Nutrition: An Exchange of Substances and the Digestive System

Posted on Dec 4, 2023 in Biology

Nutrition: an exchange of substances

In order to carry out the nutrition function, living organisms need an exchange of substances to take place between their surrounding environment and their cells. This exchange occurs in different ways according to whether the organism is unicellular or multicellular.

1.1. How substances are exchanged

The nutrition function is performed through the joint action of different systems. The systems involved in human nutrition are:

1. The digestive system: This system transforms the food we eat into nutrients, which are absorbed and passed into the blood system.

2. The respiratory system: This system takes oxygen from the air and transfers it to the blood. It also takes de carbon dioxide, produced during cell metabolism, from the blood and expels it out of the body.

3. The circulatory system: This system uses blood to carry nutrients and oxygen to the cells. It also transports waste from the cells to the excretory organs, where they are expelled.

4. The lymphatic system: This system works with the circulatory system to transport substances.

5. The excretory system:  This system expels the waste substances. Also, other organs such us sweat glands and the respiratory system are involved in the excretion process.

All five systems are intimately interlinked, fulfilling two main functions: providing cell nutrition and eliminating waste products. 

The endocrine and nervous systems control and coordinate the whole function of nutrition: they detect nutritional needs as well as alterations in the internal organism; they activate relevant organs to provide us with nutrients and oxygen; they coordinate the  transportation of nutrients and oxygen to the tissues that need them and eliminate waste products that alter the internal equilibrium.

2. Digestive system

2.1. Anatomy of the digestive system

The digestive systemis a set of organs that perform the task of digestion. It transforms food into simpler substances, which are then passed into the blood stream, meanwhile eliminating any non-digested material.

1. The digestive tract

It is about 8 metres long and its shape varies. It starts in our mouths and ends at our anus and it has the following parts: oral cavity, pharynx, oesophagus, stomach, small intestine and large intestine.

An inner cell layer called a mucous membrane makes up the gastrointestinal tract. It’s internally protected by a lubricant substance called mucus and it’s surrounded by muscular layers: a longitudinal one, a second one which is circular and an oblique third, located in the stomach. When these layers contract they push food down the digestive tract. 

The mucosa in the stomach and intestine contain glands that secrete a protective mucous and others that secrete digestive juices. Glands in the stomach produce gastric juices and glands in the intestine secrete intestinal juices.

The mouth: the entry point of the digestive tract. It contains:

• Teeth, to bite and break food into small pieces.
• A tongue, to mix and swallow food as well as enable the sense of taste.
• The salivary glands, that release saliva, a watery liquid. 

The pharynx: a cavity shared by the digestive and respiratory systems. Air passes toward the larynx and food passes toward the oesophagus. 

The epiglottisis a small, flexible cartilage that prevents food from reaching the respiratory tract.

The oesophagus: a tube with a length of 25 cm that extends down the thorax, across the diaphragm and into the stomach. It connects the pharynx with the stomach. Food travels down the oesophagus to the stomach as a result of contractions in its walls.

The stomach:it has very strong muscles in its walls, in addition to glands, which secrete gastric juices that are then mixed with food. Gastric juices contain pepsin and hydrochloric acid:

• Pepsin:is an enzyme that starts protein digestion, breaking proteins down into peptides.
• Hydrochloric acid:It activates pepsin enzymes and supports them by helping the disintegration of food fibres. Additionally, it destroys bacteria that may be present in food. Two valves regulate the flow of food: the cardia, at the entry point, and the pylorusat the exit point.

The small intestine: is a long tube. It has glands that secrete intestinal juices and folds called intestinal villi.These contain capillaries, through which blood flows. The nutrients are passed into the blood stream through these capillaries. The small intestine is divided into three sections: duodenum, jejunum and ileum.

The large intestine: it has three parts: the caecum, the colonand the rectum, which leads outside the body through the anus, where faeces are expelled.

2. The accessory glands

Unlike the gastric and intestinal glands, the accessory glands are located outside the digestive tract. They produce digestive juices and secrete them into the tract. These juices contain a great variety of substances, the most important of which are digestive enzymes.

Digestive enzymesare a type of proteins that accelerate the breakdown of complex food substances into their simplest components: nutrients.The accessory glands constitute the salivary glands, the liverand the pancreas.

The salivary glands. Glands that secrete saliva into the mouth through their ducts. The functions of saliva are the following:

• It starts the digestion of starch molecules, which are present in many foods (bread, potatoes, rice…) and transforms them into simpler sugars. In order to do this, saliva has a digestive enzyme named amylase, which breaks down the most complex sugars.
• It destroys some bacteria by using an enzyme named lysozyme. Its antibacterial action protects the mouth against infection and reduces the possibility of suffering from cavities.
• It eases the bolus into the pharynx and oesophagus. Saliva contains mucin, a mucous substance that lubricates and mixes with food. As a result, foods go through the pharynx and oesophagus easily without causing damage.

The liver. This is the largest gland in the body. It performs important functions:

• It synthesizes proteins.
• It regulates the metabolism of glucose.
• It stores vitamins and minerals.
• It processes alcohol and drugs found in the blood in order to eliminate them.
• It produces bile. This is stored in the gallbladderbefore it is secreted into the duodenum. Bile contains bile salts that facilitate the digestion and absorption of lipids, by acting as a detergent.

Pancreas.This is an arrowhead-shaped gland found underneath the stomach. It serves two functions: 

• A digestive function: it secretes pancreatic juice, which is released into the duodenum. Pancreatic juice contains enzymescapable of digesting the different types of molecules present in foods (carbohydrates, fats and proteins), as well as sodium bicarbonate, which neutralises the chyme’s acidity and stops intestinal cells from being harmed.
• An endocrine function: It produces hormones(insulin and glucagon) that regulate the quantities of glucose in our body and their accumulation in the liver. These substances are released, directly into the bloodstream.

3. Digestion

The food we eat is digested in the digestive system. The digestive process consists of various different stages: mechanical digestion, chemical digestion, absorptionand egestionor defecation.

3.1. Mechanical digestion

Mechanical digestion is a series of processes, designed to reduce the size of food particles and make them travel through the digestive system. It consists of several stages:

• Mastication.This consists of tearing, cutting, chewing and grinding the food we put in our mouths. It is carried out by the teeth and the movement of the lower jaw.
• Insalivation.This consist of mixing the ground-up food with saliva by moving the tongue. A bolusis formed as a result.
• Swallowing. The tongue pushes the bolus from the mouth into the pharynx. The bolus moves through the pharynx to the oesophagus. There, wall muscles produce wave-like contractions called peristalsis. These contractions move the bolus toward the stomach.

3.2. Chemical digestion

Chemical digestion is a series of processes that transform food into nutrients via the action of substances in the digestive juices. It takes place in different parts of the digestive system:

• In the mouth. During the formation of the bolus, the chemical actions of the enzyme amylase, present in the saliva, act on carbohydrates, breaking them down into simpler substances. 

• In the stomach.When the bolus reaches the stomach, the stomach walls secrete gastric juices that contain hydrochloric acid and enzymes like pepsin. This enzyme begins the digestion of proteins by breaking them up into short chains of amino acids. Peristaltic contractions mix the gastric juices with the bolus, turning it into a thick, very acidic liquid called chyme. 

• In the small intestine. The chyme is affected by peristaltic movement, the intestinal juices (secreted by the intestinal walls), the bile (secreted by the liver) and the pancreatic juices (secreted by the pancreas). Enzymes like the lipasesbreak the lipids into fatty acids and glycerol. The peptidasesbreak down proteins into amino acids and the amylasesbreak down starch into sugars. The chyme becomes a milky fluid called chyle that contains water, the nutrients resulting from digestion and other undigested products. 

After digestion, foods are reduced to simpler molecules that function as nutrients for cells:

• Monosaccharaides or simple sugars, such as glucose or fructose, obtained from more complex carbohydrates. 
• Glycerol and fatty acids, which are obtained from fat.
• Amino acids, obtained from the digestion of proteins. 

These nutrients must be distributed to the tissues where cells will use them to obtain energy or make new molecules

3.3 Absorption of nutrients

In this process, the nutrients obtained in digestion pass through the walls of the digestive tract to the blood where they are carried to the cells. This process take place in both the small and large intestine, so it is called intestinal absorption.

Absorption in the small intestine.

Absorption of simple glucids, fatty acids and amino acids takes place primarily in the small intestine. Water, salts, and vitamins are also absorbed here. 

Absorption of nutrients in the small intestine is a relatively quick process. This is because the intestinal wall has a large surface for absorption, which measures approximately 200 metres squared. This is because:

• It is very long, measuring between 7 and 8 metres.
• Its inside walls (known as intestinal mucus) are covered by a series of structureswhich increase the absorption surface area by about 600 times. Among these structures, the following are particularly important:

All along the small intestine, there are intestinal folds. In turn, these folds have thousands of intestinal villi, which cover the inside wall of the intestine. Inside them, there are many blood vessels and capillaries.

These villi are made up of cells with a plasma membrane that has many tiny folds known as microvilli.

The by-products of fat digestion are not directly absorbed by blood capillaries in the small intestine, but are absorbed by capillaries that are part of the lymphatic system.

Absorption in the large intestine.

Most of the water and minerals present are absorbed into the large intestine. The large intestine has a greater diameter than the small intestine, measuring approximately 1 metre in length, and does not have villi.

Intestinal flora produces nutrients such as vitamin K that are absorbed by colon cells.

3.4. Egestion or defecation

Some foods that cannot be digested are converted into waste to be eliminated by the digestive tract. This waste is compacted in the large intestine where it loses water and mineral salts. 

Gut flora(a collection of microorganisms that live in the intestine) feeds upon part of these substances, and transforms undigested products into faeces, which are expelled from the body through the anus. Faeces contain intestinal mucosa cells, bacteria and food waste.

Food contains substances that we cannot digest or, therefore, absorb. Fibre is one of theses substances. None of the digestive enzymes in our bodies can digest it and turn it into glucose. However, fibre plays and essential role in the activity of the digestive system, as it helps to move substances through the intestine and prevents constipation.

4. Principal diseases of the digestive system

Any organ in the human body can stop functioning normally at any time. This can cause problems or even disease. Sometimes the malfunction is the result of a lack of hygiene or poor eating habits.

Understanding some of the diseases of the digestive system can help to prevent them.

– Caries or dental cavities, is an infectious disease caused by bacteria that live in the mouth. They food on the remains of food found there, especially the glucids from sugary food. As a result, acids are produced that weaken dental enamel and cause small holes. The acids erode the dentine and reach the pulp.

– Gingivitis is a painful inflammation of the gums, where they bleed and redden.

– Pyorrhoeais the inflammation and infection of the ligaments and bones that support the teeth

– Peptic ulcer.  A break in the lining of the stomach or duodenum that can cause haemorrhage or perforate the stomach wall. This infection is caused by the Helicobacter pyloribacteria and can be made worse by alcohol, smoking, certain drugs or stress.

– Gastritis.  This condition occurs due to an irritation of the gastric mucosa of the stomach, causing it to become inflamed. It produces symptoms such as heartburn.

– Gastroenteritis.  An inflammation or swelling of the stomach or the small intestine generally produced by an infection. The cause of this infection can be food poisoning or contact with people already affected.

The main symptoms are vomiting and diarrhoea, stomach pains, fever, headache and chills. Dehydration is the principal problem, so the intake of liquids must be increased.

– Appendicitis.  Inflammation of the appendix characterised by intense pain, vomiting and fever. It’s caused by a blockage or obstruction usually due to bacterial infection. The consequences can be very serious if not treated quickly.

– Constipation.  Difficult defecation often caused by stress or diets lacking in fibre and water. These conditions lead to hard, dry faeces that make defecation difficult.

– Diarrhoea.  This occurs when food travels through the large intestine very fast, due to intense and rapid contractions. Faeces are very runny (liquid) because no water is absorbed.

– Colon cancer.  This is a malignant tumour that begins in the colon and may invade other organs. It often develops from polyps or bulges forming in the mucosa.

– Hepatitis.  Hepatitis is an inflammation of the liver caused by certain viruses, the abusive consumption of alcohol or certain medicines.

– Cirrhosis.  A serious illness that destroys hepatic cells

– Gallstones.  Also referred to as stones, these are solid particles composed of cholesterol or the minerals found in bile.

5. Healthy habits for the digestive system.

In order to have a healthy digestive system, follow these guidelines. Making them part of your everyday life can help to prevent disease.

– Chew food slowly, making sure that all food is crushed properly. By doing this, you’ll help the digestive process and stimulate salivation.

– Brush your teethafter each meal in order to eliminate any small pieces of food. Also, use dental floss to clean areas that the toothbrush can’t reach.

– Prevent tooth decay. Eat a healthy diet. Too much sugar is a major cause of caries.- Visit the dentistat least once a year.

– Do not eat or drink foods that are very hot. They can irritate the mucus in our mouth, pharynx and even the oesophagus. Do not eat too many spicy foodsas they can irritate the mucus in the stomach.

– Wash your handsbefore eating or preparing food. You should also wash food, especially food that is eaten raw. This way, food will not be contaminated by anything that might harm your body.

– Avoid eating out of meal timesto prevent the digestive system from functioning non-stop.

– Ensure foodsyou prepare or eat are in perfect conditionin order to prevent food poisoning.

– Eat foods that have fibreas they help intestinal movements, allowing foods and faeces to travel through our body more easily.

– Drink water.Your body needs to replace body fluids to function properly. 

– Practise sportregularly. This helps to prevent constipation.

– Do not consume alcohol. Regular alcohol consumption can cause irreversible damage to the liver and pancreas and can be responsible for stomach and oesophageal cancer.

Practise sportregularly. This helps to prevent constipation.

– Do not consume alcohol. Regular alcohol consumption can cause irreversible damage to the liver and pancreas and can be responsible for stomach and oesophageal cancer.

6. The respiratory system

As well as nutrients, cells in our body need oxygen in order to perform cell respiration. It is during this process that the combustion of nutrients takes place, releasing energy. As a result of cell respiration, waste products are created (CO2 among others) that have to be released.

The respiratory system exchanges gases with the external environment. It takes oxygen (O₂) from the air and transfers it to the blood stream, while taking carbon dioxide (CO₂) from the blood and expelling it form the body.

The respiratory system is divided into two parts: the respiratory tract and the lungs

6.1. The respiratory tract

Is a group of tubes that carry air from outside the body into the lungs and vice versa. When air reaches the lungs, it needs to be clean, humid and warm. This preparation process takes place in the respiratory tract. 

It consists of the nasal passages, pharynx, larynx, trachea, bronchi and bronchioles. 

– The nasal cavity: this is the place where air enters the body. In the first section, the internal wall has several capillaries, which warm up the air because they carry blood (blood has the same temperature as the body does). 

In addition, the presence of nooks and crannies in the nasal cavities increases the length of the journey air goes on within the body, making it warmer. Mucous purifies and humidifies the air.

– The pharynx:this area is common to the respiratory and digestive tracts. Foods continue their journey into the oesophagus, while air passes into the larynx. On the side walls of the pharynx there are tonsils, which produce white blood cells.

– The larynx:entry into the larynx is regulated by a fibrous structure known as the epiglottis, which closes when food is being ingested to prevent the organism from choking or suffocating. The walls of the larynx are formed of cartilage that continually keeps it open. From the larynx, air goes into the trachea.

Inside the larynx there are two ring-shaped folds (vocal cords). They create sounds when the passage of air makes them vibrate. These sounds change depending how the tongue, teeth and lips are positioned, and the way that air passes through the mouth or nasal passages.

– The trachea:this tube of about 12 cm has open cartilage rings at its back. There is mucous present in the interior of the trachea that traps foreign particles in the air. In addition, there are certain cells with vibrating filaments known as cilia. Cilia move mucous, filled with foreign particles, towards the larynx, where it is redirected to the oesophagus and stomach.

Bronchi and bronchioles.The trachea is divided into two bronchi, each one enters a different lung and branches into progressively smaller ducts named bronchioles. 

Bronchioles do not have cartilage rings. They end in tiny sacs, called pulmonary alveoli,which are the true protagonists of gas exchange. Each person has about 400 million alveoli.

6.2. The lungs

The group formed by bronchi, bronchioles, alveoli and an extensive network of capillaries, constitutes the lungs: highly elastic bodies whose appearance resembles that of a sponge. We have two lungs: the right lung, which has three parts called lobes and the left lung, which only has two lobes.

The lungs are located in the rib cage and separated from the abdomen by a muscle called the diaphragm.They are surrounded by a double membrane, called the pleura, which is full of pleural fluid.This liquid protects the lungs from rubbing against the rib cage and facilitates the movements involved in breathing.

6.3. how the respiratory system works

There are three stages to breathing in the respiratory system: inhalation, gas exchange and exhalation.

– Inhalation andexhalationtogether are known as pulmonary ventilation. These two movements are consecutive and involuntary.

The lungs do not have muscles. They fill with air and then expel it thanks to several muscles in the abdomen:

The diaphragm, dome-like in shape, separates the thoracic cavity from the abdominal cavity.

The intercostal muscles, located between the ribs, keep the ribs together.

1. Inhalation 

This is the respiratory movement in which oxygen-rich air from the atmosphere enters into the lungs. The intercostal muscles contract and the ribs rise. The diaphragm flattens and descends, increasing the capacity of the ribcage. The lungs become enlarged, increasing their volume, and air enters them.

2. Gas exchange

After traveling through the respiratory tract, air filled with oxygen reaches the pulmonary alveoli, where gas exchange occurs. Pulmonary alveoli only have a layer of flat cells (endothelium). The alveoli are surrounded by a network of capillaries that bring blood, rich in carbon dioxide and low in oxygen, from all organs of the body.

Gases pass between the alveoli and blood capillaries due to the large difference in the concentration of gasesbetween them. A gas always passes from a region of higher concentration to a region of lower concentration. Gasses pass through the membranes of the alveolar epithelial cells and the capillary endothelium by simple diffusion because oxygen and carbon dioxide are small molecules.

When the inhaled, oxygen-rich air reaches the pulmonary alveoli, O₂ (which is highly concentrated in the alveoli) passes through the walls of these alveoli into the incoming blood(which is low in O₂), diffusing through the thin walls of the capillaries that surround them. 

For the same reason, the abundant CO₂in this blood coming from the cells diffuses into the inside of the alveoli(where there is little CO₂) and is then expelled.

3. Exhalation

This is the respiratory movement that releases carbon dioxide-rich air from the lungs to the outside. The intercostal muscles relax and the ribs descend. The diaphragm relaxes, curves and rises as the ribs sink, reducing the ribcage capacity. As a result, the lungs reduce their volume and release the air inside them.

7. Diseases of the respiratory system

– Aphonia: Difficulty emitting sounds due to the inflammation of the vocal cords.

– Sinusitis: Inflammation of the sinus cavities of certain bones in the skull that are connected to the nostrils.

– Asthma: As a result of certain infectious processes or an allergic reaction, there is a contraction of the bronchi, complicating the passage of air and making it more difficult to breathe. This causes a feeling of breathlessness and anxiety.

– Allergic rhinitis: The presence in the air of certain substances (pollen grains, dust, fungi, among others) can trigger reactions, known as allergies. They can manifest themselves through symptoms such as a runny nose, itching or sneezing.

– Pharyngitis and tonsillitis: an inflammation of the pharynx and of the tonsils due to microorganisms. The main symptoms are: localized pain, increased mucus, fever (occasionally) and coughing.

Pneumonia: an inflammation of the lung tissue due to bacterial or virus infection the main symptoms are: difficult breathing and a high fever. It is treated with antibiotics but can cause death.

– Colds and flu: infections of the respiratory tract due to virus. Colds and flu produce nasal obstruction, sneezing, coughing, abundant mucus and general discomfort. The flu also produces muscular pain, chills and fever. Flu is a widespread disease that causes a winter pandemic every ten to fifteen years. It can be deadly to high-risk groups, such as the elderly and people who suffer from heart disease, lung infections or other chronic infections.

– Bronchitis: Irritation of the bronchial lining caused by a variety of factors including smoke and air pollutants. The disease is characterised by frequent coughing, chest pain and asthma.

– Lung and throat cancer: These are malignant tumours characterised by the uncontrolled growth of a mass of lung/throat tissue that invades and destroys adjacent tissues. The development of such tumours is closely related to smoking.

– Pulmonary emphysema: This produces an enlargement of the pulmonary alveoli, causing them to break. Breathing becomes progressively limited, almost permanently. Smoking and pollution are major causes.

8. Healthy habits for the respiratory system 

These are some of the recommendations that we can follow in order to keep our respiratory organs healthy.

Do not smoke.Smoking is an addiction that directly causes many respiratory diseases, such as cancer, chronic bronchitis or emphysema. It also makes the symptoms of many others diseases worse. That is, it has a negative effect on diseases of the respiratory, circulatory, digestive and nervous system.

– Try to avoid sudden changes of temperature,as such changes increase the risk of contracting respiratory illnesses. In addition, people that suffer from asthma can have difficulty breathing as their bronchia contracts if the temperature drops and becomes very cold.

– Do physical exercise regularly.Practising a sport increases pulmonary ventilation, as the muscles need more oxygen. Regular physical activity increases lung capacity and mobilises the mucus of the respiratory tracts.

Tr- y to avoid being in contact with dust and atmospheric contaminants.Particles floating in the air and contaminating gases irritate the respiratory mucosa and can promote the development of several throat, larynx, tracheal and bronchial conditions.