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Mandatory Activities.doc
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The Lymphatic System

The lymphatic system is a collection of special drainage vessels collecting excess tissue fluid from the different parts of the body. This excess tissue fluid re-enters the blood at the left and right subclavian veins.

Once the tissue fluid enters the lymphatic capillaries it is called lymph. Lymph is a clear liquid similar in composition to the plasma but with only 2% protein in comparisons to the plasma’s 8%.

The lymphatic capillaries unite to form lymphatics (lymph veins) and larger lymph channels. The lymphatics are richly supplied with valves. The valves keep the lymph flowing in the one direction towards the subclavian veins.

Distributed along the lymphatics are lymph nodes. The lymph nodes filter the lymph and produce lymphocytes. The lymphocytes protect against pathogens by phagocytosis and antibody production.

Lymphatics from the lower regions merge forming the thoracic duct. The thoracic duct drains lymph into the left subclavian vein. The thoracic duct also receives lymph from the lymphatics draining the upper left part of the body. The right thoracic duct collects lymph from the upper right part of the body draining it into the right subclavian vein.

The lymph is moved along the lymph vessels by the squeezing action of the skeletal muscles, pressure changes in the thorax during breathing and by the rhythmic contraction of the lymph vessel walls.

Blood is the source of the excess tissue fluid

Textbook Diagram: formation of tissue fluid and lymph.

  • As the blood enters the capillaries the pressure forces some of the plasma out through the wall.

  • The escaped fluid is called tissue fluid similar in composition to plasma with only about 2% protein.

  • The tissue fluid is the true environment of the cells of the body — our cells are aquatic.

  • At the venule end most (90%) of the escaped tissue fluid is drawn back into the blood by osmosis.

  • The escaped proteins do not return to the blood at the capillary.

  • The 10% excess tissue fluid must be drained away to allow the tissue to function normally.

  • The lymphatic system carries out this function.

Summary of the major functions of the Lymphatic System

Circulatory role

  • Return the excess tissue fluid to the blood: this maintains blood volume, pressure and concentration.

  • Collect and deliver the absorbed lipids from the small intestine to the blood (enters at the left subclavian vein).

Defence role

  • The lymph nodes filter out pathogens in the lymph.

  • Production and ‘export’ of lymphocytes to the blood system for general distribution.

  • Detection of antigens and production of specific antibodies.

Mandatory Activities

Investigate the effect of exercise on your heart rate

  • Find a strong pulse in your wrist.

  • Each pulse is a heartbeat.

  • Three times measure and record the heart rate at rest.

  • Calculate the average.

  • Count the number of pulses in 20 seconds and multiply by three - this is the heart rate-beats per minute.

  • Repeat the measurements again three times during gentle exercise — slow jogging on the spot.

  • Repeat the measurements again three times during strenuous exercise — fast jogging on the spot.

  • Rest must be taken between each exercise session.

  • Compare the results for rest, gentle and vigorous exercise.

  • Repeat many times to verify the results.

  • Conclusion: exercise increases the heart rate and the more vigorous the exercise the higher the heart rate.

Dissect, display and identify an ox’s or a sheep’s heart

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