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Lower respiratory tract
The lower respiratory tract consists of the trachea, bronchi and lungs. The air passages are lined with mucous membrane composed mainly of ciliated epithelium. Cilia constantly clean the tract and carry foreign matter upwards for swallowing or expectoration.
The trachea (windpipe) extends from the laryngopharynx at the level of the cricoid cartilage at the top to the carina (also called the tracheal bifurcation). C-shaped cartilage rings reinforce and protect the trachea to prevent it from collapsing. The carina is a ridge-shaped structure at the level of T6 or T7. The carina possesses sensory nerve endings which cause coughing if food or water is inhaled accidently.
The primary bronchi begin at the carina. The right primary bronchus - shorter, wider and more vertical than the left - supplies air to the right lung. The left primary bronchus delivers air to the left lung. Along with blood vessels, nerves, and lymphatics, the primary bronchi enter the lungs at the hilum. Located behind the heart, the hilum is a slit on the lung’s medial surface.
Each primary bronchus divides to form secondary bronchi. In each lung, one secondary bronchus goes into each lobe which means that the right lung has three secondary bronchi and the left lung has two.
Each lobar bronchus enters a lobe in each lung. Within its lobe, each of the lobar bronchi branches into segmental bronchi (tertiary bronchi). The segments continue to branch into smaller and smaller bronchi, finally branching into bronchioles. The larger bronchi consist of cartilage, smooth muscle and epithelium. As the bronchi become smaller, they lose cartilage and then smooth muscle. Ultimately, the smallest bronchioles consist of just a single layer of epithelial cells.
Each bronchiole includes terminal bronchioles and the alveolar sac - the chief respiratory unit for gas exchange. Within the acinus, terminal bronchioles branch into yet smaller respiratory bronchioles. The respiratory bronchioles feed directly into alveoli at sites along their walls.
The respiratory bronchioles eventually become alveolar ducts, which terminate in clusters of alveoli surrounded by capillaries (alveolar sacs). Gas exchange takes place through the alveoli.
Alveolar walls contain two basic epithelial cell types:
- Type I cells are the most abundant. It is across these thin, flat, squamous cells that gas exchange occurs.
- Type II cells secrete surfactant, a substance that coats the alveolus and reduces surface tension. This allows the alveoli to remain inflated so that gas exchange can occur by diffusion. Surfactant is formed relatively late in foetal life; thus premature infants born without adequate amounts experience respiratory distress and may die.
The cone-shaped lungs are located in the thoracic cavity and are surrounded by pleura. The right lung is shorter, broader and larger than the left. It has three lobes and handles 55% of gas exchange. The left lung has two lobes and contains a space for the heart (cardiac notch). Each lung’s concave base rests on the diaphragm; the apex extends about 1.5 cm above the first rib.
Pleura and pleural cavities
The pleura - the membrane that totally encloses the lung - is composed of a visceral layer and a parietal layer. The visceral pleura covers the entire lung surface, including the areas between the lobes. The parietal pleura lines the inner surface of the chest wall and upper surface of the diaphragm.
Serous fluid has serious functions
The pleural cavity - a potential space between the visceral and parietal pleural layers - contains a thin film of serous fluid. This fluid has two functions:
- It lubricates the pleural surfaces, which allows them to slide smoothly against each other as the lungs expand and contract.
- It creates a bond between the layers that causes the lungs to move with the chest wall during the mechanical breathing process.
Excerpted from Scott: Anatomy & Physiology Made Incredibly Easy! 1st UK Edition (ISBN: 978-1-901831-22-1)
The haematological system consists of the blood and bone marrow. Blood delivers oxygen and nutrients to all tissues, removes wastes, and transports gases, blood cells, immune cells, antibodies and hormones throughout the body.
Living up to their potential
The haematological system manufactures new blood cells through a process called haematopoiesis.
Multipotential stem cells in bone marrow give rise to five distinct cell types, called unipotential stem cells. Unipotential cells differentiate into one of the following four types of blood cells:
- eryhrocytes (the most common type)
Blood consists of various formed elements, or blood cells, suspended in a fluid called plasma. The RBCs of blood - and the WBCs and platelets, too Formed elements in the blood include:
- red blood cells (RBCs), or erythrocytes
- white blood cells (WBCs), or leucocytes
- platelets, or thrombocytes.
RBCs and platelets function entirely within blood vessels; some WBCs remain in the blood while others can enter tissues.
Red blood cells
RBCs mainly transport oxygen to from the lungs to the body tissues. They contain haemoglobin, the oxygen-carrying substance that gives blood its red colour. When the red cells have given up the oxygen, they are capable of transporting some carbon dioxide back to the lungs for removal.
However, carbon dioxide is mainly transported in the blood as bicarbonate.
The life and times of the RBC
RBCs have an average life span of 120 days. Bone marrow releases RBCs into circulation in immature form as reticulocytes. The reticulocytes mature into RBCs in about 1 day. The spleen removes old, worn-out RBCs from circulation.
A balance between removal and renewal
The rate of reticulocyte release usually equals the rate of old RBC removal. When RBC depletion occurs (e.g. with haemorrhage), the bone marrow increases reticulocyte production to maintain the normal RBC count.
White blood cells
Five types of WBCs participate in the body’s defence and immune systems. These five types of cells are classifi ed as granulocytes (neutrophils, eosinophils and basophils) and agranulocytes (monocytes and lymphocytes).
Granulocytes are a group of WBCs that contain granules in their cytoplasm. They can be subclassified into neutrophils, eosinophils and basophils. Neutrophils contain a multilobed nucleus, while nucleus of eosinophils and basophils are bilobed. Each cell type exhibits different properties and each is activated by different stimuli.
Haematological changes with aging
As a person ages, fatty bone marrow replaces some of the body’s active blood-forming marrow first in the long bones and later in the flat bones. The altered bone marrow can’t increase erythrocyte production as readily in response to such stimuli as hormones, anoxia, haemorrhage and haemolysis.
Vitamin B12 absorption may also diminish with age, resulting in reduced erythrocyte mass and decreased haemoglobin levels and haematocrit (packed cell volume).
The heart is a hollow, muscular organ about the size of a closed fist. Located between the lungs in the mediastinum, it’s about 12.5 cm (5”) long and 9 cm (3½”) in diameter at its widest point. It weighs between 250 and 285 g (8.8 and 10 oz).
Where’s your heart?
The heart spans the area from the second to the fifth intercostal space. The right border of the heart lines up with the right border of the sternum. The left border lines up with the left midclavicular line. The exact position of the heart may vary slightly with each patient. Leading into and out of the heart are the great vessels:
• inferior vena cava
• superior vena cava
• pulmonary artery
• four pulmonary veins.
Slip and slide
A thin sac called the pericardium protects the heart. It has an inner, or visceral, layer that forms the epicardium and an outer, or parietal, layer. The space between the two layers contains 10 to 30 ml of serous fluid, which prevents friction between the layers as the heart pumps.
The heart has four chambers - two atria and two ventricles - separated by a cardiac septum. The upper atria have thin walls and serve as reservoirs for blood. They also boost the amount of blood moving into the lower ventricles, which fill primarily by gravity.
Blood moves to and from the heart through specific pathways. Deoxygenated venous blood returns to the right atrium through three vessels:
superior vena cava - returning blood from the upper body
inferior vena cava - returning blood from the lower body
coronary sinus - returning blood from the heart muscle
Get some fresh air
Blood in the right atrium empties into the right ventricle and is then ejected through the pulmonary valve into the pulmonary artery when the ventricle contracts. The blood then travels to the lungs to be oxygenated.
Share the wealth
From the lungs, blood travels to the left atrium through the pulmonary veins. The left atrium empties the blood into the left ventricle, which then pumps the blood through the aortic valve into the aorta and throughout the body with each contraction. Because the left ventricle pumps blood against a much higher pressure than the right ventricle, its wall is three times thicker.
Valves in the heart keep blood fl owing in only one direction through the heart. Think of the valves as traffic police at the entrances to one-way streets, preventing blood from travelling the wrong way despite great pressure to do so. Healthy valves open and close as a result of pressure changes within the four heart chambers.
The heart has two sets of valves:.
atrioventricular (AV) (between atria and ventricles) – tricuspid valve on the heart’s right side and mitral valve on its left
semilunar – pulmonary valve (between the right ventricle and pulmonary artery) and aortic valve (between the left ventricle and aorta).
On the cusp
Each valve has cusps (leaflets), which are anchored to the heart wall by cords of fibrous tissue (chordae tendineae). The cusps of the valves act to maintain tight closure. The tricuspid valve has three cusps, the mitral valve has two cusps and each of the semilunar valves has three cusps
Excerpted from Medical-Surgical Nursing Made Incredibly Easy UK Edition