Ultrafiltration and Selective Reabsorption in the Nephron
Glomerular pressure drives filtration through a selective barrier. Proximal tubule cells return glucose to blood using sodium co-transport and facilitated diffusion. Glucose in urine can reflect excessive filtered glucose or reduced tubular transport.
What a learner can do afterwards
- Explains how arterial blood pressure and arteriole resistance support glomerular filtration.
- Identifies small substances that enter filtrate and large plasma proteins that are mainly retained in blood.
- Traces glucose from filtrate through proximal tubule cells back to blood and explains why glucose in urine alone does not diagnose diabetes.
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Glomerular filtration uses pressure differences, as tissue-fluid formation does. Blood enters through the afferent arteriole and leaves through the efferent arteriole. Arterial blood pressure and resistance to blood leaving help maintain high glomerular pressure. Resistance in both arterioles affects it. Pressure in Bowman's capsule and the osmotic effect of retained plasma proteins oppose filtration.
Fluid crosses the capillary endothelium, basement membrane and filtration slits between podocyte processes into Bowman's capsule. The healthy barrier retains blood cells and most large plasma proteins. Water, ions, glucose, amino acids and urea enter filtrate readily. Only part of the plasma passing through the glomerulus is filtered; not every glucose molecule in that blood leaves it.
Net pressure drives glomerular filtration. Glucose reabsorption begins in the proximal tubule. A sodium-potassium pump uses ATP to export sodium, keeping it low inside the cell. Sodium enters from filtrate down its electrochemical gradient through a sodium-glucose co-transporter, bringing glucose in. Glucose exits through another carrier by facilitated diffusion into tissue fluid, then enters nearby capillaries. Water reabsorption uses osmosis.
At normal blood glucose levels, the proximal tubule reabsorbs almost all filtered glucose. Urine normally contains only a tiny amount, usually too little for a routine urine test to detect. Glucose transport has a maximum rate. If filtered glucose arrives faster than that rate, some remains in the fluid that becomes urine. Filtration and reabsorption are separate processes.
A urine test detects glucose. High blood glucose, including that caused by diabetes, is one possible explanation: the filtered load may exceed glucose reabsorption capacity. Reduced or blocked tubular glucose transport can also allow glucose into urine without high blood glucose. The urine result alone cannot diagnose diabetes. Blood measurements and other information are needed.
Pressure forms filtrate. Tubule transport returns glucose to blood. Glucose in urine can have more than one cause.
Lesson, questions and flashcards written for this dataset. Anatomy and Physiology sections 25.4, 25.5 and 25.6 were used for factual study.
Readings and flashcards written for this dataset using Anatomy and Physiology sections 25.4, 25.5 and 25.6 for factual study. Videos by Khan Academy.
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What enters the glomerular filtrate
Blood flows into the glomerulus through an afferent arteriole and out through an efferent arteriole. Between them, capillaries lie within Bowman's capsule. Fluid filtered from these vessels collects in the capsule before entering the proximal tubule.
Arterial blood supplies pressure, while resistance to outflow helps maintain high pressure within the glomerular capillaries. Resistance in both arterioles affects this pressure. Capsule pressure and the osmotic effect of retained plasma proteins oppose filtration.
The barrier contains capillary endothelium, a basement membrane and filtration slits between podocyte processes. It retains blood cells and most large plasma proteins. Water and small dissolved substances, including glucose and urea, enter the filtrate readily.
Only part of the plasma passing through the glomerulus is filtered. Glucose being freely filtered means that it readily enters this filtered portion. It does not mean that every glucose molecule in the incoming blood leaves the circulation.
Three directions of movement in urine formation
Filtration moves water and small dissolved substances from glomerular blood into Bowman's capsule. Reabsorption returns substances from tubule fluid towards blood. Secretion moves substances from blood or surrounding tissue into tubule fluid. These directions distinguish the processes.
A substance entering filtrate is not automatically a waste product. Glucose and amino acids are normally recovered almost completely. Much of the filtered water and many ions also return to blood. The fluid's composition changes as it travels along the tubule and collecting duct.
Different movements use different mechanisms. Net pressure drives glomerular filtration. Sodium-glucose co-transport brings glucose into proximal tubule cells, while another carrier lets glucose leave by facilitated diffusion. Water reabsorption uses osmosis.
Glucose can remain in urine when its filtered arrival rate exceeds recovery capacity. Reduced tubular glucose transport can also leave it unrecovered. Urinary glucose therefore needs interpretation alongside blood measurements and other information; it does not establish diabetes by itself.
Glucose transport across a proximal tubule cell
A proximal tubule cell has a surface facing filtrate and another facing surrounding tissue fluid. Glucose crosses these surfaces through different transport proteins.
On the tissue-facing side, the sodium-potassium pump uses ATP to export sodium. This maintains low sodium inside the cell and supports an inward sodium electrochemical gradient from the filtrate.
On the filtrate-facing side, a sodium-glucose co-transporter couples sodium entry to glucose entry. Sodium moving down its gradient provides the driving energy. The co-transporter does not break down ATP directly.
Glucose leaves the cell through another carrier by facilitated diffusion. It then passes through tissue fluid into nearby peritubular capillaries. Entry into the tubule cell is only part of the return route to blood.
Normal filtered glucose loads are recovered almost completely, leaving only a tiny amount in urine. If the filtered load exceeds transport capacity, or tubular glucose transport is reduced, more glucose can remain unrecovered. The pump and co-transporter can operate together; their energy dependency does not require them to take turns.
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