Below is a diagram of the timing of events in the cardiac cycle.
1 square equals 0.1 second
Atrial Systole = 0.1 second Systole
Atrial Diastole = 0.7 seconds
Ventricular Systole = 0.3 seconds Diastole
Ventricular Diastole = 0.5 seconds
The cardiac cycle shown above is as a series of boxes representing 0.1 second each, to study the events occurring in the heart; red boxes signify when contraction is occurring and green boxes signify relaxation time. The technical term for contraction is systole and the term for relaxation is diastole. The activity of the atria is shown on the top line and the ventricles at the bottom.
The events in the cardiac cycle can be described in stages as follows:
- Both atria contract forcing blood under pressure into the ventricles.
- Ventricles are bulging with blood and the increased pressure forces the atrio-ventricular valves shut (giving rise to the first heart sound – lubb).
- Muscle in the ventricular walls begins to contract, pressure on the blood inside rises and forces open the semi-lunar valves in the aorta and pulmonary artery.
- Ventricular systole forces blood into the aorta (left side) and the pulmonary artery (right side). These arteries have elastic walls and being to expand.
- As the blood leaves the ventricles, the muscle starts to relax. For a friction of a second blood falls backwards, catching the pockets of the semi-lunar valves and making them close (the second heart sound – dup).
- With the ventricles in diastole, the atrio-ventricular valves are pushed open with the blood that has been filling the atria. When the ventricles are about 70 per cent full, the atria contract to push the remaining blood in rapidly and the next cycle has begun.
You can see that when the chambers are in diastole and relaxed, they are still filling. The heart is never empty of blood. The cycle is continuous and with a high heart rate it is the filling time which has shortened.
Structure and Functions of Blood.
Erythrocytes contain haemoglobin, a very important respiratory pigment essential for human life.
Haemoglobin is a very special iron-containing protein because:
- in an environment containing a high concentration of oxygen, the haem part of the molecule forms a strong chemical bond with oxygen, becoming oxyhaemoglobin. Oxyhaemoglobin is formed in the blood of the lung capillaries and carries oxygen to tissue cells
- in an environment containing a low concentration of oxygen, the oxygen is released to pass down a concentration gradient to body cells. Haemoglobin is now said to be reduced haemoglobin.
The Respiratory System.
Respiration can be artificially subdivided into four section to facilitate study. These are:
A External respiration comprising:
- gaseous exchange
- blood transport.
B Internal or tissue respiration carried out inside body cells.
The thorax, better known as the chest, is an airtight box containing the lungs and their associated tubes, the bronchi and the heart.
Air can enter the thorax via the nose or the mouth; the former is specially adapted for the entry of air in breathing and is the recommended route.
Role of the Air Passages in the Nose.
The nose contains fine bones on its side walls which are curled like scrolls and covered with moist ciliated mucous membrane, rich in blood capillaries. This arrangement produces a large surface area over which incoming air flows. During the through the nose, the air is warmed and moistened by the close contact with the mucous membrane and filtered by the ciliated cells. By the time the air reaches the throat, it is warmed to almost body temperature, moistened to almost saturation point and most foreign materials such as dust, carbon particles and many pathogens have been filtered out.
Structure and Function of the Trachea and Bronchi.
The trachea commences at the back of the throat, or pharynx, and divides into two mains bronchi, each serving one lung on each side of the heart. The first part of the trachea is specially adapted to produce sound and is called the larynx, or voice box. It is protected by a moveable cartilage flap, the epiglottis, which prevents food entering during swallowing. When any material, such as a crumb, manages to pass by the epiglottis it invokes an intense bout of coughing by reflex action to expel the foreign body. The trachea (or windpipe) and the bronchi have rings of cartilage to prevent them collapsing; those in the trachea are C-shaped with the gap at the back against the main food tube, the oesophagus. This is because when food is chewed in the mouth, it is made into a ball shape (called a bolus) before swallowing. The bolus stretches the oesophagus as it passes down to the stomach and the whole rings cartilage in the trachea would hamper its progress. The gap is filled with soft muscle which stretches easily, allowing the bolus to pass down the oesophagus.
Below is a diagram of a section through the thorax to show the respiratory organs.
On entering the lung, each bronchus divides and sub-divides repeatedly, spreading to each part of the lung. The tiniest sub-divisions, supplying oxygen to air sacs in the lung, are called bronchioles, and even these are held open by minute areas of cartilage. This branching arrangement is often called the bronchial tree.
The inner lining of the trachea and bronchi is composed of mucus-secreting and ciliated, columnar epithelium cells. Mucus is the sticky white gel which traps dust particles that may cause infection.
Structure and Function of the Lungs.
Each lung is a pale pink smooth structure closely mimicking the interior of half the chest in shape. Each is divided into a few lobes with a hilum, or root, that marks the entry of the bronchus, blood vessels and nerves on the inner side.
The lungs themselves have a spongy feel to them and are lined on the outside by a thin, moist membrane known as the pleura. The pleura continues around the inner thoracic cavity so that the two pleural layers slide over one another with ease and without friction. The surface tension of the thin film moisture does not allow the two layers to pull apart but does allow them to slide. This means that when the chest wall moves in breathing, the lungs move with it.
Each bronchus after repeatedly diving ends in a group of single-layered globe-shaped structures called alveoli, rather like a bunch of grapes on a stem. The walls of the alveoli consist of very thin, flat simple Squamous epithelium, and each alveolus is surrounded by the smallest blood vessels known as capillaries. The walls of the capillaries are also composed of simple squamous epithelium, in a single layer. This means that the air entering the alveoli during breathing is separated from the blood by only two single-layered, very thin walls.
There are elastic fibres round the alveoli enabling them to expand and recoil with inspiration and expiration respectively. A film of moisture lines the inside of each alveolus to enable the air gases to pass into solution. As the two layers of epithelium are very thin and semi-permeable, the dissolved gases can easily and rapidly pass through, in the process called gaseous exchange.
Below is a diagram of gaseous exchange in the alveolus.
Ventilation, or Breathing, and the Respiratory Muscles.
Ventilation is the movement of air in and out of the thorax to replenish the oxygen supply and remove surplus waste products (carbon dioxide and water).
Ventilation has two phases, namely inspiration (or inhalation) and expiration (or exhalation).
The movements are effected by respiratory muscles attached to the skeleton. Two sets of intercostal muscles run obliquely at right angles to each other between the ribs, and the diaphragm is dome-shaped muscle attached to the lower ribs and separating the thorax from the abdomen.
When the intercostal muscles contract, the ribs move upwards and outwards and at the same time the contraction of the diaphragm causes it to flatten. All these movements serve to increase the volume of the thorax and the lungs and thus reduce the pressure inside the lungs, causing air to rush in from the environment. This is known as inspired, or inhaled, air.
The main force in expiration during quiet breathing is the elastic recoil of the fibres around the alveoli and the relaxation of the diaphragm. However, during exertion, more forcible expiration can occur with the assistance of the other set of intercostal muscles contracting to move the ribs downwards and inwards. The volume of the thorax decreases, the pressure increases above that of the environmental air and air rushes out. Normal ventilation rate is from 16 to 20 breaths per minute but this rises significantly during exertion.
Below is a diagram of the process of breathing.
Nervous impulses from the brain cause the
diaphragm and intercostal muscles to contract
Diaphragm flattens and the intercostal Volume of the chest increases,
muscles cause the ribs to move upwards so the pressure inside the chest must
and outwards decrease
Surface tension between the
pleura drags the lungs with the chest
wall. As they expand, they fill with air Air containing oxygen
rushes down the trachea and bronchi
to equalise the pressure with the
external environment - inhalation
After a few seconds, the
nervous impulses stop arriving and
the elastic tissue in the lung causes
recoil: the diaphragm rises and Volume of the chest
the ribs lower decreases, so pressure
increases, causing air to rush out of the
trachea - exhalation
The cycle repeats after a few minutes because the respiratory control centre
Becomes active again, sending more nervous impulses.
The composition of inspired air, which is the air around us, and that of expired air is shown in the table below:
Although the largest component of air is nitrogen and this too passes into solution, it takes no part in the process of respiration.
Breathing in fresh air replenishes the high concentration of dissolved oxygen molecules in the lung alveoli, and the removal of diffused oxygen by the bloodstream maintains the low concentration. With carbon dioxide, the situation is reversed – the high concentration is in the blood and the low concentration is in the refreshed air, so diffusion removes dissolved carbon dioxide from the blood into the expired air from the lungs. Carbon dioxide and water are waste products from internal respiration in cells.
Diffusion occurs in liquids or gases because the molecules are in constant random motion, and diffusion is an overall ‘equalling up’ of a situation where you have a lot of molecules meeting a few molecules. Diffusion will stop in time, as the numbers of molecules become more evenly distributed. This is said to be equilibrium.
In the human body, where diffusion is a common method of transport, the state of equilibrium is not desirable as it means overall transport would cease. To prevent equilibrium being attained, the high concentration must be continually kept high and the low concentration must also be maintained.
Diffusion can only occur where there is no barrier at all to the molecules or where the barrier (in gaseous exchange, this is cell membranes) is thin. The rate of diffusion is enhanced with an increased surface area – usually by folds or similar structures to alveoli, and with temperature, since warmth increases the random motion of molecules.
The Digestive System.
The Alimentary Canal.
The alimentary canal is a tube that extends from the mouth to the anus. It is dilated, folded and puckered in various places along its length. Many glands are associated with the alimentary canal, and have important roles to play in digestion.
When food is taken into mouth it mixed with saliva, chewed or masticated by the action of the tongue and teeth, rolled into a small ball known as a bolus, and swallowed. This process is called mechanical digestion and is an important part of physically breaking the food down at an early stage.
The Salivary Glands.
Three pairs of salivary glands pour their secretions known as saliva into the mouth.
Saliva, a digestive juice, contains an enzyme known as salivary amylase, which begins the digestion of carbohydrates as well as lubricating the mouth and helping bolus formation.
Below is a diagram of the alimentary canal.
The oesophagus or gullet transports the food bolus from the back of the mouth (the pharynx) to the stomach in the abdomen. The swallowed bolus is in the oesophagus for a few seconds only and no enzymes are secreted here, although salivary amylase will continue to act during this brief journey. The oesophagus is mainly a transit for food boluses which it moves by muscular contractions known as peristalsis.
The stomach is the widest part of the alimentary canal, tucked mainly behind the rib cage under the diaphragm on the left side and receiving food from the mouth by way of the oesophagus. Food can stay in the stomach for up to three hours, with protein meal remaining the longest and food not containing protein passing through relatively quickly. During this time, the strong stomach walls roll and churn the food around and pour on secretions from the gastric glands. The resulting paste-like material is called chyme.
Gastric glands produce gastric juice that contains gastric protease and hydrochloric acid. The gastric juice works on proteins. In babies, another enzyme, rennin, solidifies and digests milk protein. The pH of the stomach is 1-2; this is strongly acidic. The epithelial lining of the stomach contains goblet cells which produce thick mucus to protect the lining from acid erosion.
The stomach empties the chime in spurts into the duodenum through the pyloric sphincter, a thick ring of muscle which alternately contracts and relaxes.
The next part alimentary canal is the small intestine, so-called because of its small diameter – certainly not its length, for it is around six meters long. The first C-shaped part, and the shortest, is called the duodenum; it is mainly concerned with digestion and helped by two large glands, the liver and the pancreas, that pour their secretions or juices into this area. The duodenal wall also contains glands which secrete enzyme-rich juices (called succus entericus) that continue the digestive process on proteins, carbohydrates and lipids, or fats. These work either on the surface or inside the epithelial lining cells.
The remainder of the small intestine, know as the ileum, is manly concerned with the adsorption of the now fully digested food. It is specially adapted for the by:
- long length
- folded interior
- lining covered in many thousands of tiny projections called villi
- epithelial cells of villi covered in microvilli, projections so small that they can only be detected using an electron microscope.
These adaptions increase the surface area for absorption of nutrients from digested food to enormous proportions.
Each villus is lined by columnar cells and goblet cells only one-cell thick with an internal extensive capillary network and a blind-ended branch of the lymphatic system called a lacteal.
The chief products of protein and carbohydrate digestion pass into the capillary network which drains to the liver by the hepatic portal vein. Products of fat digestion pass into the lacteal and eventually they pass via the lymphatic system into the general circulation.
Below is a diagram of a villi and its blood supply.
In the right hand lower corner of the abdomen, the small intestine meets the large intestine; there are two biological remnants at this point, the caecum and the appendix. In grass-eating animals the caecum is a large structure with the worm- like appendix at the end. They are known as biological or evolutionary remnants because in the human species, neither the caecum nor the appendix has any function. The appendix can become inflamed or pustulous and threaten life – a condition known as appendicitis. As well as the caecum and appendix, the large intestine consists of the colon and rectum, ending in the sphincter (the anus) for the elimination of faeces.
The colon runs up the right side of the abdomen and turns to travel across to the left side before ending at the anus. There are no enzymic juices in the large intestine.
The colon has a puckered appearance because the outer longitudinal muscle coat splits into three bands and the circular muscle bulges out between the bands. During the journey down the alimentary canal, many glands have poured watery juices onto the chyme. The body cannot afford to lose so much water and the purpose of the large intestine is to slow down the passage of food waste. (Food waste is all that is left at this stage because all the absorption of nutrients occurred in the small intestine). This means that water can be reabsorbed and the motion, or faeces, becomes semi-solid. It can then be eliminated by muscular action of the rectum and relaxation of the anus at a convenient time.
- cellulose (fibre or roughage) from plant cell walls from fruit and vegetables
- dead bacteria, including the usually harmless bacteria living in the large intestine which have died a natural death, and other bacteria, which are often killed by the hydrochloric acid in the stomach
- scraped-off cells from the gut lining.
The colour of faeces is due to bile pigments.
Mucus is secreted by enormous numbers of goblet cells in the gut lining to reduce friction as chyme and waste are moved along by peristalsis.
The liver is a large dark-red organ occupying the top right half of the abdomen and partly overlapping the stomach. It has a multitude of vital functions in the body, one of which is to produce bile. Bile flows down the bile duct into the abdomen after temporary storage in the gall bladder on the undersurface of the liver. Bile contains no enzymes at all, but it provides important bile salts that cause the emulsification of fats (lipids) in the have already experienced enzymic action. Lipids, like all fats, do not mix readily with water, so the enzymes have only a small water/lipid surface on which to work.
The emulsification results in the fats forming millions of tiny globules, each with a water/lipid surface so that enzymes can work efficiently over a massively enlarged surface area. Bile also contains bile pigments – bilirubin and biliverdin. These are the waste products of degraded haemoglobin from old, broken red blood cells. They give the brown colour to faeces. Bile is secreted continuously by the liver and temporarily stored in a sac called the gall bladder. When a lipid-rich meal arrives, the gall bladder releases bile into the small intestine.
The liver also removes glucose and other sugars from the blood coming from the small intestine and converts them into glycogen for storage. Surplus amino acids not required for manufacturing cell proteins are broken down in the liver to for glycogen and urea – a nitrogenous waste product transported by the bloodstream to the kidneys for elimination in urine.
The pancreas is a slim, leaf-shaped gland, located between the intestines and the stomach, close to the duodenum. It secretes enzyme-rich pancreatic juice as well as alkaline salts needed to neutralise the acidic secretions from the stomach. Pancreatic enzymes go to work on all three macronutrients (protein, fat and carbohydrate) and are important agents for the complete breakdown of the complex food molecules into amino acids, glucose and similar simple sugar, fatty acids and glycerol.
Breakdown and Absorption of Food Materials.
It is vital to understand that, without the organs and glands of the digestive system, we would be unable to use the substances collectively called food. Taking food in through the mouth (what we would call ‘eating’) is known technically as ingestion. Food is generally composed of large complex molecules of protein, carbohydrate and lipids (or fats) that would be unable to pass through the lining of the alimentary canal. Converting these complex molecules into simple soluble molecules enables their absorption into the bloodstream and onward transit for metabolic processes. Waste material that has not been capable of absorption is passed out through the anus periodically: the technical term for this is egestion.
Food and chyme move down the alimentary canal by a process known as peristalsis. There are two sheets of muscle surrounding the tube – one sheet runs in a circular fashion around the tube while the other runs down the tube. Behind the bolus or chyme the inner circular muscle contracts (and the longitudinal muscle relaxes) pushing material in front of it. This is rather like your fingers pushing toothpaste up the tube. In front of the material, the circular muscle relaxes and the longitudinal muscle contracts to hold the tube open to receive the food. Two sets of muscles acting in this way are said to be antagonistic.
Strong peristaltic waves will cause abdominal pain, usually called colic, and the food is hurried down the intestines.
Below is a diagram of peristalsis.
The Role of Enzymes in Digestion.
To break down large complex molecules in the laboratory we would use heat (as in cooking) or the addition of chemicals such as acids or alkalis. These processes are not possible in the human body, since cell and tissue structures would be destroyed or severely damages.
Body cells are able to produce ‘magical’ substances called enzymes that can alter the rate of chemical reactions to build up or break down other molecules without using heat or harmful chemicals.
Enzymes are biological catalysts – in simpler terms, substances that can act within living organisms to enable the breakdown or building-up of other chemicals, the enzymes themselves being unchanged at the end of the reactions or tasks.
Enzymes are specific to the material on which they act (called a substrate). For example, a protease only acts on protein and a lipase only acts on lipids or fats. You may have noted that adding – ase at the end of the substrate name signifies that it is enzyme. Not all enzymes are named in this way, but most are.
The main bulk of the human diet consists of protein, fat and carbohydrate so these are called macronutrients. They provide calories or joules of heat energy. Vitamins and mineral salts are only required in tiny amounts and are called micronutrients. They do not provide energy but are often important in energy release processes, oxygen carriage, metabolic rate, red blood cell formation and so on.
Enzyme reactions have some special features:
- Enzymes are sensitive to temperature. At low temperatures they work very slowly, or stop working; at high temperatures, they become distorted (denatured) and permanently stop working. Enzymes work best, or optimally, at body temperature.
- Enzymes are sensitive to the acidity or alkalinity of their surroundings, known as pH. Some digestive enzymes like pepsin (also know as gastric protease) work best in acid environment. The stomach lining secretes gastric protease and hydrochloric acid for maximum efficiency in breaking down proteins. Lipase prefers alkaline conditions and the pancreas secretes alkaline salts, such as sodium hydrogen carbonate, to provide optimal conditions. Salivary amylase prefers neutral or pH7 conditions. (Amylum is the Latin name for starch, so amylase works on starch.)
- Relatively few molecules of enzymes are required to break down lots of large food molecules because they are catalysts.
- Amylases work on cooked starch substrates (bread, rice, potatoes etc), converting the molecules to simple sugars like glucose.
- Proteases act on proteins, breaking them down into amino acids and peptides (two amino acids joined together chemically).
- Lipases convert lipids to fatty acids and glycerol.
Source: Stretch B and Whitehouse M – BTEC National Health and Social Care Book 1 (Heinemann, 2007) Accessed - 10.09.2010.
There are three systems which are involved in energy supply to the cells around the body; these are the cardiovascular, digestive and respiratory. Both the digestive and respiratory systems are important in the production of energy’s main ingredients, glucose and oxygen; they make energy by cellular respiration. This is the process in which the chemical bonds of energy-rich molecules are converted into energy that can be used for life processes. The digestive system is responsible for consuming food and water, using enzymes, breaking up complex molecules into simple soluble materials that are capable of passing into the neighbouring capillaries of the cardiovascular system.
The cardiovascular system then transports all these resources to the liver and body cells through the bloodstream, which happens because of the pumping action of the heart. As this is happening the respiratory system is persistently refreshing and reloading oxygen into the lungs, and disposing of carbon dioxide and water via the process of breathing. Dissolved oxygen passes through the lean alveolar walls in the lungs into the bloodstream and is transported to all the other cells in the body. This means that all body cells have a constant relief of glucose, oxygen and other raw materials so that the breakdown process of glucose oxidation (C6H12O6 + 6O2 → 6CO2 + 6H2O) can occur and release energy. This takes place in the cytoplasm and is completed in the mitochondria.
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