UNIT 3: THE DIGESTIVE AND RESPIRATORY SYSTEMS.Human nutrition.In multicellular organisms like humans, not all cells have direct access to nutrients.
Nutrients are complex mixtures of many different substances which have to be
transformed by our bodies to use them.
So it is necessary to prepare nutrients, distribute them to all the cells and expel the waste
products from cellular activity. For that our body needs the following systems:
1. Digestive system: it processes foods, transforming them so that they can be
distributed to all cells, and expels the undigested products (egestion).
2. Respiratory system: it captures oxygen needed for cells and eliminates carbon
dioxide released during cell metabolism.
3. Circulatory system: It takes oxygen and nutrients to the cells and takes away
waste products.
4. Excretory system: It expels waste products released during cell metabolism and
transported via the circulatory system.
The digestive system.Anatomy.The digestive system is made up of:
- The digestive tract with the oral cavity (mouth), pharynx, oesophagus, stomach,
small intestine and large intestine.
It is a large tube about eight metres long. It starts in the mouth and ends at the
anus.
- The accessory glands with the salivary glands (in the mouth), the gastric glands
(in the stomach), the intestinal glands (in the intestine), the liver and the
pancreas.
They are organs that release secretions (enzymes) into the digestive tract, that
aid digestion.
Physiology.The main functions of the digestive system are:
1. Consumption, mastication, insalivations and swallowing.
- Consumption and mastication: the food goes into the mouth and mastication (or
chewing) happens there.
Mastication is a mechanical process in which food is crushed and turned into
smaller pieces to make it easier to digest.
Mastication is carried out by the teeth.
Teeth are a set of very hard structures made of calcium and fluoride compounds.
They are found inside the mouth cavity. There are different types of teeth:
. Milk teeth: they are the children’s first teeth. There are twenty, and there are
three types: incisors, canines and premolars.
. Permanent teeth: milk teeth are gradually replaced by permanent teeth.
There are thirty two permanent teeth and they are incisors, canines, premolars
and molars.
The incisors are used to cut food, the canines are used to tear food and the
premolars and molars are used to crush food.
In a tooth we can see the following parts:
. Crown: the part of a tooth that we can see.
. Root: The part of a tooth inside the gum.
. Cervix: part between the crown and the root.
. Cementum: the union between the tooth and the gums.
. Enamel
. Dentine
. Dental pulp
The tongue also plays a role in mastication moving food around the different
teeth.
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Insalivations:
At the same time the food is crushed it is mixed with saliva. The tongue helps
with the process.
Saliva is a watery substance secreted by the salivary glands.
There are three pairs of salivary glands: the parotid, submandibular and
sublingual glands.
Saliva has the following functions: it starts digesting sugar molecules, destroys
some of the bacteria in food and helps the food in its movements.
The food crushed by the teeth mixes with saliva to form a mass known as the
food bolus.
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Swallowing:
The food bolus passes down the pharynx (throat), and the oesophagus helps the
tongue in a process called swallowing.
The muscular layers in the walls of the oesophagus contract to move the food
bolus downward. This movement is called peristalsis.
The digestive process.The previous process prepares the food and afterwards digestion starts.
Digestion in stomach:
The stomach is connected to the oesophagus by the cardia and with the small intestine
by the pylorus.
In the stomach the food bolus mixes with gastric juice produced by the gastric glands.
The resulting mixture is a mass called chyme.
Gastric juice contains pepsin, an enzyme that begins the digestion of proteins. It also
contains hydrochloric acid, which facilitates the action of pepsin and destroys bacteria.
The inside wall of the stomach is covered in mucus that protects it.
Digestion in the small intestine:
The small intestine is a tube around six or seven metres long between the pylorus and
the ileocaecal valve, which separates it from the large intestine.
It is divided into three sections: duodenum, jejunum and ileum.
In the small intestine the most important part of the digestive process happens,
particularly in the duodenum.
In the duodenum, chyme comes into contact with digestive juices:
- The intestinal juice secreted by the intestinal glands.
- The pancreatic juice secreted by the pancreas.
The pancreas is an organ behind and underneath the stomach that has two
functions: it produces hormones (insulin and glucagon) which regulate the
amount of glucose in the blood, and produces pancreatic juice with digestive
enzymes.
- The bili secreted by the liver and stored in the gall bladder.
The liver has a lot functions, and one of them is to produce bili. Bili does not
contain digestive enzymes, but it contains substances called bile salts which help
to digest fats by emulsifying them.
At the end of the process in duodenum we obtain chyle, a mixture of nutrients
(monosaccharide, glycerol and fatty acids and amino-acids) and undigest substances.
Afterwards the nutrients are absorbed: the nutrients from digestion pass into the blood
from the intestine in a process called absorption. This happens in the jejunum and the
ileum.
For absorption a big surface is necessary, so in the wall of the small intestine there are a
lot of intestinal villi and microvilli to provide a large area.
The large intestine.Once the nutrients have been absorbed, any undigested food moves through the
ileocaecal valve into the large intestine, which is wider and shorter than the small
intestine.
The large intestine is made up of three sections:
- Caecum connected to the appendix
- Colon: subdivided into three sections: ascending, transverse and descending.
- Rectum, which leads to the anus.
In the large intestine water and mineral salts are absorbed, and waste products are
compacted to form faeces.
The colon contains symbiotic bacteria which make up the intestinal flora and produce
certain vitamins.
The waste products are moved through the intestine by peristalsis and expelled from the
body via the anus, in a mechanism called egestion.
THE RESPIRATORY SYSTEM.In addition to the nutrients obtained through digestion, the cells in our body need
oxygen. This molecule is essential for cellular respiration a process that burns food and
turns it into energy.
Cellular respiration creates waste products, including carbon dioxide. These waste
products have eliminated from the body.
The respiratory system captures oxygen from the air and collects carbon dioxide from
the blood and expels it from the body.
The respiratory system is made up of:
- The airways: tubes that collect air with the lungs. They include the nasal cavity,
pharynx, larynx, trachea, bronchi and bronchioles.
- The lungs: where gases are exchanged between the blood and the air. Inside the
lungs are bronchioles and pulmonary alveoli.
Nasal cavity: two cavities where air enters the system. Inside the nasal cavity air is
cleared, moistened and warmed, and we perceive smells.
Pharynx: this tract is shared by the digestive and respiratory systems.
The tonsils (amygdales), which produce white blood cells, are on the sides of the
pharynx.
Larynx: the entrance to the larynx is controlled by the epiglottis.
The wall of the larynx is made up of rings of cartilage which keep it open all the
time.
The vocal cords are the larynx. When air passes through these cords they vibrate and
emit sounds.
Trachea: this tube is about 12 cm long and has C-shaped rings of cartilage around
the back.
The mucus and the cilia in the trachea trap any foreign particles present in the air.
Bronchi and bronchioles: the trachea divides into two brochi. Each brochus goes
into a lung and branches out into progressively smaller tubes, the bronchioles. The
final branches of the bronchioles are tiny sacs, the pulmonary alveoli, where gas
exchange takes place.
Lungs: They are two elastic sponge-like organs: the right lung and the left lung.
The right lung has three parts, called lobes, and the left lung has two lobes.
The lungs are surrounded by pleurae, two membranes separated by a fluid layer that
keeps them together and allows them to slide over one another.
The lungs are protected too by the cutlets that delimit the thoracic cage.
The functions of the respiratory system.The respiratory system has two functions:
1. Pulmonary ventilation: It is the renovation of the air in the lungs by two
movements caused by the muscles in the thoracic cage and the diaphragm (the
muscle that separates the thorax and the abdomen). These movements are:
- Inspiration: when air enters the lungs.
- Expiration: when air is expelled from the body after gas exchange has taken
place.
2- Gas exchange: It is the exchange of gases between two places: the pulmonary
alveoli and the blood capillaries by diffusion.