Blood Through the Organs
The same litre of blood that just dropped off oxygen in your leg muscle is, seconds later, picking up glucose in your gut and dumping urea at your kidney. Blood is not a passive river, its chemical makeup changes profoundly depending on which organ it has just passed through. Read the blood at any point in the body and you can deduce exactly where it has been.
Orient yourself
Predict, scan the goals and key terms, and clear the misconception this lesson exists to fix.
Before You Read
warm-upBlood arrives at your lungs carrying lots of carbon dioxide and little oxygen. It leaves carrying lots of oxygen and little carbon dioxide.
Before reading: predict how blood changes as it passes a working muscle, and as it passes the small intestine after a meal. For each, say what is added and what is removed.
Learning Intentions
goalsKnow
- What each organ adds to or removes from the blood
- The roles of the lungs, liver, gut, kidneys and muscles in changing blood content
- The difference between gases, nutrients and wastes in blood
Understand
- Why blood concentration changes depend on the organ's function
- Why exchange always follows a concentration gradient
- How the liver buffers nutrient levels after a meal
Can Do
- Account for changes in blood gases, nutrients and wastes across each organ
- Predict the composition of blood leaving a named organ
- Identify an organ from a description of blood entering and leaving it
Scan these before reading
vocabMisconceptions To Fix
watch outOdd one out: Three of these statements about blood composition are correct. Which one is wrong?
One Rule Explains Every Change
Organs add what they make and remove what they need
Core Content
One Rule Explains Every Change
+5 XPOrgans add what they make and remove what they need
Every change in blood composition follows one simple logic: an organ removes from the blood whatever it consumes, and adds to the blood whatever it produces or absorbs.
Substances move down their concentration gradient. Where an organ uses a substance (so its concentration inside cells is low), that substance leaves the blood; where an organ produces or releases a substance, it enters the blood.
We track three categories: gases (oxygen and carbon dioxide), nutrients (glucose, amino acids) and wastes (carbon dioxide, urea). At any organ, the blood loses what the organ takes up and gains what the organ releases.
So predicting how blood changes never requires memorising lists, it only requires asking: "what does this organ do, and therefore what does it take from or give to the blood?"
Pause, copy the highlighted rule and the three categories into your book before moving on.
Substances move between blood and tissue by moving down their concentration _____.
Organ by Organ
Lungs, gut, liver, kidneys and muscles each change the blood
Organ by Organ
+5 XPLungs, gut, liver, kidneys and muscles each change the blood
We just saw the single rule. That raises a question: what does each specific organ add and remove? This card answers it → an organ-by-organ account you can reproduce in an exam.
Applying the rule to each organ in turn gives a complete map of how blood content rises and falls around the body.
Text description
- Lungs: O₂ added, CO₂ removed.
- Small intestine: glucose and amino acids added.
- Liver: stores glucose; urea added.
- Muscle: O₂ and glucose removed; CO₂ added.
- Kidney: urea removed; excess salts and water removed.
- Key: + means added to the blood; − means removed from it.
Lungs: O₂ up, CO₂ down. Small intestine: glucose and amino acids up. Liver: stores/releases glucose, makes urea (urea up). Muscles: O₂ and glucose down, CO₂ up. Kidney: urea, excess salts and water down.
Pause, copy the highlighted organ-by-organ changes into a table in your book.
Compared with blood entering it, blood leaving a working muscle has:
Read the Blood, Name the Organ
Do the activity in your book, then check what you have understood.
Activity 1
Read the Blood, Name the Organ
Pattern B, Reverse reasoning
For each clue, name the organ the blood has just left and justify it. (a) Blood high in glucose and amino acids just after a meal. (b) Blood low in urea after being high. (c) Blood high in oxygen and low in carbon dioxide. (d) Blood high in carbon dioxide and low in glucose. Explain your reasoning each time using "the organ took up / released…".
Match each organ to the change it makes to the blood passing through it:
- Lungs
- Small intestine, after a meal
- Liver
- Kidney
- Glucose buffered toward normal, urea added
- Oxygen added, carbon dioxide removed
- Urea and excess salts and water removed
- Glucose and amino acids added by absorption
The Liver: The Blood's Manager
Why the liver receives blood straight from the gut
The Liver: The Blood's Manager
+5 XPWhy the liver receives blood straight from the gut
We just mapped each organ. That raises a question: why is the liver special, and why does blood from the gut go to it first? This card answers it → the liver smooths out the spikes in nutrients and makes urea.
After a meal, nutrient levels could swing wildly, the liver exists to buffer these swings so that the rest of the body receives a steady supply.
Nutrient-rich blood from the small intestine travels in the hepatic portal vein directly to the liver before reaching the rest of the body. When glucose is high, the liver stores the excess as glycogen; when blood glucose falls, it releases glucose back. This keeps blood glucose roughly steady regardless of when you last ate.
The liver also handles excess amino acids, which cannot be stored. Through deamination it removes their amino groups, producing ammonia that is converted to urea. Urea then enters the blood, which is why blood leaving the liver carries more urea, and is later removed by the kidneys.
Blood from the gut reaches the liver first via the hepatic portal vein. The liver buffers blood glucose (stores it as glycogen, releases it when low) and deaminates excess amino acids to make urea, which it adds to the blood for the kidneys to remove.
Add the liver's two jobs, glucose buffering and urea production, to your notes before the check below.
Blood leaving the lungs has more oxygen and less carbon dioxide than blood entering them.
The liver produces urea, so blood leaving the liver tends to carry more urea.
The kidney adds urea to the blood as blood passes through it.
Why blood tests reveal organ problems
Where this biology is doing real work outside the classroom.
Why blood tests reveal organ problems
Because each organ changes specific substances in the blood, a simple blood test can flag where something is wrong. High blood urea can signal that the kidneys are not clearing it; raised liver enzymes appear when liver cells are damaged; abnormal blood glucose points to problems with how the liver and pancreas regulate sugar. Doctors essentially do exactly what you practised in this lesson, reading the blood's composition to work out which organ is involved, which is why a blood test is one of the most powerful diagnostic tools in medicine.
Account for the changes in the concentration of gases, nutrients and wastes in the blood as it travels through the small intestine, then the liver, then a working leg muscle, then the lungs, one hour after a large meal.
Account for means give the reasons for the changes, not merely state them. Every rise and fall must be attributed to something the organ does, which is what turns a list into an explanation.
6 marks across four organs. Use one rule throughout: an organ removes from the blood whatever it consumes and adds whatever it produces or absorbs, with substances moving down their concentration gradients.
Substances move between the blood and the tissues down their concentration gradients. Where an organ consumes a substance its concentration inside the organ's cells is low, so that substance leaves the blood; where an organ produces, releases or absorbs a substance, it enters the blood. Predicting any change therefore only requires asking what the organ does.
One hour after a large meal, chemical digestion is complete and absorption is at its peak. Glucose and amino acids are absorbed across the villi into the blood, so the concentration of nutrients in the blood leaving the small intestine rises sharply. Oxygen falls slightly and carbon dioxide rises slightly, because the intestinal wall is respiring like any other tissue.
Nutrient-rich blood travels from the small intestine directly to the liver in the hepatic portal vein rather than going to the rest of the body first. The liver buffers the spike: with glucose high, it stores the excess as glycogen, so the glucose concentration in blood leaving the liver is brought back toward normal. Excess amino acids cannot be stored, so the liver deaminates them, removing the amino group to produce ammonia which is converted to urea. Urea therefore enters the blood, and the concentration of urea in blood leaving the liver rises.
A working muscle is respiring rapidly, so it consumes oxygen and glucose from the blood and produces carbon dioxide. Oxygen and glucose concentrations therefore fall in the blood leaving the muscle, and carbon dioxide rises. Urea is unchanged here, since the muscle neither makes nor removes it.
At the lungs, the blood arrives low in oxygen and high in carbon dioxide. Oxygen diffuses from the high concentration in the alveolar air into the blood, and carbon dioxide diffuses from the blood into the alveolar air, both moving down their concentration gradients across the thin alveolar wall. The concentration of oxygen therefore rises and that of carbon dioxide falls in blood leaving the lungs. Urea is unchanged, because it is removed at the kidneys and not here.
Substances move between blood and tissue down their concentration gradients, so an organ removes from the blood whatever it consumes and adds whatever it produces, releases or absorbs. One hour after a large meal, absorption at the small intestine is at its peak, so glucose and amino acids pass across the villi into the blood and the concentration of nutrients in blood leaving it rises sharply, while oxygen falls slightly and carbon dioxide rises slightly because the intestinal wall is itself respiring. That nutrient-rich blood travels directly to the liver in the hepatic portal vein, where the spike is buffered: with glucose high the liver stores the excess as glycogen, so glucose in blood leaving the liver is brought back toward normal, while excess amino acids, which cannot be stored, are deaminated to produce ammonia that is converted to urea, so the concentration of urea in blood leaving the liver rises. At a working leg muscle, rapid respiration consumes oxygen and glucose and produces carbon dioxide, so oxygen and glucose fall and carbon dioxide rises in the blood leaving it, while urea is unchanged because the muscle neither produces nor removes it. At the lungs the blood arrives low in oxygen and high in carbon dioxide; oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses out of the blood into the alveolar air, both down their concentration gradients across the thin alveolar wall, so oxygen rises and carbon dioxide falls, while urea is again unchanged since it is removed at the kidneys rather than here.
Blood is sampled from two vessels in the same person moments apart. Sample A is high in urea; sample B is low in urea. Both samples are low in oxygen. Deduce which organ each sample was taken from, and account for the difference in urea concentration.
Deduce asks you to reach a conclusion from the information given, by reasoning rather than recall. The evidence is the blood composition; the conclusion is the organ. Show the reasoning, because that is what is being marked.
5 marks. Work out what each substance tells you, use urea as the discriminator, identify each organ, then account for the difference by what each organ does.
Both samples are low in oxygen, so both were taken from blood that has already passed through a respiring tissue and not yet returned to the lungs. That is consistent with almost any organ other than the lungs themselves, so oxygen alone cannot distinguish the two; the urea concentration must do the work.
Sample A is high in urea, so it was taken from blood leaving an organ that adds urea. Only one organ produces urea: the liver. Sample A was therefore taken from the vessel leaving the liver, the hepatic vein.
Sample B is low in urea, so it was taken from blood leaving an organ that removes urea. That organ is the kidney, so sample B came from the renal vein.
Excess amino acids absorbed from the diet cannot be stored by the body. The liver removes their amino groups in a process called deamination, producing ammonia, which is highly toxic and is immediately converted into the much less toxic urea. That urea is released into the blood for transport, which is why blood leaving the liver carries a raised urea concentration.
The kidney removes urea from the blood by filtering it out into the filtrate, from which it is not reabsorbed, so it passes into the urine and leaves the body; blood leaving the kidney is correspondingly low in urea. The two samples therefore capture the two ends of one pathway: the liver produces urea and loads it into the blood, the circulatory system carries it, and the kidney clears it. This is also why a failing liver and a failing kidney produce opposite blood-urea results, and why blood urea is used clinically to assess kidney function.
Both samples are low in oxygen, so both come from blood that has passed through a respiring tissue and not yet returned to the lungs; that is true of almost any organ, so oxygen cannot distinguish them and the urea concentration must. Sample A is high in urea, so it left an organ that adds urea to the blood, and the only organ that produces urea is the liver, so sample A was taken from the hepatic vein. Sample B is low in urea, so it left an organ that removes urea, which is the kidney, so sample B was taken from the renal vein. The liver raises blood urea because excess amino acids absorbed from the diet cannot be stored: the liver deaminates them, removing the amino group to produce ammonia, which is highly toxic and is immediately converted to the far less toxic urea, and that urea is released into the blood for transport. The kidney lowers blood urea because it filters urea out of the blood into the filtrate, from which it is not reabsorbed, so the urea passes into the urine and leaves the body while the blood leaving the kidney is correspondingly depleted. The two samples therefore capture opposite ends of one pathway, in which the liver produces urea and loads it into the blood, the circulatory system transports it and the kidney clears it — which is also why blood urea is used clinically to assess kidney function.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 3 questions · 14 marks
+5 XPAnalyseBand 5(5 marks) 1. Account for the changes in the concentration of gases, nutrients and wastes in the blood as it passes through (i) the lungs and (ii) the small intestine after a meal.
Lungs: 1 mark O₂ added, 1 mark CO₂ removed, 1 mark reason (gas exchange down gradients). Small intestine: 1 mark nutrients added by absorption, 1 mark explanation linking to digestion/absorption.
AnalyseBand 5(4 marks) 2. Blood is sampled from two vessels in the same person. Sample A is high in urea; sample B, taken moments later from a different vessel, is low in urea. Identify which organ each sample was taken after, and account for the difference.
1 mark: sample A identified as leaving the liver · 1 mark: sample B identified as leaving the kidney · 1 mark: explains that the liver deaminates excess amino acids and produces urea, adding it to the blood · 1 mark: explains that the kidney filters urea out of the blood into the filtrate, removing it
ApplyBand 5–6(5 marks) 3. Account for the changes in the concentration of gases, nutrients and wastes in the blood as it travels through the small intestine, then the liver, then a working leg muscle, then the lungs, one hour after a large meal.
1 mark: small intestine, glucose and amino acids rise by absorption · 1 mark: liver, glucose buffered toward normal by glycogen storage, urea added · 1 mark: muscle, oxygen and glucose fall, carbon dioxide rises · 1 mark: lungs, oxygen rises and carbon dioxide falls by diffusion down gradients · 1 mark: every change attributed to what the organ does, rather than listed
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (5 marks): (i) At the lungs, the concentration of oxygen in the blood increases and the concentration of carbon dioxide decreases. Oxygen diffuses from the high concentration in the alveoli into the blood, while carbon dioxide diffuses out of the blood into the alveoli, both moving down their concentration gradients during gas exchange. (ii) At the small intestine after a meal, the concentration of nutrients in the blood increases: glucose and amino acids produced by chemical digestion are absorbed across the villi into the blood. Wastes are largely unchanged here, but the blood becomes nutrient-rich, which is why it is carried first to the liver (via the hepatic portal vein) for processing. In both organs the change reflects the organ's function, the lungs exchange gases, the small intestine absorbs nutrients.
Q2 (4 marks): Sample A was taken from blood leaving the liver and sample B from blood leaving the kidney. The difference is accounted for by what each organ does with urea. The liver produces it: excess amino acids cannot be stored, so the liver removes their amino groups by deamination, producing ammonia which is converted to the less toxic urea, and that urea is released into the blood. Blood leaving the liver therefore carries more urea than blood entering it. The kidney removes it: the nephrons filter urea out of the blood at the glomerulus and it is not reabsorbed, so it leaves the body in the urine. Blood leaving the kidney therefore carries less urea than blood entering it. This is the general rule for every organ, blood loses what an organ consumes or removes and gains what an organ produces or absorbs, and the liver and kidney are simply the clearest illustration of the two directions.
Q3 (5 marks): At the small intestine, the products of chemical digestion are absorbed across the villi into the blood, so the concentration of glucose and amino acids rises sharply. At the liver, which receives this nutrient-rich blood directly through the hepatic portal vein, glucose is brought back toward its normal level as the excess is stored as glycogen, and the excess amino acids, which cannot be stored, are deaminated, so the concentration of urea rises. At the working leg muscle, respiration is consuming oxygen and glucose rapidly and producing carbon dioxide, so oxygen and glucose concentrations fall while carbon dioxide rises; each substance moves down its own concentration gradient between the blood and the muscle cells. At the lungs, oxygen diffuses from the high concentration in the alveolar air into the blood and carbon dioxide diffuses out of the blood into the alveoli, so oxygen rises and carbon dioxide falls. Every one of these changes follows the same rule: an organ removes from the blood what it consumes and adds to the blood what it produces or absorbs, and the direction of each movement is set by the concentration gradient.
Retrieve and reflect
Use the spaced-review session, then compare how your thinking has changed since your opening prediction.
Check what actually stuck
A short retrieval session mixing this lesson with earlier ones, so the ideas move into long-term memory instead of fading by the exam.
Lock it in
bossYou can now account for how blood content changes across each organ using one rule. Next lesson looks at the vessels that carry this blood, how arteries, capillaries and veins are built for their jobs.
↩ Back to focus-area hubHow did your thinking change?
At the start you predicted the blood changes at a muscle and at the gut. Check them: at a working muscle, blood loses oxygen and glucose and gains carbon dioxide; at the small intestine after a meal, blood gains glucose and amino acids. Both follow the single rule, organs remove what they consume and add what they release.
If you wrote that "blood is the same everywhere", revisit the misconception box: composition depends on the organ just passed.