Human Gas Exchange System

Gas Exchange in Humans

• Organisms obtain oxygen for aerobic respiration through gas exchange, which involves the exchange of gases.
• The efficiency of gas exchange is influenced by the surface area to volume ratio
• Large animals like fish, amphibians, and mammals use special organs for gas exchange because their external surfaces are insufficient due to their size and thickened skin.
• Lungs and gills have enlarged surface areas, a rich blood supply, and thin coverings to facilitate efficient gas exchange.
• In humans, the absorption of atmospheric oxygen and the removal of carbon dioxide occur in the alveoli (air sacs) of the lungs.

The Human Gas Exchange System

• The organs involved in gas exchange include the lungs in the thorax and the air passages leading to them.
• The air passages consist of the nasal passages, pharynx, larynx, trachea, bronchi, and bronchioles.
• The thoracic cavity, ribs, diaphragm, and related muscles are vital for gas exchange.
• Air enters the body through the two external nostrils, which have hairs.
• The nasal passages are lined with a moist mucous membrane.
• Nasal passages warm and moisten incoming air.
• Dust and foreign particles are trapped by hairs and mucus in the nasal passages.
• Sensory cells in the mucous membrane can detect harmful chemicals.
• From the nasal passages, air passes through the pharynx, then into the larynx, and finally into the trachea via an opening known as the glottis.

Trachea

• The trachea is supported by C-shaped rings of cartilage, which keep the airway open.
• The epithelium lining the trachea consists of goblet cells and ciliated cells.
  • Goblet cells: secrete mucus to trap dust particles and bacteria.
  • Ciliated cells: have hair-like structures (cilia) that sweep the mucus up to the trachea, expelling trapped pathogens and particles.

Bronchi and Bronchioles

• The trachea divides into two tubes called bronchi.
• Each bronchus carries air into a lung and has a similar structure to the trachea.
• Each bronchus branches repeatedly into numerous bronchioles.
• Bronchioles are very fine tubes that end in clusters of air sacs called alveoli.

Alveoli

• Gas exchange occurs through the walls of these tiny air sacs.
• Numerous alveoli in the lungs provide a large surface area for efficient gas exchange.
• Walls of the alveolus are only one cell thick; this provides short diffusion distances for gases.
• A thin film of moisture covers the surface of the alveolus, allowing oxygen to dissolve in it.
• The walls of the alveoli are richly supplied with blood capillaries.
• The flow of blood maintains the concentration gradient required for gas exchange.

Gas Exchange in the Alveoli

• Gas exchange in the lungs occurs by diffusion.
• Blood entering the lungs has a lower concentration of oxygen and a higher concentration of carbon dioxide compared to the inhaled air.
• Oxygen dissolves in the thin film of moisture on the alveolus wall
• Dissolved oxygen then diffuses into the blood capillaries and combines with hemoglobin in red blood cells to form oxyhemoglobin.
• Carbon dioxide diffuses out of the blood capillaries into the alveolar air.
• Concentration gradients of oxygen and carbon dioxide between the alveolar air and the blood are maintained by a continuous flow of blood through blood capillaries and the movement of air in and out of the alveoli.

Oxygen Transport

• In the lungs, where oxygen concentration is high, oxygen combines with hemoglobin in red blood cells to form oxyhemoglobin.
• Oxyhemoglobin is transported from the lungs to other parts of the body via the bloodstream.
• When blood passes through an organ or tissue with low oxygen concentration, oxyhemoglobin releases oxygen, which can be used by the respiring cells.

Breathing Mechanisms

• The internal and external intercostal muscles and diaphragm are involved in lung ventilation.
• Ribs are attached dorsally (at the back) to the vertebral column and ventrally (in front) to the sternum.
• Humans have 12 pairs of ribs, but only the first 10 pairs are attached to the sternum; the remaining pairs are free ribs.
• Internal and external intercostal muscles are found between the ribs.
• The diaphragm is a dome-shaped sheet of muscle and elastic tissue that separates the thorax.
• Intercostal muscles and diaphragm work together to change the volume and pressure in the thorax, resulting in the ventilation of the lungs.

Inspiration (Inhalation)

• Diaphragm muscle contracts, causing the diaphragm to flatten.
• External intercostal muscles contract, while internal intercostal muscles relax.
• Ribs move upwards and outwards, and the sternum moves upwards and forward.
• The volume of the thoracic cavity increases.
• Lungs expand, increasing their volume and decreasing the air pressure inside.
• Atmospheric pressure becomes higher than the pressure in the lungs.
• Air from the external environment is forced into the lungs.

Expiration (Exhalation)

• Diaphragm muscle relaxes, and the diaphragm arches upwards.
• Internal intercostal muscles contract, while external intercostal muscles relax.
• Ribs move downwards and inwards, and the sternum moves downwards to its original position.
• The volume of the thoracic cavity decreases.
• Lungs are compressed, decreasing the volume in the lungs and increasing the air pressure inside them.
• Pressure within the lungs becomes higher than the atmospheric pressure.
• Air is forced out of the lungs into the external environment.
• Oxygen is absorbed into the bloodstream while carbon dioxide is released from the blood.
• Expired air has a lower concentration of oxygen but a higher concentration of carbon dioxide than inspired air.

Factors Affecting Breathing

• The rate and depth of breathing increases during exercise.
• Breathing rate can be measured by counting the number of breaths taken in a minute.
• The stimulus for breathing is the high concentration of carbon dioxide in the blood.
• Breathing decreases when the concentration of carbon dioxide in the blood goes down.
• Low levels of concentration of oxygen will stimulate breathing.