The Lymphatic System

Formation of the Interstitial Fluid
When blood flows from arteries into capillaries, there is higher hydrostatic pressure at the arterial end of the capillaries. This high pressure forces some fluid out through the capillary walls into the intercellular spaces between the cells. Once the fluid leaves the capillary walls, it is called interstitial or tissue fluid. Interstitial fluid is similar in composition to blood plasma but it has no erythrocytes, platelets or large protein molecules as these are too large to pass through the capillary walls.

Importance of the Interstitial Fluid
Interstitial fluid is important because it forms the internal environment of the body. It bathes the cells and supplies them with their requirements. Oxygen and nutrients diffuse from the blood through the interstitial fluid and into the cells. Excretory waste products, such as carbon dioxide and urea, diffuse out of the cells into the interstitial fluid. The internal environment of the body is kept within a normal range by homeostatic processes.

Fate of the Interstitial Fluid
Approximately 90% of the interstitial fluid flows back into the venous end of the capillary system where the hydrostatic pressure is low. The remaining 10% of the interstitial fluid enters the lymphatic capillaries and is called lymph. If excess interstitial fluid is unable to return to the blood circulatory system, it will accumulate and cause tissue swelling. This condition is called oedema.

Structure of the Lymphatic System
Lymph is the colourless fluid found in the lymphatic vessels. Lymph is similar in composition to blood plasma but has no erythrocytes, platelets or large protein molecules. Lymph contains a higher number of lymphocytes than blood. Lymph travels through the lymphatic vessels by the contraction of the surrounding skeletal muscles. The lymph flows in one direction. One end of the vessels is closed and back flow is prevented by valves present in the larger vessels. The smaller lymphatic vessels join form larger vessels. 

The vessels from the left side of the body, the alimentary canal and the right side of the lower part of the body flow into the thoracic duct. The thoracic duct is the largest lymphatic vessel in the body. It carries lymph to the left subclavian vein and back into the bloodstream. The right lymphatic duct transports lymph from the right side of the head and chest into the right subclavian vein. Lymph nodes are mainly found in the neck, armpits and the groin. The lymphatic system consists of a network of lymphatic capillaries, lymphatic vessels, lymph nodes and certain organs such as the thymus gland, spleen and tonsils.

Role of the Lymphatic System in Transport
The lymphatic system collects the interstitial fluid and returns it to the circulatory system. Lacteals are lymphatic capillaries in the villi of the ileum. They absorb fats and fat-soluble vitamins and transport them to the blood circulatory system. The lymph nodes filter out bacteria and other foreign particles. Phagocytes present in the nodes engulf and destroy these foreign particles. Lymphocytes in the lymphatic tissues produce antibodies which aid in the destruction of pathogens and the neutralisation of toxins.

Consequences of an Impaired Blood Clotting Mechanism

Some people lack the gene for the production of certain clotting factors, for example, factor VIII. They suffer from a disease called haemophilia. This is an impaired clotting mechanism which causes serious bleeding particularly in the joints. In severe cases haemophiliacs may die of internal or external bleeding. Certain clotting factors such as factor VIII can now be produced by genetic engineering and are used in the treatment of haemophilia.
 

Sometimes a local blood clot (thrombus) is formed on the damaged rough inner wall of the artery. This may cause blockage of the artery, a condition known as thrombosis. When the thrombus dislodges and is carried away by blood circulation, it is known as an embolus. The embolus may be trapped in the small artery where it blocks the blood flow. This condition is called embolism.
 

If the coronary artery is partially blocked, it can cause chest pains called angina. A total blockage, which cuts off the supply of oxygen and nutrients to the heart muscles, causes heart attack (myocardial infarction). The affected heart muscles are damaged. If only a small area of the muscle cells died, the victim can recover. Extensive heart muscle damage can cause death. A stroke occurs if there is a blockage of blood to the brain cells. Strokes are commonly caused by an embolus blocking the flow of blood to a portion of the brain.

Clotting of Blood

Necessity of Blood Clotting
Blood clotting is necessary to prevent blood loss from the body when there is damage to the blood vessels. It prevents blood pressure from falling to a low level as pressure is needed to maintain proper blood circulation. A blood clot seals a wound and prevents the entry of microorganisms and foreign particles into the body through the wound.

Blood Clotting Mechanism
What happens when you accidentally cut your fingers and bleeding occurs? In your blood there are cells called the platelets which help in blood clotting. When you get a cut, the blood vessels around the wound immediately constrict to reduce blood loss. The platelets in the blood become sticky and clump together to plug the wound.

Clotting factors are released by the platelets and damaged tissues which set off a chain of reactions. Thrombokinase, in the presence of factor VIII, converts prothrombin into thrombin. The formation of prothrombin in the liver requires vitamin K. thrombin converts a soluble plasma protein, fibrinogen, into insoluble fibrin fibres which form a meshwork of threads over the wound. As the blood flows out, erythrocytes and platelets are trapped in the fibrin fibres and blood clot forms. It dries to form a scab which covers the wound. When the wound heals, new skin is formed and the scab peels or falls off.

Prevention of Cardiovascular Diseases

Cardiovascular diseases are disorders of the heart and the blood circulatory system. Some common disorders are hypertension, atherosclerosis, thrombosis, embolism, angina and heart attack (myocardial infarction). Major risk factors for cardiovascular diseases are high levels of blood cholesterol, family history, age, cigarette smoking obesity, diabetes mellitus and a sedentary lifestyle.

Some cardiovascular diseases begin early in life, practising a healthy lifestyle is an important preventive measure. A good balanced diet low in saturated fats and cholesterol and higher in unsaturated oils should be practised. The diet should have sufficient green leafy vegetables and fruit. Excessive intake of salt and smoking should be avoided. Regular exercise such as brisk walking, moderate jogging, cycling or swimming, together with maintaining an optimal body weight and a stress-free lifestyle, will help to reduce the risk of cardiovascular diseases.

Regulatory Mechanism of Blood Pressure

Blood pressure is the force of the blood exerted on the walls of the arterial blood vessels. Arterial blood pressure is highest during ventricular systole, and lowest during diastole. Normal blood pressure is 120 (systolic)/80 (diastole) mm Hg. Stretch-sensitive receptors known as baroreceptors are located in the walls of the aorta and carotid arteries branch out form the aorta. They monitor the pressure of blood flowing to the body and to the brain. An increase in blood pressure stretches the baroreceptors. Impulses are sent to the cardiovascular control centre in the medulla oblongata of the brain. From there, impulses are then sent via the parasympathetic nerve to the heart. This slows down the heartbeat, resulting in a decrease in blood pressure.

A decrease in blood pressure increases stimulation of the sino-atrial node by the sympathetic nerve. This increases the contraction of the cardiac muscles of the heart and the smooth muscles of the arteries. The blood pressure increases and returns to its normal level. A person’s blood pressure can be measured by using an instrument called a sphygmomanometer.

The Muscular Heart

Blood can carry out its transportation function only if it can circulate in the human body. The organ responsible for generating the pressure to pump the blood through the vessels is the muscular heart. The heart is a dark red cone-shaped muscular organ found in the thoracic cavity. It is the size of a clenched fist and weighs from 350 to 450 grams in an average adult. The heart is located between the lungs with its apex slightly oriented to the left. The heart has four chambers: two upper thin-walled atria and two lower thick-walled ventricles. The septum separates the right chambers from the left chambers.

The valves in the heart ensure that blood flows in one direction. Heart disease is the number one killer in many countries in the world. An important preventive measure is to educate people about heart fitness and a healthy lifestyle. The valve between the left atrium and left ventricle is the bicuspid valve. The valve between the right atrium and right ventricle is the tricuspid valve. The valves at the base of the aorta and pulmonary artery are the semilunar valves. The heart mainly made up of myogenic cardiac muscles. The heart muscles contract and relax automatically throughout life and are not controlled by the nervous system.The heart functions like two pumps with different pressure system, the right pump forces deoxygenated blood to the lungs. The left pump forces oxygenated blood to other parts of the body.

The sino-atrial node (SAN) is a group of specialised cells located in the right atrial wall, near the entrance of the superior vena cava. It acts like a pacemaker which initiates the heartbeat. The SA node generates a wave of excitatory impulses which spread to the two artria, causing them to contract simultaneously. Blood is then forced from the atria into the ventricles.The second node, the atro-ventricular node (AVN), lying at the base of the right atrium, is then stimulated. Impulses from the AV node are conducted by specialised muscle fibres called bundle of His and Purkinje fibres to the ventricular walls. This causes the contraction of both ventricles to pump the blood out of the heart.

 The right ventricle pumps the blood into the pulmonary artery, which forces the blood to the lungs. The left ventricle, which is thicker and more muscular than the right ventricle, generates greater pressure to pump blood through the aorta to the other arteries in the body.
  
The cardiac cycle is the series of events that occur during one complete heartbeat. It includes the contraction (systole) and relaxation (diastole) of both the atria and the ventricles. The average heart rate is about 72 heartbeats per minute.
  
The pumping of the heart generates sufficient force to move the blood through the arteries, arterioles and capillaries. However, when the blood reaches the veins, the pressure produced by the heart is insufficient to force it back into the heart. The blood on the veins also has to flow against gravitational pull. How does the blood in the veins flow back to the heart? When the body moves, the skeletal muscles around the veins contract and press on the veins. The blood pressure increases, forces open the valves and pushes the blood towards the heart. The valves in the veins prevent the blood from flowing backwards.
  
The sino-atrial node can initiate the heartbeat on its own. However, the heart rate may be modified by certain external factors. The sympathetic nerve carrying impulses to the heart can increase the heart rate and the parasympathetic nerve can slow it down. When a person is excited, an increase in the secretion of the hormone, adrenaline, causes the heart to beat faster. An increase in the partial pressure of carbon dioxide in the blood also increases the heart rate. Heart rate also increases when body temperature is elevated. The human heart is a very hard-working organ. It works non-stop day and night. The average heart beats about 72 times per minute. In an average lifetime, the human heart beats more than two and a half billion times. In total it pumps 34 million litres of blood throughout the circulatory system in the body! (1 billion = 1 000 000 000).

Structure of Human Blood Vessels

Blood vessels are tubes that transport blood from one part of the human body to another. Arteries are blood vessels that carry blood away from the heart. Arteries branch out into small vessels called arterioles. The arterioles branch to form tiny vessels with thin walls called capillaries. Capillaries are the sites for the exchange of respiratory gases, nutrients and wastes. Capillaries join with another to form venules. Venules join together to form veins. Veins transport blood back to the heart. It estimated that if all the blood vessels in a human body were to be arranged end to end, the vessels would measure about 100 000 km in length.


  Table 1.Comparison between arteries, capillaries and veins.