The function of the respiratory system (Figure 38.1) is to deliver oxygen from the atmosphere to the bloodstream and to deliver carbon dioxide from the bloodstream to the atmosphere. The structures that make up the respiratory tract constitute the means by which this exchange of gases occurs. The respiratory system consists of cavities and conducting airways that begin at the nasal and oral cavity and end at the alveoli, the functional unit of the respiratory system. The larger airways are composed of cartilage and smooth muscle that maintain their patency, and are gradually replaced with smooth muscle in the terminal airways, which allow alterations in airway diameter and ventilation. The two lungs are located in the thoracic cavity, encased by a double membrane known as the pleura, and are separated by the mediastinal cavity that contains the heart and great vessels. The thoracic cavity has ribs that aid in ventilation and protect the lungs from damage. The diaphragm and the internal and external intercostal regions are composed of skeletal muscle and constitute the main muscles of ventilation; other muscles are used when required for more forceful inhalation or expiration.

Figure 38.1 The respiratory tract

NASAL CAVITIES

The nose is a bony cartilaginous structure divided into a right and left nasal cavity by the nasal septum. The anterior portion of the septum is cartilage and the posterior portion is bone, formed by the vomer and part of the ethmoid bone. Inside each nostril (nares) is a vestibule lined by skin containing sebaceous and sweat glands and coarse hairs that act as filters. Apart from the vestibules, all other areas of the nasal cavity are lined by mucous membrane. In most of the cavity the membrane is covered by ciliated epithelium with many ‘goblet’ cells. Mucous cells are also present in the underlying connective tissue. The nose consists of these chambers, with specific structures and cells that have the following functions:

• Hairs and cilia line the nasal cavities and filter foreign particles and pathogens from the inhaled air

• Mucus secreted by the mucosa traps substances in inhaled air, and the cilia move particles of mucus towards the pharynx to be swallowed or expectorated

• Inhaled air is warmed and moistened as it passes over the mucosa. The three nasal turbinate bones in each cavity cause air flow to become turbulent, which enhances contact of air with the mucosa

• Sensory organ for the sense of smell (olfaction).

THE PHARYNX

The pharynx is a muscular tube about 12 cm long, lying in front of the cervical vertebrae and behind the nose, mouth and larynx. It is lined with mucous membrane and has three sections:

1. The nasopharynx, which is continuous with the nasal cavity above and with the oropharynx below. Its functions include:

— warming and moistening inhaled air

— protection from infection, by patches of lymphoid tissue such as the adenoids

— equalisation of pressure in the middle ear with the atmospheric pressure by allowing air to travel along the eustachian tubes to the middle ear

2. The oropharynx, the section that lies behind the mouth and is separated from the cavity of the mouth by two folds of mucous membrane (the fauces). Between these folds lie the oral tonsils, which are patches of lymphoid tissue involved in the immune system. The oropharynx provides a common passage for air, food and fluids. The uvula is a muscular projection of the soft palate in the middle of the arch formed by the fauces, and prevents the entry of food and fluid into the nasal cavity

3. The laryngopharynx, which is the section that opens into both the larynx and the oesophagus. It contains the epiglottis (see next section).

THE LARYNX

The larynx is situated in the upper region of the neck and extends from the pharynx above to the trachea below. It is composed of pieces of cartilage connected by membranes and provides a passageway for air between the pharynx and trachea. As air passes through, it is further moistened, warmed and filtered. The main cartilages that form the larynx are:

• The thyroid cartilage, which is the largest and forms a prominence known as the ‘Adam’s apple’

• The epiglottis, a leaf-shaped cartilage attached to the upper part of the thyroid cartilage. During swallowing, the larynx rises and the epiglottis covers its opening, directing food and fluid into the oesophagus and preventing its entry into the trachea and subsequent aspiration into the lungs

• The cricoid cartilage, which lies below the thyroid cartilage, and is shaped like a wide banded ring.

The larynx is lined with mucous membrane, which becomes ciliated in the lower part. In the upper part, two folds of membrane containing embedded fibrous and elastic tissue form the vocal cords. The vocal cords extend from the anterior wall to the posterior wall of the larynx to form the glottis, or voice box, which produce sounds. The nerve supply to the larynx is from the laryngeal and recurrent laryngeal nerves, which are branches of the vagus nerve.

Voice production

The vocal cords are apart during normal breathing. Contraction of muscles attached to the cords brings them closer together, and expired air is used to cause vibration of the cords. The brain, tongue, lips, nasal cavity and facial muscles all help to convert the resultant sounds into speech. The pitch of the voice depends on the length and tightness of the cords, and the air sinuses in the skull bones influence the resonance of the voice. The vowels and consonants that make up speech are formed by various positions of the lips and tongue. Speaking requires coordination of the larynx, mouth, lips, tongue, throat, lungs and abdomen.

THE TRACHEA

The trachea is about 12 cm long and lies in front of the oesophagus, extending from the larynx to the mid-thorax, where it divides into a right and a left bronchus. The trachea consists of 15–20 C-shaped rings of cartilage joined by involuntary muscle and fibrous tissue. Posteriorly the trachea lacks cartilage and is replaced with smooth muscle to enable the oesophagus to expand, while the cartilages maintain the patency of the airway. It thus provides a permanently open passageway for air travelling to and from the lungs. The trachea is lined with ciliated epithelium containing mucus-secreting goblet cells. The cilia sweep the mucus, cell debris and any foreign particles that enter the trachea up into the pharynx to be swallowed or expectorated. Air travelling through the trachea is further warmed and moistened to prevent drying and damage to tissues.

BRONCHI

At about the middle of the thorax, the trachea divides to form the right and left bronchus. The bronchi enter the lungs; the right bronchus dividing into three, and the left bronchus dividing into two branches. There are three lobes in the right lung and two lobes in the left lung, therefore one branch of each bronchus enters each lobe. The left bronchus is longer than the right because of the position of the heart. Smooth involuntary muscles surround the airways to allow for alteration in airway diameter.

BRONCHIOLES

Bronchioles are the smallest branches of the bronchi, and their walls consist of involuntary muscle with elastic fibrous tissue, allowing for expansion and constriction. They divide to form terminal bronchioles that give rise to microscopic alveolar ducts, which terminate in clusters of air sacs called alveoli.

ALVEOLI

Alveoli are microscopic air sacs in the lungs. Their walls are composed of one layer of type I, simple squamous epithelial cells, and type II cells that produce surfactant that maintain alveolar expansion by reducing surface tension. Macrophages are present and their role is to phagocytose cell debris and pathogens. The alveoli form a surface area of about 70 m² for semi-permeable membrane diffusion of gases. The alveoli are surrounded by networks of capillaries, arising from the pulmonary arteries and their tributaries. The function of alveoli is the interchange of oxygen and carbon dioxide between the air in the alveoli and the blood in the capillaries (Figure 38.2).

Figure 38.2 Exchange of gases in the alveoli

LUNGS

The two lungs lie in the thoracic cavity on either side of the mediastinum. The mediastinal cavity contains the heart, major blood vessels and the oesophagus. The lungs are light and spongy and consist of the bronchioles, alveoli and blood vessels and are supported by areolar tissue. There is also a great deal of elastic tissue to enable the lungs to expand and recoil freely during respiration. The base of each lung rests on the diaphragm and the apex of each extends to just above each clavicle. The right lung has three lobes and is shorter and wider than the left lung, which has two lobes (Figure 38.1). Each lobe is made up of lobules, each with its own blood, nerve and lymph supply. On the medial side of each lung is a depression called the hilus, through which the bronchi, lymphatic vessels and blood vessels enter and exit.

PLEURA

The pleura (Figure 38.3) comprise a double layer of serous membrane, consisting of the visceral pleura, which adheres to the surface of the lungs, and the parietal pleura, which lines the thoracic cavity and covers the superior surface of the diaphragm. The pleura secrete a thin film of serous fluid, maintained at about 50 mL, which lies between the two layers and prevents friction between the surfaces. The pressure within the pleura is 2 mmHg below atmospheric pressure to prevent lung collapse.

Figure 38.3 The lungs and pleura

MUSCLES OF VENTILATION

The main muscles responsible for ventilation are the diaphragm and internal and external intercostal muscles. During difficult or forced breathing, accessory muscles are used, such as the muscles of the neck, thorax (e.g. sternocleidomastoid, anterior serrate, scalene) and abdominal muscles, including the rectus abdominus and transverse abdominus.

SCIENTIFIC PRINCIPLES RELATED TO THE RESPIRATORY SYSTEM

Air is moved into and out of the lung by alterations in pressures in different areas in relation to the atmosphere. An understanding of pressure relationships and laws concerning gases will assist in the understanding of ventilation and respiration discussed later in this chapter.

PRESSURE

Pressure may be defined as force (or stress) per unit area applied to a surface. The concept of pressure, or force, applies equally to solids, liquids and gases. Described below are some of the principles and concepts relating to pressure that are relevant to nursing.

ATMOSPHERIC PRESSURE

Atmospheric pressure arises by virtue of the weight of the air above the earth. Atmospheric pressure decreases as altitude increases because of the reduced amount of air above. Even at a particular altitude, atmospheric pressure is not constant, but varies according to atmospheric conditions. The total pressure exerted by the atmosphere is about 6.8 kg per 25 mm² of surface area at sea level. The atmosphere consists principally of nitrogen (N₂) 79.03%, oxygen (O₂) 20.93%, and carbon dioxide (CO₂) 0.0004%, which totals 99.9604%. Other gases such as carbon monoxide (CO) are present in minute quantities. Oxygen, a colourless and odourless gas, is essential in sustaining most forms of life.

Water vapour and gases in the atmosphere have weight and, at sea level, exert a pressure defined as 1 atmosphere of pressure (‘atmospheric pressure’) equivalent to 760 mmHg. The terms negative and positive pressure are used to compare a pressure to normal atmospheric pressure at sea level. Any pressure above normal atmospheric pressure is regarded as a positive pressure, and any pressure below normal atmospheric pressure is regarded as a negative pressure.

The following three laws of physics define the characteristics of gases:

1. Pascal’s principle, which states that a confined liquid transmits pressure, applied to it from an external force, equally in all directions

2. Boyle’s law, which states that the volume of a given mass of gas is inversely proportional to the pressure to which it is subjected, provided that the temperature remains constant

3. Charles’ law, which states that the volume of a given mass of gas is directly proportional to its absolute temperature, provided that the pressure remains constant; for example, as the temperature of a gas is increased (at constant pressure) the gas expands.

The combined effects of atmospheric pressure and the application of the gas laws above provide the basis for the operation of many common devices, and also of the lungs. Boyle’s law refers to pressure differentials and is able to be applied to the process of breathing. By changing the volume of the thoracic cavity, the air pressure in the lungs can be made lower or higher than atmospheric pressure, leading to inhalation or exhalation, respectively.

REGULATION OF VENTILATION

SENSORY MECHANISMS OF THE RESPIRATORY SYSTEM

Every cell in the human body requires oxygen for normal metabolism and must excrete the metabolic waste product, carbon dioxide. To maintain homeostasis, cells in different locations of the body react to changes in oxygen and CO₂ levels. Sensory cells include chemoreceptors, pressorreceptors and baroreceptors.

CHEMORECEPTORS

Chemoreceptors are specialised cells located centrally in the upper medulla of the brainstem and peripherally in bodies located in the carotid and aortic arteries. They respond to slight increases in arterial or cerebrospinal fluid CO₂ pressure (P_CO2) and acidity (an increased concentration of hydrogen [H⁺] ions). Regulation of ventilation depends mainly on the level of CO₂ in the blood. A slight increase in CO₂ concentration stimulates chemoreceptors to increase the respiratory rate and depth until the excess CO₂ is eliminated. Conversely, a decreased CO₂ level slows the ventilatory rate. Oxygen levels are normally sensed by the carotid bodies, which are sensitive to a fall in oxygen concentration of less than 50%. Stimulation of the carotid receptors increases the respiratory rate, the exception being clients with chronic hypercapnia such as emphysema.

PRESSORRECEPTORS

Pressorreceptors, or mechanoreceptors, are stretch receptors present in lung tissue and within the thoracic wall. The bronchioles and alveoli also have stretch receptors that respond to extreme over-inflation as well as extreme deflation. When over-inflation occurs, impulses are transmitted from the stretch receptors to the medulla by the vagus nerve, the expiratory centre is activated and exhalation occurs. When extreme deflation occurs impulses from the lungs activate the inspiratory centre, and inhalation occurs.

BARORECEPTORS

Baroreceptors are cells sensitive to blood pressure, and which normally monitor changes in blood pressure. When blood pressure increases, impulses are sent to the respiratory centres to cause a decrease in respiratory rate. Rate, depth and rhythm of respirations are further affected by reflex responses, chemical signals and voluntary control; for example, during actions such as swallowing, impulses from gustatory centres are conveyed to the respiratory centre, and breathing stops temporarily.

Any abnormal mechanical disturbance, such as the presence of chemical substances such as cigarette smoke, causes excitement of the lung irritant receptors, which induces hyperventilation and a reflex bronchoconstriction. Information from other parts of the body may also be received by the respiratory centres; for example, a rise in body temperature initiates an increase in the rate of ventilation, while a sudden cooling of the body induces a sudden inhalation followed by hyperventilation.

THE RESPIRATORY CENTRES

Respiration is controlled both voluntarily and involuntarily. The automatic control of breathing is regulated by three respiratory centres, known as the medullary centre, located in the medulla oblongata, the apneustic centre in the pons, and the pneumotaxic centre in the upper pons of the brainstem. These centres receive stimuli from sensory cells described above, and from each other. Their function is to control the rate, rhythm and depth of ventilation. Impulses travel from the respiratory centres along separate nerves that exit the spinal cord at different levels to separately innervate and control the diaphragm and internal and external intercostal muscles. Impulses are also transmitted to the other centres and cause stimulation of the respiratory muscles via the phrenic nerves, to stimulate the diaphragm to contract, and the intercostal nerves, which stimulate the intercostal muscles.

VENTILATION AND RESPIRATION

Respiration is the term used to describe an interchange of gases. The main purpose of respiration is to supply the body with oxygen and dispose of carbon dioxide. The four processes involved are:

1. Ventilation: movement of air containing different gases into the respiratory tract

2. External respiration: exchange of oxygen and CO₂ between the blood and the alveoli (Figure 38.2)

3. Internal respiration: exchange of oxygen and CO₂ between the bloodstream and the tissues

4. Cellular respiration: exchange of gases by cells.

Respiration

External respiration

External respiration is the exchange of gases between air in the alveoli, and the blood travelling through the capillaries surrounding the alveoli. Branches of the pulmonary artery bring deoxygenated blood to the capillaries surrounding each alveolus. During gas exchange, gases normally diffuse through the semi-permeable walls of the alveoli and capillaries to the area of lowest concentration of each gas, as each gas diffuses independently of other gases, until the pressure is equal on both sides. Thus, oxygen moves from an area of higher concentration in the alveoli to an area of lower concentration in the blood capillaries, while CO₂ moves from an area of a higher concentration in blood capillaries to an area of lower concentration in the alveolar air. Pulmonary venules then collect the blood rich in oxygen from the capillaries and unite to form the two pulmonary veins which leave each lung to enter the left atrium of the heart. Table 38.1 illustrates the concentration and movement of gases in the alveoli and capillary blood. Table 38.2 illustrates the approximate composition of inspired and expired air.

TABLE 38.1 CONCENTRATION AND MOVEMENT OF GASES IN THE ALVEOLI AND CAPILLARY BLOOD

TABLE 38.2 COMPOSITION OF INHALED AND EXHALED AIR (APPROXIMATE)

Substance	Inhaled air	Exhaled air
Nitrogen	78.62%	74.5%
Oxygen	20.84%	15.7%
Carbon dioxide	0.04%	3.6%
Water vapour	0.50%	6.2%
Total	100.0%	100.0%

Internal respiration

Internal respiration is the exchange of gases between the bloodstream and the tissues (Figure 38.5). During this exchange, the gases diffuse through the semi-permeable walls of the capillaries to equalise the concentration of gases on both sides. Oxygen moves from the blood into the tissues, down a concentration gradient, to replenish oxygen used in cellular metabolism. Carbon dioxide moves from the tissues into the blood, down a concentration gradient, to rid the tissues of waste produced by cellular metabolism. Table 38.3 illustrates the concentration and movement of gases in the tissues and capillary blood.

Figure 38.5 Internal respiration

TABLE 38.3 INTERNAL RESPIRATION

Cellular respiration

The gases that diffuse from the interstitial tissue into cells are used by the cells for metabolism. The metabolism of nutrients uses oxygen and produces CO₂, which diffuses from cells into the interstitial tissue. This level of respiration is termed cellular respiration.

STRUCTURE OF THE CARDIOVASCULAR SYSTEM

The structures that make up the cardiovascular system are the:

• Heart, which acts as a pump to circulate the blood through the body

• Blood, which carries essential substances to cells and carries wastes away from cells

• Blood vessels, which contain and transport the blood throughout the body

• Lymphatic system, which transports tissue fluid containing electrolytes, proteins and some waste products, recognises and destroys pathogens before they reach the bloodstream, and delivers nutrients from the digestive tract into the cardiovascular system.

BLOOD

Blood, which is classed as a connective tissue, constitutes about one-twelfth of the weight of the body. It is a viscous substance composed of a fluid portion (plasma) and formed elements (cells and cell fragments). Depending on the weight of the individual, the average total volume of blood is about 5–6 L. Blood varies in colour, from bright red when it has a high oxygen content, to dark red when the oxygen content is low. Arterial blood normally has a pH range of 7.35 to 7.45.

Constituents of blood

Plasma and formed elements make up the components of blood. Plasma, the fluid part of blood, is a straw-coloured watery fluid in which blood cells are suspended. Plasma forms about 55% of the blood volume and contains:

• Water: about 90–92% of the plasma is composed of water, which is important in the maintenance of all body fluids and in the production of secretions

• Proteins: albumin, globulin, fibrinogen, prothrombin and heparin are some of the proteins found in plasma. The liver normally produces proteins, with the exception of serum globulin, which is derived from lymphocytes. Plasma proteins have several important functions: they assist in retaining water in the plasma and interstitial tissue; factors such as prothrombin and fibrinogen are essential for blood clotting; proteins such as heparin help to prevent abnormal clotting of blood in the blood vessels

• Mineral salts: the main mineral salts found in blood plasma are sodium chloride, iodine, potassium, phosphorus, calcium, iron, magnesium and copper. Mineral salts are necessary for the regulation of cellular functioning and electropotentials, and maintenance of the blood pH

• Nutrients: those found in blood plasma are numerous and include amino acids, glucose, fatty acids, glycerol and vitamins. They have been reduced to their simplest form by the digestive processes and absorbed from the alimentary tract into the blood and lymph for circulation to the cells

• Waste products resulting from fat and protein metabolism, including urea, uric acid and creatinine

• Gases, including oxygen, nitrogen and carbon dioxide. Oxygen and nitrogen enter the bloodstream after inhalation of air, and carbon dioxide is an end product of oxidation in the cells

• Hormones: chemical substances secreted directly into the bloodstream by endocrine glands and carried to the areas of the body where they are required to stimulate activity

• Antibodies and antitoxins: complex protein substances produced by the body in response to an invasion by a foreign protein (antigen). They are part of the body’s defence mechanism

• Enzymes, produced by the body, which initiate or accelerate chemical reactions.

Blood group types

Human blood is grouped into four classifications based on immune reactivity. The groups are O, A, B, AB. The Rhesus factor (either negative or positive) is also determined. Eighty-five percent of the population has Rh antibodies on the surface of the red blood cell (that is RH positive). Generally speaking the blood of any one group is incompatible with the blood group of another. Therefore blood transfusions should be an exact match to the client’s blood group and Rh factor. When blood transfusions occur with mismatched blood a haemolytic reaction can occur (refer to Table 38.4 Preparing and monitoring a client undergoing a blood transfusion) (Tollefson 2004).

TABLE 38.4 PREPARING AND MONITORING A CLIENT UNDERGOING A BLOOD TRANSFUSION

Review and carry out the steps in Appendix 1

Action	Rationale
Check medical orders to ascertain type, frequency and amount of fluid to be administered, and time prepared	Ensures correct quantities are given to client
Explain procedure to client	Reduced anxiety/apprehension and gains client’s trust and cooperation
Measure and record blood pressure and vital signs	Provides a baseline of the client’s haemodynamic health status
Prepare equipment
Wash and dry hands	Prevents cross infection and contamination of blood and tubing
Don appropriate equipment and clothing as per infection control guidelines
Gather equipment including: • Sphygmomanometer (aneroid or mercury) efficiency, reduce apprehension on the • Stethoscope increase the confidence in the nurse • Blood or blood product • Client’s blood order sheet	Ensures all equipment is at
Establish an IV infusion with normal saline	IV access is established by a doctor or accredited Registered Nurse (RN). Normal saline is the solution used during a blood transfusion because it is compatible with blood and does not cause red blood cell lysis
Identify the client and the blood product according to policy	Group and type of blood product matches on the order and the product
Two nurses check the: • Client’s armband • Blood order sheet • Information on the blood product • Expiry date
The blood transfusion must be initiated within 15 minutes of arrival to the ward	Minimises risk of bacterial infection
Initiate the blood transfusion • Gently invert the blood product several times • Wearing gloves, close clamp on saline infusion and attach blood product to the short tubing on the administrating set • Tip the blood and gently squeeze to fill the filter • Place blood on IV stand and open the clamp, squeeze the drip chamber and release it until blood begins to flow	Ensures cells and plasma are mixed
Initiate the transfusion slowly	Most reactions occur within the first 10 minutes. Beginning the transfusion slowly reduces the amount of blood for the system to react against reducing the severity of the reaction
Monitor the client	Vital signs are taken as per facility guidelines. Most are generally taken every 15 minutes for the first hour of the infusion then hourly for the remainder of the infusion
Observe for reactions such as:	Signs of a febrile reaction. Slow infusion rate
• Fever, chills, headache, malaise
Observe for reactions such as:
• Flushing of the skin, urticaria, wheezing, itchy rash	Signs of an allergic reaction. Stop infusion, flush with normal Notify doctor, give IV corticosteroids as ordered Restart the transfusion with a new blood product
Observe for reactions such as:	Signs of a haemolytic reaction
• Restlessness, anxiety, chest pain, tachypnoea, tachycardia, nausea, shock, haematuria	Stop transfusion, start new giving set with normal saline Commence oxygen therapy Notify doctor, give adrenaline and IV corticosteroids as ordered
Complete the transfusion	Ensures client receives the entire transfusion
Dispose of the blood unit as per facility guidelines	Some facilities require all blood units (including the giving set) to be returned to blood bank and kept for 24 hours in case of a delayed reaction
Monitor client	Clients can have a delayed reaction to the blood product for up to 24 hours
Monitor client	Vital signs are often recorded hourly for 4 hours then 4 hourly for 24 hours
Report and document the procedure and any complications	Most blood products have a peel off identification tag that is identical to the blood unit ID number, grouping and Rh factor so that errors in transcription are avoided. This tab should be removed and placed in the

BLOOD CELLS

The blood cells and fragments are suspended in the plasma and are called formed elements. The three types of blood cells or fragments of cells are erythrocytes, leucocytes and thrombocytes.

Erythrocytes (red cells) are biconcave non-nucleated discs measuring about 7 microns in diameter. In adults, erythrocytes are produced in the red bone marrow of cancellous bone tissue, where they pass through several stages of development. They begin as large nucleated cells but when mature (after they have produced haemoglobin) they lose the nucleus and are liberated into the circulation. Haemoglobin is a complex protein composed of four different ‘haem’ chains, each containing a central atom of iron and a globulin protein. It has a strong affinity for both oxygen and carbon monoxide and gives the blood its colour. The normal haemoglobin level is about 14–16 g/100 mL of blood.

The number of erythrocytes is about 5 000 000/mm³ of blood, and their average life span is 100–120 days. As their nucleus is absent, they are unable to repair damage and become worn out in circulation, and are destroyed in the spleen and liver. The haemoglobin is split; its iron is stored by the liver for future use, and the pigment is used by the liver in the production of bile. The primary function of erythrocytes is to carry oxygen. In the lungs, oxygen combines with haemoglobin to form oxyhaemoglobin, making the blood bright red in colour. As blood circulates through the tissues, the oxygen is released, forming deoxyhaemoglobin, and the blood becomes dark red in colour.

Leucocytes (white cells) measure about 10 microns in diameter. They differ from erythrocytes in that they are larger, possess a nucleus and are less numerous. They also have the power of independent movement, known as diapedis, or emigration, which erythrocytes do not possess. There are two main types of leucocytes: granulocytes and agranular leucocytes. Granulocytes contain granules of enzymes and are classified as neutrophils, basophils or eosinophils. Neutrophils are the most numerous of the leucocytes and are important to the body in defence against bacteria, as they have the ability to engulf phagocytose and digest them. Neutrophils also play an important part in the inflammatory response. Injured tissues, and other leucocytes, secrete substances that stimulate the bone marrow to release increased numbers of neutrophils. Basophils release substances in infected tissue that are toxic to many microorganisms. They also play a part in the allergic response and act to limit the inflammatory response. Eosinophils are also involved in phagocytosis, as they ingest antigen–antibody complexes and parasites. They also play a role in clot retraction.

Agranular leucocytes lack granules of enzymes and are classified as monocytes or lymphocytes. Monocytes have the ability to move into the tissues, where they become macrophages and are capable of phagocytosis. They also secrete a variety of substances involved in the body’s defence, and play a role in the immune response. Lymphocytes are either T lymphocytes or B lymphocytes, both of which divide when stimulated by antigens. T lymphocytes are responsible for cellular immunity, and adhere to cells identified as foreign to the body. They secrete cytotoxic substances that kill the foreign cells. B lymphocytes are involved in humoral immunity, as they produce antibodies and are also responsible for immunoglobulin production. While the life span for granular leucocytes is only about 21 days, lymphocytes may survive for up to 100 days.

The total number of leucocytes is about 8000–10 000/mm³ of blood, but this number increases considerably (leucocytosis) when there is any infection in the body. The life span of a leucocyte is variable and depends to some extent on the degree of activity.

Thrombocytes (platelets) are colourless microscopic fragments of the megakaryocyte cell. Measuring about 3 microns in diameter, they do not possess a nucleus. Thrombocytes are produced in the red bone marrow, which is present in cancellous bone tissue. The number of thrombocytes is about 250 000–300 000/mm³ of blood, and the average life span of a thrombocyte is 5–9 days. The function of thrombocytes is to play a major role in the clotting of blood to reduce blood loss when a vessel wall is injured. The process involves many substances (clotting factors) which are produced by the liver and circulate in the plasma, as well as some substances released by the platelets and injured tissues. Normally a blood clot will form within 2–6 minutes after a blood vessel wall has been damaged.

The mechanism of clotting (haemostasis) involves three phases: vasoconstriction, formation of a temporary platelet plug and formation of a clot. When a small vessel becomes damaged:

• Local vasoconstriction occurs, which reduces blood flow and therefore blood loss

• The vessel wall becomes ‘sticky’ and platelets adhere to the damaged area

• The platelets release serotonin and adenosine diphosphate (ADP), which attract other thrombocytes, leading to the formation of a temporary platelet plug

• The temporary platelet plug is converted into a clot by the deposition of fibrin, which is formed from fibrinogen. The conversion of fibrinogen to fibrin involves a ‘cascade’ of reactions that requires a number of plasma factors (numbered I to XIII). A series of reactions culminate in the conversion of prothrombin to thrombin, which converts fibrinogen to fibrin. This conversion requires the presence of platelets, Factor V, Factor X and calcium ions. Vitamin K is also necessary for the conversion of Factors VII, IX and X

• The fibrin forms a meshwork of fibres that traps the erythrocytes and form the basis of a clot

• The clot plugs the injured blood vessel, drawing the edges together.

The clotting mechanism is a complex one that will not occur if any of the necessary elements are reduced, defective or missing.

BLOOD VESSELS

Blood is circulated throughout the body within vessels that form a closed continuous system (Figure 38.6). The walls of blood vessels have three layers: an outer coat of fibrous tissue, a thick middle layer of involuntary muscle with elastic fibrous tissue and an inner lining of endothelium to form a smooth surface for contact with blood (Figure 38.7). Blood vessels include the arteries, veins and capillaries (Figure 38.8).