The Circulatory System
What is the basic structure and function of the circulatory system?
The circulatory (or cardiovascular) system includes:
The respiratory passage.
The lungs.
The heart.
The blood vessels.
The blood circulated throughout the entire body.
Respiratory Passage
Air passes through the following process:
During inhalation, air enters the nostrils (preferable to the mouth), as nasal breathing warms and filters the air in the nasal cavities.
Air then passes through the nasopharynx, and then the oropharynx.
Air passes by the larynx, the organ connecting the lower part of the pharynx with the trachea. The larynx guards the air passages, especially during swallowing, for the maintenance of open airway and for vocalisation.
Air then passes through the glottis, at which point the epiglottis closes. The glottis is made of the vocal folds and expands whilst inhaling for air to enter the trachea. The epiglottis is a leaf-shaped flap projecting out of the larynx, and it flattens to cover the opening of the larynx so that food passes into the oesophagus rather than the trachea
Air passes down the trachea, a cylindrical pipe around 10–13 cm in length that is kept open by a series of rings of cartilage.
Air then passes into the right and left bronchi, which branch into the Bronchioles, each of which terminates in a cluster of alveoli, the site of gaseous exchange.
During exhalation, air moves in the reverse direction.
Inhaled and exhaled air has the following proportion of atmospheric gases:
Inhaled Air:
Nitrogen: 79%
Oxygen: 21%
Carbon Dioxide: <1%
Trace gasses: <0.001%
Exhaled Air:
Nitrogen: 79%
Oxygen: 16%
Carbon Dioxide: 4%
Trace gases: <0.001%
Note that 1% of exhaled air is lost to evaporated water on exhalation.
Lungs
The two lungs located near the backbone on either side of the heart are primary organs of the respiratory system.
Their function is to extract oxygen from the air and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange.
The inspiration and expiration of gases in the lungs is caused by changes in lung-tissue pressure.
These changes come about as a result of the contraction and relaxation of the muscles surrounding the thorax and ribs.
Breathing rate goes on continuously and for much of the time unconsciously. It is controlled by the respiratory centre of the brain, which responds very quickly to slight changes in the blood concentration of carbon dioxide. Even a small increase in the carbon dioxide level can greatly increase the volume of air breathed in and out. Lack of oxygen also affects the breathing rate, but is detected mainly by sensors in the large blood vessels in the neck.
Inspiration
An inward breath is started when the diaphragm contracts and descends. This increases the thoracic volume.
The external intercostals contract, causing the ribcage to rise and flare, creating a further increase in thoracic volume, which expands the lungs.
The pressure within the lungs drops to below atmospheric pressure.
External air flows into the body until the pressures are equal.
Expiration
An outward breath starts with the relaxation of the diaphragm and external intercostals.
The ribcage drops and the diaphragm rises back into its original position.
This decreases the intrathoracic volume, and the elastic fibres in the lung tissue recoil to decrease the lung volume.
The resulting increase in intrapulmonary pressure causes the expulsion of air.
Residual Volume refers to the minimum of approximately one litre of air that the lungs always contain. If the lungs contained no air at all they would collapse.
Tidal Volume refers to the approximately half a litre of air flowing into and out of the respiratory passages when the body is at rest. An adult's average respiratory rate would be approximately 12 breaths per minute.
Ventilation = Tidal Volume x Number of Breaths Per Minute = 500 ml x 12 /minute = 6000 ml/minute (6 litres/minute).
Exhaling normally and then breathing in as much air as possible would give a measure referred to as Forced Maximum Inspiration.
After maximum forced inspiration, breathing out as much air as possible (Maximum Forced Expiration) would give a measure referred to as Vital Capacity.
The residual volume can be added to the vital capacity to calculate Total Lung Capacity. In the average adult this would be approximately 6000 ml or 6 litres.
Gas exchange describes the exchange of gases, namely oxygen and carbon dioxide, across cell surfaces. It is the driver of energy production in the cells or ‘respiration’ (not to be confused with breathing). Changes in pressures and concentrations within body cavities and tissues drive this process.
Diffusion refers to the movement of the ventilatory gases:
A gas will move from an area where its pressure or concentration is higher (meaning there is lots of it) to an area where its pressure or concentration is lower (there is less of it).
In addition, the greater the pressure difference between the two areas, the more rapid the movement of gases.
Pulmonary ventilation provides air to the alveoli for this gas exchange process to occur: where the alveolar and capillary walls meet, gases move across the membranes, with oxygen entering the bloodstream and carbon dioxide exiting.
It is through this mechanism that blood is oxygenated and carbon dioxide, the waste product of cellular respiration, is removed from the body.
Both alveoli and capillaries have walls that are only one cell thick and allow gases to diffuse across them, which is ideal for facilitating gas exchange:
Alveoli :
Outside: Capillary blood higher in carbon dioxide, lower in oxygen.
Inside: Alveolar air sacs higher in oxygen, lower in carbon dioxide.
Diffusion: Oxygen in, Carbon dioxide out.
Respiring / working cells of body:
Outside: Capillary blood lower in carbon dioxide, higher in oxygen.
Inside: Muscle fibre cells higher in carbon dioxide, lower in oxygen.
Diffusion: Carbon dioxide out, Oxygen in.
In sum, the change in pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs, where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs and oxygen into the blood.
The Heart
The heart divides into four chambers:
Upper left atrium.
Upper right atrium.
Lower left ventricle.
Lower right ventricle.
These act as a double-action pumping system: the two atria contract whilst the ventricles relax, then when the two ventricles contract, the two atria relax.
Blood moves between the chambers of the heart and this flow is regulated by the various valves. In a healthy heart, blood flows one way through the valves which prevent backflow.
The left ventricle has a thicker muscular wall than the right ventricle to pump blood more forcefully to the working muscles.
Two Systems of Circulation
The circulatory systems divides into two systems of circulation:
Pulmonary circulation
Systemic circulation
Pulmonary circulation refers to the flow of blood from the heart to the lungs and back again:
To the lungs:
Right atrium
Right ventricle
Pulmonary arteries
Alveoli (Lungs)
From the lungs:
Alveoli (Lungs)
Pulmonary veins
Left atrium
Left ventricle
Within the heart, the right atrium receives the deoxygenated blood from the body through the inferior and superior vena cavae, where it is then pumped into the right ventricle through an atrioventricular valve called the tricuspid valve - which prevents back flow of blood between the right atrium and ventricle.
From here it is then pumped through a semi lunar valve called the pulmonary valve and sent to the lungs for oxygenation via the right and left pulmonary arteries.
Oxygenated blood then flows from the lungs into the left atrium via the pulmonary veins.
Systemic circulation refers to the flow of blood from the heart to the rest of the body and back again:
To the body:
Left atrium
Left ventricle
Aorta
Body tissues
From the body:
Body tissues
Venae cavae
Right atrium
Right ventricle
From the left atrium, blood flows into the left ventricle, through another atrioventricular valve called the bicuspid or mitral valve (preventing backflow of blood between the left atrium and ventricle) where it is then pumped through another semi lunar valve called the aortic valve (which prevent back flow of blood from the aorta to the left ventricle) to the rest of the body via the aorta.
It then returns to the heart via the inferior and superior vena cavae, and the whole process continues.
Blood flows through the system of blood vessels because of different pressures in different regions of the system.
Blood Pressure is a measure of the pressure exerted by blood on the walls of the arteries to keep the blood circulating in the blood vessels. It is measured using a Sphygmomanometer and is taken on the Brachial Artery, slightly superior to the elbow joint on the anterior surface of the arm.
There are two readings which are taken to measure blood pressure:
1. Systolic Pressure is the pressure exerted when blood is forced out of the left ventricle into the aorta. The normal range for systolic pressure would be 110 to 130 mmHg.
2. Diastolic Pressure is the pressure of blood in the arteries when the left ventricle is in diastole or relaxation. The normal range for diastolic pressure would be 70 to 90 mmHg.
Blood pressure readings are always expressed as Systolic Pressure / Diastolic Pressure (Systolic 'over' Diastolic).
The amount of blood ejected from the left ventricle into the aorta per minute is referred to as the ‘cardiac output’ (CO or Q). It is calculated by multiplying the ‘stroke volume’ (SV), which is the amount of blood pumped from the left ventricle measured in millilitres, by the heart rate (HR) or the number of heartbeats per minute (bpm).
CO or Q also increases in proportion to cardiorespiratory training intensity.
For example:
If SV = 70 ml, and HR = 75 bpm
Then, Q = 70 ml x 75 bpm = 5,250 ml/minute
Factors that affect SV have a proportionate effect on cardiac output. A more-efficient heart has a greater SV; therefore, if the heart had the ability to pump a greater volume of blood with each beat, it would be able to pump or beat at a slower rate to produce the same level of blood flow through the body.
In the average adult, SV is approximately 70–80 ml per beat; in trained individuals, it can be as high as 100–110 ml per beat. SV increases with activity intensity and can be as high as 200 ml per beat.
A healthy adult has a resting heart rate of 60–80 bpm. A well-trained athlete could have a resting heart rate of 35–50 bpm. Resting heart rate, however, does increase with age.
Blood Vessels
There are over 60,000 miles of blood vessels in the body, including:
Arteries
Arterioles (small arteries)
Capillaries
Veins
Venules (small veins)
Each of these vary in size and in function, with the main function being to transport blood around the body:
Arteries carry blood from the heart to other parts of the body.
Veins transport blood from other parts of the body to the heart.
The walls of arteries, veins and their smaller counterparts contain smooth muscle, which provides wave-like contractions to assist in the flow of blood.
It is within the capillaries that nutrients, oxygen, carbon dioxide and metabolic waste are passed into and out of cells.
Blood vessels can increase and decrease in diameter, referred to as ‘vasodilation’ and ‘vasoconstriction’, respectively.
Exercise stimulates vasodilation, and this is thought to be one of the major reasons why exercise reduces the risk of cardiovascular disease.
Blood
Blood is a body fluid in the circulatory system that delivers necessary substances, such as nutrients and oxygen, to the cells and transports metabolic waste products away from those same cells. It consists of:
Red blood cells (erythrocytes)
In a drop of blood there is approximately 240-270 million red blood cells (RBCs). They are produced in the soft red bone marrow, and contain a protein called haemoglobin (Hb) which binds to the oxygen (O2) allowing O2 to be carried in the blood. Haemaglobin is the pigment that gives the RBCs its red colour.
White blood cells (leukocytes)
White blood cells (WBCs) are transparent, fewer in quantity, and are also produced in the red bone marrow. These are the cells of the immune system that fight infection, destroy bacteria and other harmful organisms, and protect the body by removing diseased or injured tissue.
Platelets (thrombocytes)
Platelets are slightly different as they are cell fragments. They aid in preventing blood loss by forming a 'plug'. Platelets aid in blood clotting by releasing chemicals, which stimulate the first stage of repair from injury.
Plasma
Plasma is the straw coloured liquid found in the blood and makes up 55% of blood volume. It is predominately water and is also made up of solutes, such as proteins, electrolytes, nutrients, gases, hormones, enzymes, vitamins and waste products.
This summarises the basic structure and function of the circulatory system.
