Lymphatic system in vertebrates, Gaseous exchange through gills and Mechanism of Inhalation and Exhalation


Lymphatic system in vertebrates:
It begins with small vessels called lymphatic capillaries which are in direct contact with extra cellular fluid surrounding tissues.
(1) To collect and drain most of the fluid that sweeps from the blood stream and accumulates in extra cellular fluid.
(2) To return small amount of proteins that have left the cells.
(3) To transport lipids that have been absorbed from small intestine.
(4) To transport foreign particles and cellular debris to disposal centres called lymph nodes.
Small lymphatic capillaries merge to form larger lymphatic vessels called lymphatic. Lymphatic are thin walled vessels with valves that ensure the one way flow of lymph. Lymph (clear water) is extra cellular fluid that accumulates in lymph vessels. These vessels pass through lymph nodes on their way back to the heart. Lymph nodes concentrate in several areas of the body and play important role in body’s defense against disease.

Gaseous exchange through gills:
Gas exchange across internal gill surfaces is extremely efficient. It occurs as blood and water move in opposite direction on either side of lamellar epithelium e.g. the water that passes over a gill first encounters vessels that are transporting blood with low oxygen partial pressure into the body.
Thus oxygen diffuses into the blood. What then passes over the vessels carrying blood high in oxygen. More oxygen diffuses inward because this blood still has less oxygen than the surrounding water. Carbon dioxide also diffuses into water because its pressure is higher in the blood than in water. This counter current exchange mechanism provides efficient gas exchange by maintaining a concentration gradient between blood and water over the length of capillary bed.

Mechanism of Inhalation and Exhalation:
Inhalation: Several sets of muscles, the main ones being the diaphragm and intercostal muscles contract. The intercostal muscles stretch from rib to rib and when they contract, they pull the ribs close together enlarging the thoracic cavity.
Thoracic cavity further enlarges when the diaphragm contracts and flattens. The increased size of thoracic cavity causes pressure in the cavity to drop below the atmospheric pressure. Air rushes into the lungs and the lungs inflate.
Exhalation: The intercostal muscles and the diaphragm relax, allowing the thoracic cavity to return to its original smaller size and increasing the pressure in the thoracic cavity.
Abdominal muscles contract, pushing the abdominal organs against the diaphragm, further increasing the pressure within the thoracic cavity. The action in this step causes the elastic lungs to contract and compress the air in the alveoli with this compression alveolar pressure becomes greater than atmospheric pressure causing air to be expelled from the lungs.

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