The Digestive System

The digestive system is responsible for breaking down food into small, soluble molecules that can be absorbed into the bloodstream and used by cells for energy, growth, and repair. This involves both mechanical digestion (physical breakdown) and chemical digestion (enzymes breaking chemical bonds).

Digestive System Overview

Mouth

Mechanical: Teeth grind and tear food into smaller pieces. Chemical: Salivary amylase begins breaking down starch into maltose. The tongue mixes food with saliva to form a bolus.

Oesophagus

A muscular tube that moves the bolus from the mouth to the stomach via rhythmic muscle contractions called peristalsis. No chemical digestion occurs here.

Stomach

Produces hydrochloric acid (pH ~2) to kill bacteria and provide the optimal pH for protease (pepsin), which breaks down proteins into smaller peptides. Muscles churn the food into a liquid called chyme.

Small Intestine

The main site of chemical digestion and absorption. Enzymes from the pancreas and bile from the liver complete digestion. Villi (tiny finger-like projections) provide a large surface area for absorbing nutrients into the bloodstream.

Large Intestine

Absorbs water and mineral ions from the remaining indigestible material. Bacteria in the large intestine also produce some vitamins (e.g. vitamin K). The solid waste (faeces) is stored in the rectum and excreted through the anus.

Enzymes and Chemical Digestion

Enzymes are biological catalysts — they speed up chemical reactions without being used up. Digestive enzymes break down large, insoluble food molecules into small, soluble molecules that can pass through the intestinal wall into the blood.

Enzyme	Substrate	Product	Where Found
Amylase	Starch (carbohydrate)	Maltose (sugar)	Mouth, small intestine
Protease (pepsin)	Proteins	Amino acids	Stomach, small intestine
Lipase	Fats (lipids)	Fatty acids + glycerol	Small intestine

Role of bile: Bile is produced by the liver and stored in the gall bladder. It emulsifies fats (breaks large fat droplets into smaller ones), increasing the surface area for lipase to work on. Bile is not an enzyme.

The Circulatory System

The circulatory system transports oxygen, nutrients, hormones and waste products around the body. It consists of the heart (pump), blood vessels (tubes), and blood (transport medium). Humans have a double circulatory system: blood passes through the heart twice per complete circuit.

The Heart (4 chambers)

Note: In diagrams, right and left are shown from the patient's perspective (mirrored).

Blood Vessels

Feature	Arteries	Capillaries	Veins
Direction	Away from heart	Connect arteries to veins	Back to heart
Walls	Thick, muscular, elastic	One cell thick	Thin walls
Lumen (opening)	Narrow	Very narrow	Wide
Valves	No (except heart)	No	Yes (prevent backflow)
Blood Pressure	High	Low	Low

Blood Components

Red Blood Cells

Contain haemoglobin that binds to oxygen. Biconcave disc shape gives a large surface area. No nucleus to carry more haemoglobin.

White Blood Cells

Part of the immune system. They defend the body against pathogens by engulfing bacteria (phagocytosis) or producing antibodies.

Platelets

Cell fragments that help blood clot at wound sites. They form a mesh to stop bleeding and prevent infection.

Plasma

The liquid component of blood (~55%). Transports dissolved nutrients, hormones, CO₂, urea, antibodies, and heat around the body.

Key Vocabulary

Peristalsis

Rhythmic, wave-like muscle contractions that push food along the digestive tract.

Villi

Tiny finger-like projections lining the small intestine that increase surface area for nutrient absorption.

Haemoglobin

The iron-containing protein in red blood cells that binds to oxygen, forming oxyhaemoglobin.

Double Circulation

Blood passes through the heart twice per circuit: once to the lungs (pulmonary) and once to the body (systemic).

Worked Examples

1

Trace the path of a sandwich from mouth to absorption.

Step 1: Mouth — Teeth mechanically break the bread and filling. Salivary amylase begins breaking starch into maltose.

Step 2: Oesophagus — The bolus is pushed to the stomach by peristalsis.

Step 3: Stomach — HCl kills bacteria. Protease (pepsin) breaks down the protein from the filling. Churning creates chyme.

Step 4: Small intestine — Bile emulsifies fats. Pancreatic enzymes (amylase, protease, lipase) complete chemical digestion. Nutrients are absorbed through villi into the bloodstream.

Step 5: Large intestine — Water is reabsorbed. Remaining waste forms faeces, stored in rectum, excreted via anus.

2

Describe the path of blood through the heart.

Step 1: Deoxygenated blood enters the right atrium via the vena cava.

Step 2: It flows into the right ventricle, which pumps it to the lungs via the pulmonary artery.

Step 3: In the lungs, blood picks up oxygen and releases CO₂ (gas exchange).

Step 4: Oxygenated blood returns to the left atrium via the pulmonary vein.

Step 5: It flows into the left ventricle (thickest wall, most powerful), which pumps it to the body via the aorta.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which enzyme breaks down starch into sugars?

Amylase Lipase Protease Bile

Question 2

Where is the main site of nutrient absorption in the digestive system?

Stomach Small intestine Large intestine Oesophagus

Question 3

Which blood vessel carries oxygenated blood away from the heart to the body?

Vena cava Pulmonary artery Aorta Pulmonary vein

Question 4

What is the function of red blood cells?

Fight infection Clot blood at wounds Transport dissolved nutrients Transport oxygen using haemoglobin

Question 5

Why does the left ventricle have a thicker wall than the right ventricle?

It pumps blood at higher pressure to the entire body It receives blood from the lungs It contains more blood than the right ventricle It stores oxygenated blood longer

Body Systems: Digestive & Circulatory