M3 Exchange Surfaces and Breathing Flashcards
Why is diffusion alone enough to supply the needs of single-called organisms?
- The metabolic activity of a single-called organism is usually low, so the oxygen demands and carbon dioxide production of the cell are relatively low.
- The surface area to volume ratio of the organism is large.
Why do large organisms need specialised exchange surfaces?
- Large, multicellular organisms have evolved specialised systems for the exchange of the substances they need/must remove.
- Multicellular organisms have a higher metabolic rate, and the distance between cells and the supply of oxygen is too far for effective diffusion. The bigger the organism the smaller the SA:V ratio so gases can’t be exchanged quickly enough.
What are the characteristics of effective exchange surfaces?
- Increased surface area provides the area needed for exchange and overcomes the limitations of the SA:V ratio of larger organisms.
- Thin layers mean the distances that substances have to diffuse are short, making the process fast and efficient.
- Good blood supply, the steeper the concentration gradient the faster diffusion takes place, having a good blood supply ensures substances are constantly delivered and removed from the exchange surface. This maintains a steep concentration gradient for diffusion.
- Ventilation to maintain diffusion gradient, for gases a ventilation system also helps maintain concentration gradients and makes the process more efficient.
What is the pathway of the gaseous exchange system?
Nasal cavity
Trachea
Bronchus
Bronchioles
Alveoli
Describe the naval cavity
- a large surface area with 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
After passing through the nasal cavity, the air entering the lungs is a similar temperature and humidity to the air already there.
Describe the trachea
- The trachea is the main airway carrying clean, warm, moist air from the nose down into the chest.
- It is a wide tube supported by incomplete rings of strong, flexible cartilage, which stop the trachea from collapsing. The rings are incomplete so that food can move easily down the oesophagus and behind the trachea.
- The trachea and it’s branches are lined with a ciliated epithelium with goblet cells (which secrete mucus onto the lining of the trachea, to trap dust and microorganisms) between and below the epithelial cells.
- The cilia beat and move the mucus, along with any trapped first and microorganisms, away from the lungs. Most of it goes into the throat and is swallowed. Cigarette smoke stops these cilia beating.
Describe the bronchus
- 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 (plural bronchi).
- They are similar in structure to the trachea, with the same supporting rings of cartilage, but they are smaller.
Describe bronchioles
- In the lungs the bronchi divide to form many bronchioles. The smaller bronchioles have no cartilage rings.
- The walls of the bronchioles contain smooth muscle. When the smooth muscle contracts, the bronchioles constrict (close up). When it relaxes, the bronchioles dilate (open up). This changes the amount of air reaching the lungs.
- Bronchioles are lined with a thin layer of flattened epithelium, making some gaseous exchange possible.
Describe the alveoli
- The alveoli are tiny air sacs, which are the main gas exchange surfaces of the body.
- Each alveolus has a diameter of around 200-300 micrometers and consists of a layer of thin, flattened epithelial cells, along with some collagen and elastic fibres (composed of elastin). These 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. This is elastic recoil of the lungs.
What are the main adaptations of the alveoli for effective gaseous exchange?
- Large surface area
- Thin layers, both the alveoli and the capillaries that surround them have walls that are only a single epithelial cell thick, so the diffusion distances between the air in the alveolus and the blood in the capillaries are very short.
- Good blood supply, maintaining a steep concentration gradient for both oxygen and carbon dioxide between the air in the alveoli and blood in the capillaries.
- Good ventilation, breathing moves air in and out of the alveoli, helping maintain steep diffusion gradients for oxygen and carbon dioxide between the blood and the air in the lungs.
- The inner surface of the alveoli is covered in a thin layer of a solution of water, salts and lung surfactant, making it possible for alveoli to maintain inflated. Oxygen dissolves in the water before diffusion into the blood, but water can also evaporate into the air in the alveoli.
How does air enter the lungs?
- Air is moved in and out of the lungs as a result of pressure changes in the thorax (chest cavity) brought about by the breathing movements. This movement of air is called ventilation.
- The rib cage provides a semi-rigid case within which pressure can be lowered with respect to the air outside it.
- The diaphragm is a broad, domed sheet of muscle, which forms the floor of the thorax. The external 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 easily over each other as you breathe.
Describe inspiration
Inspiration is an energy-using process:
- The dome-shaped diaphragm contracts, flattening and lowering.
- The external intercostal muscles contract, moving the ribs upwards and outwards.
- The volume of the thorax increases so the pressure in the thorax is reduced.
- It is now 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 the pressures inside and outside the chest.
Describe expiration
Normal expiration is a passive process:
- The muscles of the diaphragm relax so it moves up into it’s resting dome shape.
- The external intercostal muscles relax so the ribs move down and inwards under gravity.
- The elastic fibres in the alveoli of the lungs return to their normal length.
- The effect of all these changes is to decrease the pressure inside the thorax. Now the pressure inside the thorax is greater than the pressure of the atmospheric air, so air moves out of the lungs until the pressure inside and out is equal again.
- You can exhale forcibly using energy. The internal intercostal muscles contract, pulling the ribs down hard and fast, and the abdominal muscles contract forcing the diaphragm up to increase the pressure in the lungs rapidly.
