Filtration, reabsorption and secretion · Biology Class… | StemPy Lab
Course 15.2·Lesson 1 of 3
Filtration, reabsorption and secretion
Urine forms in three steps. The glomerulus filters the plasma, the tubule takes back almost everything that the body needs, and the tubule adds a few more wastes. This lesson gives the numbers of the filtration and explains what each part of the tubule does.
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Three steps
The kidneys filter about 180 litres of fluid a day, and you pass only about 1.5 litres of urine. So where does the rest go? The answer is in the three steps of urine formation.
Glomerular filtration. The glomerulus filters the plasma.
Reabsorption. The tubule takes back what the body needs.
Secretion. The tubule adds some more wastes and ions to the fluid.
Glomerular filtration
The first step is the filtration of blood by the glomerulus. The pressure of the blood in the glomerular capillaries pushes the plasma out. The blood is filtered through three layers.
The endothelium of the glomerular blood vessels.
The basement membrane.
The epithelium of the Bowman's capsule. Its cells are called podocytes, and they are arranged so as to leave minute spaces, the filtration slits or slit pores.
A drawing of the wall between a glomerular capillary and the space of the Bowman's capsule, seen in section. On the left is the blood in the capillary, with red cells and blue dots for plasma proteins. Three layers lie between the blood and the capsule space. First is the endothelium of the capillary, drawn as pink blocks with gaps that are pores. Second is the basement membrane, a thick grey band. Third is the epithelium of the capsule, made of purple cells called podocytes, whose small foot like processes leave narrow filtration slits. Green arrows show water, glucose, amino acids, urea and salts passing through all three layers into the space of the capsule on the right, which holds the filtrate. Blood cells and proteins stay in the capillary.
The blood is filtered so finely that almost every constituent of the plasma passes into the capsule, except the proteins. This is called ultrafiltration. Water, glucose, amino acids, urea and salts pass through. Blood cells and proteins stay in the capillary. The fluid in the capsule is the filtrate.
The glomerular filtration rate
The amount of filtrate that the kidneys form in a minute is the glomerular filtration rate, or GFR. In a healthy person it is about 125 mL a minute, which is 180 litres a day.
Check this: 125mL/min×1440min=180,000 mL, which is 180 L. The plasma of the body is only about 3 L, so the kidneys filter all of it about 60 times a day.
The kidneys receive about 1100 to 1200 mL of blood a minute, which is about 600 mL of plasma. So about one fifth of the plasma that reaches the kidneys is filtered. This share is called the filtration fraction. It is an extra idea that is not in the NEET syllabus text, but it checks the numbers.
The kidneys keep the GFR steady. The part that does this is the juxtaglomerular apparatus, which you will study in the third lesson.
Reabsorption
You pass about 1.5 L of urine, so the tubules must take back about 99 per cent of the filtrate. This is reabsorption.
A single horizontal bar for one day. The long blue part is the 178.5 litres that are reabsorbed, which is 99.2 per cent of the 180 litres of filtrate. A very small orange part at the end is the 1.5 litres of urine, which is 0.8 per cent. A line of text says that the filtrate forms at 125 millilitres a minute, and that about 1.04 millilitres a minute is passed as urine.
The cells of the tubule do this by two ways.
Active transport takes back glucose, amino acids and Na+ against their concentration gradient, with the use of energy.
Passive transport takes back the nitrogenous wastes in part, and water, by diffusion and osmosis. Water is reabsorbed passively in the initial segments of the nephron.
Tubular secretion
The tubular cells also secrete substances into the filtrate. They secrete H+, K+ and ammonia. This is an important step in urine formation, because it helps to keep the ionic balance and the acid-base balance of the body fluids.
What each part of the tubule does
A drawing of one nephron over a cortex band above and a medulla band below, with a box of text for each part. Proximal tubule: absorbs all the essential nutrients and 70 to 80 per cent of the electrolytes and water, secretes hydrogen ions, ammonia and potassium ions, and absorbs bicarbonate. Descending limb of the loop of Henle: permeable to water and almost impermeable to electrolytes, so the filtrate gets concentrated. Ascending limb: impermeable to water, and electrolytes pass out actively or passively, so the filtrate gets diluted. Distal tubule: conditional reabsorption of sodium ions and water, and of bicarbonate, and secretion of hydrogen ions, potassium ions and ammonia. Collecting duct: large amounts of water are reabsorbed, a little urea enters the medulla, and hydrogen and potassium ions are secreted.
Part
What it does
Proximal convoluted tubule (PCT)
absorbs nearly all the essential nutrients and 70 to 80 per cent of the electrolytes and water; secretes H+, ammonia and K+; absorbs HCO3−
Loop of Henle, descending limb
permeable to water, almost impermeable to electrolytes; the filtrate gets concentrated
Loop of Henle, ascending limb
impermeable to water; electrolytes pass out, actively or passively; the filtrate gets diluted
Distal convoluted tubule (DCT)
conditional reabsorption of Na+ and water; absorbs HCO3−; secretes H+, K+ and ammonia
Collecting duct
takes back large amounts of water; lets a little urea into the medulla; secretes H+ and K+
The PCT is lined with a simple cuboidal brush border epithelium, which increases the surface area for reabsorption. In the loop of Henle, the reabsorption is minimum. Its work is to keep the medulla salty, as you will see in the next lesson.
A note on the numbers. NCERT says that the PCT takes back 70 to 80 per cent of the electrolytes and water. Many physiology books give about 65 per cent. In a NEET question, use the NCERT range.
Beyond the syllabus: why glucose can appear in urine
This part is not in the NEET syllabus text. It is in physiology books, and it explains why glucose appears in the urine of a person with diabetes mellitus. The carriers for glucose in the PCT are limited in number. Take a normal blood glucose of about 90 mg in every 100 mL. The GFR of 125 mL a minute is 1.25 dL a minute. The filtered glucose is 90mg/dL×1.25dL/min=112.5mg/min, and all of it is taken back. When the blood glucose rises above about 180 mg in 100 mL, the carriers cannot keep up, and the extra glucose passes into the urine.
You should find that more than 99 per cent of the filtrate is reabsorbed at the normal rate. If the urine volume rises, the share reabsorbed falls only a little, because the filtrate is so large.
Try it in Python
The program works out the filtrate in a day and the part of it that is reabsorbed.
Practise with new numbers
Common mistakes
Saying that proteins pass into the filtrate. Blood cells and plasma proteins stay in the blood.
Saying that filtration needs energy from the tubule. It is driven by the pressure of the blood in the glomerulus.
Saying that glucose is reabsorbed passively. It is reabsorbed actively.
Mixing up reabsorption and secretion. Reabsorption takes substances back into the blood. Secretion adds substances from the blood to the filtrate.
Saying that the loop of Henle takes back most of the filtrate. It is the PCT that does so.
Three layers: endothelium, basement membrane, podocytes with filtration slits. Everything except proteins and cells passes.
GFR is about 125 mL/min, or 180 L a day. About 99 per cent is reabsorbed. About 1.5 L of urine is passed.
Active: glucose, amino acids, Na+. Passive: nitrogenous wastes in part, water in the initial segments.
Secreted: H+, K+, ammonia. The PCT: nutrients, 70 to 80 per cent of electrolytes and water.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the three layers through which the blood is filtered.
What is the GFR in a healthy person, and how many litres of filtrate is that in a day?
About what share of the filtrate is reabsorbed, and why is the urine so small a volume?
Name three substances that the tubule secretes into the filtrate. Why is secretion important?
Which part of the tubule has a brush border, and what does it take back?
References
NCERT (2023). Biology, Textbook for Class XI, chapter 16, Excretory Products and Their Elimination.
Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapters on glomerular filtration and on tubular reabsorption and secretion.
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 25, The Urinary System.
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