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Human Respiratory System


Function of the Respiratory System

The function of the human respiratory system is to transport air into the lungs and to facilitate the diffusion of Oxygen into the blood stream. Its also receives waste Carbon Dioxide from the blood and exhales it.

Respiratory System ?

The respiratory system consists of the following parts, divided into the upper and lower respiratory tracts:

Parts of the Upper Respiratory Tract :

Mouth, nose & nasal cavity: The function of this part of the system is to warm, filter and moisten the incoming air


Pharynx: Here the throat divides into the trachea (wind pipe) and oesophagus (food pipe). There is also a small flap of cartilage called the epiglottis which prevents food from entering the trachea


Larynx: This is also known as the voice box as it is where sound is generated. It also helps protect the trachea by producing a strong cough reflex if any solid objects pass the epiglottis.

Parts of the Lower Respiratory Tract :

Trachea: Also known as the windpipe this is the tube which carries air from the throat into the lungs. It ranges from 20-25mm in diameter and 10-16cm in length. The inner membrane of the trachea is covered in tiny hairs called cilia, which catch particles of dust which we can then remove through coughing. The trachea is surrounded by 15-20 C-shaped rings of cartilage at the front and side which help protect the trachea and keep it open. They are not complete circles due to the position of the oesophagus immediately behind the trachea and the need for the trachea to partially collapse to allow the expansion of the oesophagus when swallowing large pieces of food.


Bronchi: The trachea divides into two tubes called bronchi, one entering the left and one entering the right lung. The left bronchi is narrower, longer and more horizontal than the right. Irregular rings of cartilage surround the bronchi, whose walls also consist of smooth muscle. Once inside the lung the bronchi split several ways, forming tertiary bronchi.


Bronchioles: Tertiary bronchi continue to divide and become bronchioles, very narrow tubes, less than 1 millimeter in diameter. There is no cartilage within the bronchioles and they lead to alveolar sacs.


Alveoli: Individual hollow cavities contained within alveolar sacs (or ducts). Alveoli have very thin walls which permit the exchange of gases Oxygen and Carbon Dioxide. They are surrounded by a network of capillaries, into which the inspired gases pass. There are approximately 3 million alveoli within an average adult lung.


Diaphragm: The diaphragm is a broad band of muscle which sits underneath the lungs, attaching to the lower ribs, sternum and lumbar spine and forming the base of the thoracic cavity.


The Mechanics of Breathing :


The action of breathing in and out is due to changes of pressure within the thorax, in comparison with the outside. This action is also known as external respiration. When we inhale the intercostal muscles (between the ribs) and diaphragm contract to expand the chest cavity.


The diaphragm flattens and moves downwards and the intercostal muscles move the rib cage upwards and out. This increase in size decreases the internal air pressure and so air from the outside (at a now higher pressure that inside the thorax) rushes into the lungs to equalise the pressures.

When we exhale the diaphragm and intercostal muscles relax and return to their resting positions. This reduces the size of the thoracic cavity, thereby increasing the pressure and forcing air out of the lungs.


Breathing Rate :

The rate at which we inhale and exhale is controlled by the respiratory centre, within the Medulla Oblongata in the brain. Inspiration occurs due to increased firing of inspiratory nerves and so the increased recruitment of motor units within the intercostals and diaphragm. Exhalation occurs due to a sudden stop in impulses along the inspiratory nerves.

Our lungs are prevented from excess inspiration due to stretch receptors within the bronchi and bronchioles which send impulses to the Medulla Oblongata when stimulated.


Breathing rate is all controlled by chemoreceptors within the main arteries which monitor the levels of Oxygen and Carbon Dioxide within the blood. If oxygen saturation falls, ventilation accelerates to increase the volume of Oxygen inspired.

If levels of Carbon Dioxide increase a substance known as carbonic acid is released into the blood which causes Hydrogen ions (H+) to be formed. An increased concentration of H+ in the blood stimulates increased ventilation rates. This also occurs when lactic acid is released into the blood following high intensity exercise.


Respiratory Volumes :


Respiratory volumes are the amount of air inhaled, exhaled and stored within the lungs at any given time

Tidal Volume: The amount of air which enters the lungs during normal inhalation at rest. The average tidal volume is 500ml. The same amount leaves the lungs during exhalation.

Inspiratory Reserve Volume: The amount of extra air inhaled (above tidal volume) during a deep breath. This can be as high as 3000ml.

Expiratory Reserve Volume: The amount of extra air exhaled (above tidal volume) during a forceful breath out.

Residual Volume: The amount of air left in the lungs following a maximal exhalation. There is always some air remaining to prevent the lungs from collapsing.

Vital Capacity: The most air you can exhale after taking the deepest breath you can. It can be up to ten times more than you would normally exhale.

Total Lung Capacity: This is the vital lung capacity plus the residual volume and is the total amount of air the lungs can hold. The average total lung capacity is 6000ml, although this varies with age, height, sex and health.

Blood


Blood has many functions. These include:

Transportation: The blood carries other substances around the body inside Arteries, Veins and Capillaries. These include gasses (Oxygen and Carbon Dioxide), waste products (water, urea), hormones, enzymes and nutrients (glucose, amino acids, vitamins and minerals). The blood flows through the Circulatory System.
Maintaining Homeostasis: Altering the blood flow to the skin can help to reduce body temperature. Transportation of enzymes which are used to maintain our internal environments.
Immunity and defence: White blood cells fight infection and platelets help repair damage and clot the blood

Constituents of blood :

Blood is made up of a number of types of cells:

Plasma: Plasma is a straw-coloured fluid in which blood cells are suspended. It is made up of approximately 90% water as well as electrolytes such as sodium and potassium and proteins.
Red Blood Cells (Erythrocytes): The main function of red blood cells is to carry oxygen. RBC's contain a protein called Haemoglobin. This combines with oxygen to form Oxyhaemoglobin. Each RBC has a lifespan of approximately 120 days before it gets broken down by the spleen. New RBC's are manufactured in the bone marrow of most bones. There are approximately 4.5-5 million RBC's per micro-litre of blood.
White Blood Cells (Leucocytes): There a number of types of white blood cells, although the function of all of them is to help fight disease and infection. They typically have a lifespan of a few days and there are only 5-10 thousand WBC's per micro-litre of blood.
Platelets (Thrombocytes): Platelets are disc shaped cell fragments which are involved in clotting the blood to prevent the excess loss of body fluids.

The Human Circulatory System :


What is the Human Circulatory System ?

The main organ of the circulatory system is the Human Heart. The other main parts of the circulatory system include the Arteries, Arterioles, Capillaries, Venules, Veins and Blood. The lungs also play a major part in the pulmonary circulation system.

The Functions of the Circulatory System ?

The function of a humans circulatory system is to transport blood around the body. The blood itself also carries numerous other substances which the body requires to function.

The main substance being Oxygen, carried by a protein called haemoglobin, found inside red blood cells. White blood cells are also vital in their role of fighting disease and infection. Blood contains platelets which are essential for clotting the blood, which occurs follwing an injury to stop blood loss. Blood also carries waste products, such as Carbon Dioxide away from muscles and organs in order to be dispelled by the lungs.

How the Circulatory System Works :

There are three circulatory processes occurring simultaneously within the body. Firstly systemic circulation carries blood around the body, pulmonary circulation carries blood to the lungs and coronary circulation provides the heart with its own supply of blood.

Systemic Circulation :

At the start of the blood circulatory cycle the heart pumps oxygenated blood out of the left ventricle, through the Aorta (the largest artery in the body). The aorta divides into smaller arteries, then arterioles and finally into microscopic capillaries, found deep within muscles and organs. Here the Oxygen (and other nutrients) passes through the thin capillary walls, into the tissues where it can be used to produce the energy muscles require to contract. (See the diagram below)

A waste product of energy production (metabolism) is Carbon dioxide and in order to be removed, it too passes across the walls of the capillaries, into the blood stream. The blood continues back towards the heart, through venules and then veins, into the right atrium.

Pulmonary Circulation :

Once blood returns to the heart it is then pumped from the right ventricle through the Pulmonary arteries to the lungs, where the waste carbon dioxide can be expelled and more Oxygen collected. The Pulmonary vein carries oxygenated blood back to the left atrium of the heart, where the cycle starts again.

The Heart


The heart is a strong, powerful organ, consisting of cardiac muscle. The heart pumps continuously, without resting and without becoming fatigued. Its function is to pump blood to the lungs and around the body. The heart is one of the key organs in the Circulatory System.








Anatomy of the heart :

The heart consists of four chambers and is divided into left and right by a wall of muscle called the septum. The right side of the heart consists of an atrium which recieves blood returning from the body, and the right ventricle, which then pumps blood out to the lungs, via the pulmonary artery.

The left side again contains an atrium and a ventricle. The left atrium recieves the oxygenated blood returning from the lungs and the ventricle then pumps this blood around the body.

Due to the distance which the blood being pumped from the left ventricle has to travel, a more forceful contraction is required. For this reason the muscular wall of the left side is thicker than that of the right side.


The atria and ventricles are separated by valves known as Atrioventricular, or AV valves. The purpose of these valves is to prevent blood from flowing in the wrong direction. Following the movement of blood from the atrium, into the ventricle, the AV valve snaps shut which causes the first heart sound of the heart beat (often described “lub dub”, with the closing of the AV valves being the “lub”)

The “dub” sound is caused by the closing of two other valves, known as the Semilunar or SL valves. These are located between each ventricle and the artery leaving the heart, and again prevent the blood flowing backwards.The way in which the cardiac muscle contracts in order to force blood around the body is highly specialised and will describe later .

The Heart Beat :


The heart beat is caused by impulses arising from two specialised groups of cells within the heart muscle. The Sino-Atrial (SA) node, situated in the wall of the right atrium initiates the beat, and the Atrioventricular (AV) node which is positioned between the ventricles and continues to distribute the wave of impulses.

There are several distinct stages which form a full heart beat. Cardiac Systole describes the period at which the heart contracts and cardiac diastole describes the period of relaxation, between beats. They can however be further divided into diastole and systole of the atria and ventricles.

The phases of a heart beat can also be divided into sections relating to the shape of the electrical signals produced when viewing the heart beat via an ECG (Electrocardiogram). This traces the electrical activity of the heart. The wave shape produced is called the QRS wave, with each part of the wave being labelled to help describe what is happening at each stage.

TP Interval (Ventricular Diastole)
Atria and ventricles are relaxed, blood is flowing into the atria from the veins. As the atrial pressure increases above that of the ventricle, the AV valves open, allowing blood to flow into the ventricle

P Wave (Atrial Systole)
The SA node fires and the atria contract causing atrial systole which forces all blood into the ventricles, empyting the atria.

QR Interval (End of Ventricular Diastole)
The AV valves remain open as all remaining blood is squeezed into the ventricles. The electrical impulse from the SA node reaches the AV node which spreads the signal throughout the walls of the ventricles via specialised cells called bundles of His and Purkinje fibres. The R peak is the end of ventricular diastole and the start of systole.

RS Interval (Ventricular Systole)
As the blood is now all within the ventricles and so pressure is higher here than in the atria, the AV valves close. The ventricles start to contract although pressure is not yet high enough to open the SL valves

ST Segment (Ventricular Systole)
Pressure increases until it equals Aortic pressure, when the SL valves open. The blood is ejected into the Aorta (and pulmonary artery) as the ventricles contract. At this time the atria are in diastole and filling with blood returning from the veins.

T Wave (Ventricular Diastole)
Ventricles relax, the ventricular pressure is once again less than the aortic pressure and so the SL valves close. The cycle continues.

video.about.com/heartdisease/How-the-Heart-Functions.htm

Thank you dear Dr. James.
I'm sharing a video, it is amazing to see what you are describing.