersed but not dissolved in a liquid. osmotic pressureThe force exerted by water moving across a membrane due to osmosis..  Sodium and potassium(K⁺): Major ICF cation; essential for muscle and nerve function. cations and chloride anions are all reabsorbed here in the ascending loop.  There are active transporters embedded in the cells of the loop. These transporters can move 1 sodium cation, 1 potassium cation, and 2 chloride anions at once.  But, the problem is, I don’t really want the potassium.  I want the sodium and the chloride, please, but not the potassium.  So….the Na/K pump secretes the potassium back into the filtrate.  Now, since our solutes are being reabsorbed, we expect that water is following them because water follows salt, right?  Well, it does, but not here.  As these ions move from the filtrate to the capillary, water does not follow. This happens because the cells of the ascending loop do not have aquaporins.  They don’t have any channelsProtein passages in the cell membrane that allow specific molecules to pass through. or transports that water can use.  The ascending loop is impermeable to water!  It is becoming dilute from the loss of solutes. 

Distal Convoluted Tubule

There is a lot of action at the DCT.  Not as much action as the PCT.  In the DCT, we have simple cuboidal cells with some, not a lot, but some villi.  Again, this surface area-expanding feature allows the DCT to move lots of substances, in either direction.  We still have the collecting ductA duct in the nephron that collects urine from multiple nephrons and adjusts water reabsorption. to do. However, the DCT is really the last chance for us to adjust anything other than water. It also allows us to adjust pHA measure of hydrogen ion concentration in a solution..  So, if there’s glucose in the filtrate, here’s where you take it back.  If there’s too much potassium in the blood, here’s where you get rid of it.  Are you dehydrated? Well, then, let’s take back some water.  What about pH?  Can we reabsorb or secrete hydrogen cations?  What about bicarb?  Do we need more of that?

The DCT and CD have receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter that allow them to respond to ADH, the anti-diuretic hormone, and aldosteroneA hormone that increases sodium and water reabsorption in the kidneys, helping regulate blood pressu.  If the DCT receives ADH, it will insert aquaporins in the simple cuboidal cells and start reabsorbing water as told.  If the DCT receives aldosterone, it inserts little Na/K pumps to reabsorb the sodium and secrete the potassium.  These are talked about more in the mini lecture on water reabsorption.

Collecting Duct

The collecting due has a high diameter because it is collecting filtrate from many nephrons, not just one.  The collecting duct has two populations of cells, which are also present in the DCT, I should mention.  Principle cells are able to respond to ADH and aldosterone, as described for the DCT.  Intercalated cells are involved in pH balance.  Here in the collecting duct, it’s our last chance to deal with hydrogen cations and bicarbonate(HCO₃⁻) – A crucial buffer in blood that helps maintain pH balance; formed when carbon dioxide anions.  These intercalated cells secrete hydrogen cations, making you less acidic and making your urine more acidic.  If your urine is acidic, these cells are working to prevent YOU from being acidic.  Here’s the problem, these intercalated cells will only secrete a hydrogen cation if it’s traded for a reabsorbed potassium cation.  Remember, we always want to be secreting potassium.  I feel the CD can undo the good work. The DCT has done well on reabsorbing sodium and secreting potassium.  All collecting ducts descend into the renal pyramids. They converge on the renal papillaThe tip of a renal pyramid where urine drains into the minor calyx. where they weep urine into a minor calyxA small cavity in the kidney where urine from a renal pyramid collects before moving to the major ca. This urine is collected in a major calyxA large cavity in the kidney where urine from the minor calyces collects before entering the renal p and then the renal basin.