7.2. MAMMALIAN GASEOUS EXCHANGE SYSTEM

Question 1

why do mammals have a high metabolic rate?

Answer 1

• as they are active and maintain their body temperature independent to the environment

Question 2

how does metabolic rate link to gas exchange?

Answer 2

• due to high metabolic rates, mammals need lots of oxygen for cellular respiration and they produce carbon dioxide, which needs to be removed.
• this takes place in the lungs

Question 3

what are the key structures in the human gaseous exchange system?

Answer 3

• nasal cavity
• trachea
• bronchus
• bronchioles
• alveoli

Question 4

what are the important features of the nasal cavity?

Answer 4

• large surface area with a good blood supply, which warms the air to body temperature
• a hairy lining, which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from irritation and infection
• moist surfaces, which increase the humidity of the incoming air, reducing evaporation from the exchange surfaces

Question 5

what is the trachea?

Answer 5

• main airway carrying clean, warm, moist air from the nose down into the chest.

Question 6

what are the key features of the trachea?

Answer 6

• wide tube supported by incomplete rings of strong, flexible cartilage, which stops it from collapsing
• these rings are incomplete so that food can move easily down the oesophagus behind the trachea
• trachea and its branches are lined with ciliated epithelium with goblet cells between and below the epithelial cells

Question 7

what is the role of goblet cells in the trachea?

Answer 7

• secrete mucus into the lining of the trachea, to trap dust and microorganisms that have escaped the nose lining

Question 8

what is the role of the cilia in the trachea?

Answer 8

• beat and move the mucus, along with trapped dirt and microorganisms, away from the lungs
• most of it goes into the throat and is swallowed and digested
• cigarette smoke stops the cilia from beating

Question 9

what is the bonchus?

Answer 9

• in the chest cavity, the trachea divides to form the left bronchus, leading to the left lung, and the right bronchus, leading to the right lung.
• similar in structure to trachea, with same supporting rings of cartilage, but they are smaller.

Question 10

what are brochioles?

Answer 10

• in the lungs, the bronchi divide to form many small bronchioles
• no cartilage rings, but contain small muscle
• when the smooth muscle contracts, the bronchioles constrict (close up), and when it relaxes, the bronchioles dilate (open up), this changes the amount of air reaching the lungs
• lined with a thin layer of epithelium, making some gaseous exchange possible

Question 11

what is the alveoli?

Answer 11

• tiny air sacs, which are the main gas exchange surfaces of the body
• unique to mammalian lungs
• each alveolus has a diameter of around 200-300μm
• consists of a layer of thin, flattened epithelial cells, along with some collagen and elastic fibres

Question 12

what is elastic recoil of the lungs?

Answer 12

• the elastic tissues allow the alveoli to stretch as air is drawn in
• when they return to their resting size, they help squeeze the air out

Question 13

what are the main adaptations of the alveoli for effective gaseous exchange?

Answer 13

• large surface area, 300-500 million alveoli per adult resulting in 50-75m2 surface area per adult
• thin layers, very short diffusion distance
• good blood supply, maintains a steep concentration gradient for both the carbon dioxide and oxygen between the air in the alveoli and the blood in the capillaries
• good ventilation, breathing moves air in and out of the alveoli helping maintain a steep concentration gradient for O2 and CO2.

Question 14

what is lung surfactant?

Answer 14

• inner surface of alveoli is covered in a thin layer of a solution of water (lung surfactant)
• makes it possible for alveoli to remain inflated
• oxygen dissolves in the water before diffusing into the blood, but this water can also evaporate into the air of the alveoli (several of the adaptations of the human gas exchange system are to reduce this loss of water)

Question 15

how does air move in and out of the lungs?

Answer 15

• due to a pressure change in the thorax (chest cavity) brought about by the breathing movements
• this is called ventilation

Question 16

what is the thorax and how does it work?

Answer 16

• the rib cage provides a semi-rigid case within which pressure can be lowered with respect to the air outside it
• the diaphragm (a broad, domed sheet of muscle) forms the floor of the thorax
• the external and internal intercostal muscles are found between the ribs
• the thorax is lined by the pleural membranes, which surround the lungs
• the space between them, the pleural cavity, is usually filled with a thin layer of lubricating fluid so the membranes slide over each other as you breathe

Question 17

what is inspiration?

Answer 17

• taking air in or inhalation
• it is an energy using process

Question 18

what happens during inspiration?

Answer 18

• dome-shaped diaphragm contracts, flattening, and lowering
• the external intercostal muscles contract, moving the ribs upwards and outwards
• volume of the thorax increases so the pressure in the thorax is reduced, lower than the pressure of the atmospheric air, so air is drawn through the nasal passages, trachea, bronchi and bronchioles into the lungs
• this equalises pressure inside and outside the chest

Question 19

what is expiration?

Answer 19

• breathing out or exhalation
• passive process

Question 20

what happens during exhalation?

Answer 20

• muscles of the diaphragm relax so it moves up into its resting dome shape
• external intercostal muscles relax so ribs move down and inwards under gravity
• elastic fibres of the alveoli return to their normal length
• this decreases volume of the thorax, increasing pressure inside it to greater than the pressure of the atmospheric air, so air moves out of the lungs until the pressure inside and out is equal again

Question 21

how to you exhale forcibly (using energy)?

Answer 21

• internal intercostal muscles contract, pulling the ribs downwards hard and fast
• the abdominal muscles contract forcing the diaphragm up to increase pressure in the lungs rapidly

Question 22

what happens during an asthma attack?

Answer 22

• cells lining the bronchioles release histamine which make the epithelial cells become inflamed and swollen, and stimulate the goblet cells to produce excess mucus, and the smooth muscle in the bronchioles contract
• airways narrow and fill with mucus, making it difficult to breathe

Question 23

how are asthma attacks being treated/ prevented?

Answer 23

• drugs are delivered straight into the breathing system using an inhaler
• i.e. relievers give immediate relief from symptoms (chemicals similar to hormone adrenaline) and attach to active sites on bronchiole smooth muscle cells, making them relax and dilate the airways.
• i.e. or preventers (often steroids) which are taken everyday to reduce the sensitivity of the lining of the airways