For a spoonful of rice to be used by the body, it has to pass through three organ systems in turn.
The digestive system breaks food into nutrients small enough to enter the blood, the circulatory system carries those nutrients and oxygen to every cell, and the excretory system filters out the wastes cells leave behind and sends them out of the body.
The first diagram traces the path food takes from mouth to anus, and the small intestine is where the broken-down nutrients are taken in.
Switching the buttons shows the circulation that carries nutrients and the kidneys that filter urea out of the blood to make urine.
From Food to Energy
🍚 Three Systems in a Relay
①Digestive system: breaks food into small nutrients such as glucose and amino acids and absorbs them in the small intestine
②Circulatory system: loads the absorbed nutrients and the oxygen from the lungs into the blood and carries them to every cell
③Cells use nutrients and oxygen to release energy (cellular respiration), producing wastes such as CO₂ and urea
④Excretory system: the kidneys filter urea out of the blood and send it out as urine
Three Systems Working Together
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Digestive organs from mouth to anus and the job each one does
Digesting the Three Main Nutrients
Digesting carbohydrates
Starch → maltose → glucose
Amylase in saliva and pancreatic juice turns starch into maltose, and carbohydrate enzymes in the small intestine turn maltose into glucose
Digesting proteins
Protein → shorter chains → amino acids
Pepsin in the stomach cuts proteins first, then trypsin from the pancreas and enzymes in the small intestine finish the job down to amino acids
Digesting fats
Fat → fatty acids + monoglycerides
Bile breaks fat into tiny droplets (emulsifies it) so that lipase from the pancreas can break it down. Bile itself contains no digestive enzyme
The Core of Circulation and Excretion
❤️ Blood Travels Two Loops
①Pulmonary: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium. The lungs remove CO₂ and add O₂
②Systemic: left ventricle → aorta → body → venae cavae → right atrium. Cells receive O₂ and nutrients and hand back CO₂ and wastes
③Arterial blood is rich in O₂ and bright red; venous blood is rich in CO₂ and dark red
💧 How the Kidneys Make Urine
①The working unit of the kidney is the nephron (glomerulus + Bowman's capsule + tubule)
②Filtration: blood pressure pushes small substances in the glomerulus (water, glucose, urea, salts) into Bowman's capsule
③Reabsorption: all the glucose and amino acids, and most of the water and salts, return from the tubule to the capillaries
④Secretion: wastes that were not filtered are moved from the capillaries into the tubule
If any one stops, cells go without nutrients or oxygen, or wastes build up
🎯 Common Test Points
①The small intestine is the center of digestion and absorption, and its villi give it a huge absorbing surface
②Bile is not an enzyme; it emulsifies fat
③Water-soluble nutrients enter capillaries; fat-soluble nutrients enter lacteals
④Kidney: filtration at the glomerulus, reabsorption and secretion at the tubule
⑤Releasing feces is not excretion. Excretion means removing wastes made by cells, such as urea and CO₂
Examples and Unit Review
Example 1
Which enzymes break down starch, protein and fat, and what are the final products?
1
Starch becomes glucose through amylase and the carbohydrate enzymes of the small intestine; protein becomes amino acids through pepsin, trypsin and the protein enzymes of the small intestine.
2
Fat is emulsified by bile and then broken down by lipase into fatty acids and monoglycerides.
Each nutrient has its own enzymes, and bile is not an enzyme: it scatters fat into droplets so lipase can work.
Example 2
How are digested nutrients absorbed at the villi of the small intestine, and where do they go?
1
The folds of the intestinal wall and its countless villi greatly enlarge the surface, so nutrients are absorbed quickly.
2
Water-soluble nutrients such as glucose and amino acids enter the capillaries in each villus, while fat-soluble nutrients such as fatty acids and monoglycerides enter the lacteal.
▸ The villi enlarge the surface for absorption; water-soluble nutrients go into capillaries and fat-soluble nutrients into lacteals
Nutrients taking either route eventually pass through the heart and ride the systemic circulation to every cell.
Unit Test
Which statement about digestion and absorption is correct?
①Pepsin breaks down starch in the mouth
②Fat is broken down by lipase into fatty acids and monoglycerides
③Fatty acids absorbed in the small intestine enter the capillaries
④The kidney is an organ that digests food
⑤Arterial blood is rich in carbon dioxide
▸ ② Fat is broken down by lipase into fatty acids and monoglycerides
1
Pepsin breaks down protein in the stomach (① wrong), and fatty acids are absorbed into the lacteal or lymph vessel (③ wrong).
2
The kidney filters blood and removes wastes as an excretory organ (④ wrong), and arterial blood is rich in oxygen (⑤ wrong); fat becomes fatty acids and monoglycerides through lipase (② correct).