Digestion · Circulation · Excretion

Digestion, Circulation & Excretion

Food is broken into nutrients in the digestive system, then the circulatory system delivers them to every cell. The excretory system removes leftovers and wastes, so the three systems work together. Starch becomes glucose, protein and fat each have their own enzymes, and absorption happens at the villi of the small intestine. Here you can follow the path from mouth to anus and the essentials of circulation and excretion step by step.

Food's Journey to Energy

🍚 Three Systems Cooperate

①Digestive: breaks food down into nutrients

②Circulatory: delivers nutrients and oxygen throughout the body

③Excretory: removes leftover waste

Three Body Systems

Digestion of the Three Macronutrients

Carbohydrate Digestion

Starch → maltose → glucose

Enzymes: amylase (saliva, pancreatic juice) → maltase (small intestine)

Protein Digestion

Protein → polypeptide → amino acids

Enzymes: pepsin (stomach) → trypsin (pancreatic juice)

Fat Digestion

Fat → fatty acids + glycerol

Bile (emulsifies) + lipase (pancreatic juice)

Circulation & Excretion Essentials

❤️ Blood-Circulation Path

①Pulmonary: heart → lungs (absorb O₂, release CO₂) → heart

②Systemic: heart → body (supply O₂, collect CO₂) → heart

③Arterial blood (rich O₂, bright red) ↔ venous blood (rich CO₂, dark red)

💧 Excretion Core — Kidney

①Kidney filters blood and removes waste

②Nephron: basic unit (glomerulus → Bowman's capsule → tubules)

③Reabsorption: glucose, water etc. return to the blood

Summary

Digestion · Circulation · Excretion Core

Digest (break down) → Circulate (transport) → Respire (energy) → Excrete (remove)

Four processes cooperate to sustain life

🎯 Exam Key Points

①Small intestine is the main site for digestion & absorption (villi maximize surface area)

②Each nutrient has its own enzymes

③Pulmonary: RV→lungs→LA / Systemic: LV→body→RA

④Kidney: glomerulus filters, tubules reabsorb

⑤Wastes: urea (protein), CO₂ (respiration), water, salts

Examples and Unit Test

Example 1

Which digestive enzyme breaks down starch, protein, and fat, and what is the final product of each?

Starch is broken down by amylase into glucose in the end, and protein is broken down by pepsin and trypsin into amino acids.

Fat is first emulsified by bile and then broken down by lipase into fatty acids and glycerol.

▸ Starch →(amylase)→ glucose / protein →(pepsin, trypsin)→ amino acids / fat →(lipase)→ fatty acids + glycerol

Each nutrient has its own enzyme, and bile is not an enzyme but emulsifies fat.

Example 2

How are digested nutrients absorbed at the villi of the small intestine, and where do they go?

The many villi on the inner wall of the small intestine greatly enlarge the surface area and raise absorption efficiency.

Glucose and amino acids enter the capillaries inside a villus, while fatty acids and glycerol enter the lacteal (lymph vessel).

▸ Villi enlarge the surface area / glucose and amino acids → capillaries, fatty acids and glycerol → lacteal (lymph vessel)

Thanks to its villi, the small intestine is the key place where digestion and absorption happen together.

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 glycerol

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 glycerol

Pepsin breaks down protein in the stomach (① wrong), and fatty acids are absorbed into the lacteal or lymph vessel (③ wrong).

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 glycerol through lipase (② correct).

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