Structure and Function of the Pulmonary System

Pulmonary System

• Made up of two lungs– Where gas exchange takes place

• Airways– To get air to lungs

• Blood vessels– To circulate oxygen and carbon dioxide

• Chest wall – To move air in and out of lungs through

pressure changes and protect lungs

Airways• Can divide respiratory system into two

parts:– Upper respiratory tract : nasal cavity, pharynx– Lower respiratory tract: larynx, trachea,

bronchi, bronchioles, alveoli

• Upper respiratory tract serves to conduct air into the body, and filter, warm and humidify the air.

Lungs

• Two lungs

• Right lung has 3 lobes and two fissures

• Left lung has 2 lobes and one fissure

• Blood vessels and bronchi enter at hilus

Alveoli

• Small, thin air sacs surrounded by a “hair net” of capillaries

• Most of the walls of the alveoli are made up of simple squamous epithelium, called Type 1 alveolar cells. Gas exchange takes place through these cells

Other cells of the alveoli

• Type II alveolar cells produce surfactant (lipoprotein) to decrease the surface tension of water in the lungs– Premature infants – respiratory-distress

syndrome of the newborn

• Alveolar macrophages, or “dust cells” phagocytize foreign particles and microbes

Respiratory membrane• Made of alveolar type I cell, endothelium

of capillary and their basement membranes.

• As thin as possible for gas exchange which takes place by diffusion from areas of high partial pressure to low partial pressure

• Any disorder that thickens the membrane decreases gas exchange.

• Usually, about one third of the pulmonary vessels are perfused at one time, so if right heart output increases does not increase arterial pressure in lungs, simply perfuse more vessels.

• Lungs are surrounded by pleural membranes : parietal pleura and visceral pleura

• Separate by the mediastinum

• Enclosed in the thoracic cavity.

Pulmonary ventilation (breathing)

• Takes place by decreasing the pressure inside the thoracic cavity, and allowing atmospheric pressure to force air into lungs.

• Thoracic cavity is expanded primarily by the contraction and lowering of the diaphragm, but is aided by expansion of chest by contraction of the external intercostal muscles.

Expiration

• Is normally a passive process due to the relaxing of the inspiratory muscles and the elastic recoil of the lungs.

Respiratory volumes

• Amount of air moved in and out of lungs

• Tidal volume is the amount of air moved with a normal breath

• Minute volume of respiration = tidal volume X breaths per minute

Peripheral Chemoreceptors

• Carotid bodies and aortic bodies

• Stimulated by oxygen concentration decrease

• Send impulses to respiratory centers, and breathing increases

• Not triggered until O2 is very low (50 mm Hg)

Oxygen transport• Gases move down partial pressure gradient.

• 98 % of oxygen is bound to the iron in hemoglobin as oxyhemoglobin (HbO2)

• Rest is dissolved in plasma

• 4 Iron atoms per hemoglobin molecule

• 1 hemoglobin molecule can hold up to 4 molecules of oxygen

• Amount of oxygen on hemoglobin is determined by partial pressure of oxygen.

Carbon Dioxide Transport• 7 % is dissolved in plasma

• 23 % combines with the amino groups of the hemoglobin forming carbaminohemoglobin

• 70 % is converted to bicarbonate ions

Control of pulmonary circulation• In most areas of the body, hypoxia causes

vasodilation.

• Low oxygen concentrations in the alveoli of the lungs cause vasoconstriction.

• This sends blood to areas of the lung that have higher oxygen concentrations

• Chronic alveolar hypoxia can lead to permanent pulmonary artery hypertension, which can lead to right heart failure.

• Acidemia also causes pulmonary artery constriction