Chapter 38 OXYGENATION
The role of the cardiac and respiratory systems is to supply the body’s oxygen demands. Cardiopulmonary physiology involves delivery of oxygenated blood from the lungs to the left side of the heart and thence to the tissues, and deoxygenated blood from the tissues to the right side of the heart and thence to the pulmonary circulation for re-oxygenation. Blood is oxygenated through ventilation, perfusion and transport of respiratory gases.
The overall function of the cardiovascular system, which includes the lymphatic system, is to move blood around the body. Blood is circulated by the heart through blood vessels to transport oxygen, nutrients and other substances to the cells and to transport wastes away from the cells. Blood also assists in protecting the body against infection and distributing heat evenly throughout the body, and prevents its own loss by means of a built-in clotting mechanism. The respiratory system provides the body with the ability to absorb oxygen and excrete carbon dioxide and other waste products from the body. The two systems work in conjunction to maintain homeostasis.
Every cell in the body requires a constant supply of nutrients and oxygen, and every cell must rid itself of waste products. The cardiovascular, lymphatic and respiratory systems are the means by which these activities are achieved. The overall function of the circulatory system is transportation of substances to and from the cells.
The respiratory system is the means by which oxygen from the atmosphere is delivered to the bloodstream and carbon dioxide is diffused out from the bloodstream. This is achieved through the capillary alveoli membrane in the lungs. Ventilation is the method of delivering air into and out of the lungs. Respiration, which is the intake and use of oxygen and the elimination of carbon dioxide to the atmosphere, is achieved by the respiratory system and the cardiovascular system.
An adequate supply of blood is necessary for the normal function of every cell. Cells temporarily deprived of blood or oxygen will not function normally, and continued disruption of blood supply causes irreversible damage or cell death. Any disorder that interferes with the distribution or delivery of blood to tissues or the uptake or excretion of gases in respiration is a potential harm to body cells and may have permanent effects on a part, or all, of the body. The most common complication of a respiratory disorder is carbon dioxide retention. This can be a result of alveolar hypoventilation, or a cardiovascular disorder altering the ventilation or the perfusion of the lungs and other tissues.
Homeostasis depends on the ability of the heart to adequately circulate the required volume of blood and oxygen to the tissues. One of the most important aspects of nursing care is the maintenance or restoration of a clear airway, which includes measures directed at removing secretions by the use of suction via the nasal, oropharyngeal or endotracheal routes or by tracheostomy. Education is essential to promote exercise, which maintains optimal circulation of blood, and deep breathing and coughing exercises are encouraged to minimise the retention of secretions and secondary infections. Circulation can be assisted by changes in diet, fluid, exercise, medications and positioning. The patency of airways can be assisted by the use of humidification, nebulisation and physiotherapy using isotonic or hypotonic solutions or certain medications.
Although disorders of the cardiovascular and the respiratory system are common in most communities, the incidence of cardiac and respiratory disease is controlled to some extent by: legislation to minimise airborne irritants; immunisation programs; and health education regarding risks such as bad diet, smoking, hypertension and environmental pollution.
My dad had a pulmonary embolism, which caused him to collapse 4 days after an appendectomy. The day before it happened, I can remember he was complaining of pain in his leg, and they were actually taking him down to the X-ray department when he collapsed. Now, 18 years later, dad is on warfarin to keep his blood from clotting and has to wear antiembolic stockings. He has clots all up his right leg, which then leads to lymphoedema and, occasionally, leg ulcers. Dad is also often breathless — just doing little things — and he easily becomes puffed out. Mum and I were concerned about this, then dad’s doctor showed us an X-ray of dad’s lungs. It looked like his lungs had bullet holes in them — from the embolism. This explained the breathlessness.
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.
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:
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 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.
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 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.
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 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 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).
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.
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.
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.
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 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 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 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.
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 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 (PCO2) 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, 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 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.
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.
During inhalation the diaphragm contracts and flattens, enlarging the thoracic cavity lengthwise, particularly in males. In females normal inhalations occur primarily by contraction of the external intercostal muscles, which raise the ribs and sternum, thus increasing the size of the thoracic cavity from side to side and front to back. As the chest wall moves up and outward, the parietal pleura moves with it and, because of the 2 mmHg negative pressure within the pleura, the visceral pleura follows the parietal pleura. This causes stretching of the lungs, which expand to fill the enlarged thorax, and air is pushed into the respiratory passages.
Exhalation is normally a more passive process than inhalation except in exercise and respiratory conditions in which active expiration occurs via internal intercostal and accessory muscles. During exhalation the diaphragm relaxes thus decreasing the size of the thoracic cavity. The external intercostal muscles also relax, allowing the ribs and sternum to return to their former position, further decreasing the size of the thoracic cavity. The elastic tissue of the lungs allows for recoil, further forcing air out of the respiratory passages.
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.
|Substance||Inhaled air||Exhaled air|
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.
The passage of oxygen from the atmosphere to the alveoli in the lungs, and the passage of carbon dioxide from the alveoli to the atmosphere, requires an unobstructed airway. In addition, the process of respiration requires:
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.
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:
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).
|Review and carry out the steps in Appendix 1|
|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|
|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:||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:|
|The blood transfusion must be initiated within 15 minutes of arrival to the ward||Minimises risk of bacterial infection|
• Wearing gloves, close clamp on saline infusion and attach blood product to the short tubing on the administrating set
|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|
|Observe for reactions such as:|
|Observe for reactions such as:||Signs of a haemolytic reaction|
|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|
|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|
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.
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).
Arteries carry blood away from the heart (efferent). All arteries carry oxygenated (bright red) blood, with the exception of the two pulmonary arteries which carry deoxygenated (dark red) blood from the heart to the lungs. Arteries vary in size, and large arteries divide to form smaller arteries. Further division, or branching, occurs to form the smallest arteries, called arterioles, which divide into capillaries. Arteries and arterioles have the same tissue structure that allows them to stretch and recoil as the heart pumps the blood into them.
Veins carry blood towards the heart (afferent). All veins carry deoxygenated (dark red) blood, with the exception of the four pulmonary veins, which carry oxygenated blood from the lungs to the heart. Veins vary in size, and large veins divide to form smaller veins. The smallest veins are called venules, which divide into capillaries. Venules carry deoxygenated blood away from the capillary beds and unite to form veins. The walls of veins are composed of the same three layers as those of arteries, but the walls are thinner and have less elastic and muscular tissue. Veins join up until the two largest veins are formed — the superior and inferior vena cavae. These two veins empty their contents into the right atrium of the heart.
The larger veins possess pocket-like valves on their inner surfaces. These valves aid the unidirectional flow of blood towards the heart, and prevent a backward flow of blood. Skeletal muscle activity also helps venous return. As the muscles surrounding the veins contract and relax, the blood is ‘milked’ through the veins towards the heart.
Capillaries are microscopic vessels about 5–7 microns in diameter and are composed of a single layer of endothelium, with little surrounding connective tissue. They form closed networks through all tissues and are structurally adapted for their role in the rapid diffusion of substances between the plasma and interstitial fluid. This allows water, oxygen, nutrients and other essential substances to pass rapidly from the blood to the tissue cells, and waste products from the tissue cells pass through the capillary walls to the blood.
The heart is a hollow, conical muscular organ situated obliquely in the thoracic cavity between the lungs and behind the sternum. One third of the heart lies to the right, and two thirds lie to the left of the median plane. Its base is uppermost and points towards the right shoulder, and its apex is below, pointing to the left. The adult heart is about 12 cm × 8 cm × 6 cm, and weighs about 300 g.
The heart is divided into a right, and a left side by a muscular partition called the septum. Each side is further divided into an upper receiving chamber, the atrium, and a lower distributing chamber, the ventricle (Figure 38.9). The walls of the heart consist of the pericardium, myocardium and endocardium: