SS2 Biology Lesson Note on Respiration in Higher Animals

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RESPIRATION IN HIGHER ANIMALS

This type of respiration takes place through the lungs. All mammals have a [air of lungs. lungs adaptations that allow animals to invade the terrestrial environment. Some fishes, amphibians, reptiles, birds, and mammals, all have their lungs located internally. Mammals’ breathing system comprises the air passages (nostrils, pharynx, larynx, and trachea), lungs, and diaphragm. Oxygen enters the nasal passage through the nostrils into the nasal cavity where it is moistened and kept warm. The nasal cavity has lining cells, which secretes mucus that traps most of the dust and microorganisms present in the air before it diffuses into the lungs.

Nasal cavity leads into the pharynx, to the trachea (windpipe), which divides into two bronchi before it enters into the lungs; the cartilages support the wall of the trachea and bronchus to prevent them from collapsing when the air pressure in them is reduced. Within each lung, the bronchus divides repeatedly and becomes progressively narrower forming numerous tiny tubules called bronchioles, which end in pockets of the air sacs called alveoli. Alveoli are surrounded by a dense network of capillaries, which enhances gas exchange.

Breathing in Humans

As the diaphragm contracts and moves downward, the pectoralis minor and intercostal muscles pull the rib cage outward. The chest cavity expands, and air rushes into the lungs through the trachea to fill the resulting vacuum. When the diaphragm relaxes to its normal, upwardly curving position, the lungs contract, and air is forced out.

The alveoli give the lungs a very large surface area for gas exchange. The lungs are located in the thorax or thoracic cavity, which comprises the breast bone or sternum in front, and the back bone (vertebrae) at the back. At the sides are twelve (12) pairs of ribs and intercostals muscles, which are attached to the spine and the sternum, thereby, forming a bony cage. The diaphragm is a sheet of muscle at the floor of the thorax.

Human Lungs

In humans the lungs occupy a large portion of the chest cavity from the collarbone down to the diaphragm. The right lung is divided into three sections, or lobes. The left lung, with a cleft to accommodate the heart, has only two lobes. The two branches of the trachea, called bronchi, subdivide within the lobes into smaller and smaller air vessels known as bronchioles. Bronchioles terminate in alveoli, tiny air sacs surrounded by capillaries. When the alveoli inflate with inhaled air, oxygen diffuses into the blood in the capillaries to be pumped by the heart to the tissues of the body. At the same time carbon dioxide diffuses out of the blood into the lungs, where it is exhaled.

MECHANISM OF BREATHING IN MAMMALS

INHALATION:

1. The intercostals muscles contract, while the ribs move upward and outwards.
2. The diaphragm contracts and flattens out.
3. The thoracic cavity volume increases, while its air pressure decreases.
4. Air is then forced from outside into the lungs through the air passage.

EXHALATION:

1. The intercostals muscles relax, while the ribs cage is lowered.
2. The diaphragm relaxes and returns to its original dome-shaped position.
3. The thoracic cavity volume decreases, while the lungs shrink.
4. The lung air pressure increases, and a lot of air is expelled from the lungs through the air passage.

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