Unit Two‎ > ‎

Transport Mechanisms

The Cardiac Cycle is the process of contraction and relaxing of the heart's atria and ventricles. Although when we talk about the heart we use the words systole instead of contraction and diastole instead of relaxing.

The heart is divided into four chambers consisting of two atria and two ventricles. The right atrium receives deoxygenated blood from the superior and inferior Vena Cava. This blood then passes into the right ventricle by atrial systole, which is then pumped to the lungs through the pulmonary artery, by ventricular systole. This blood becomes oxygenated as the haemoglobin in the red blood cells combines with the oxygen, creating oxyhaemoglobin. This blood is transported back to the heart via the pulmonary vein into the left atrium. This is pumped into the left ventricle during atrial systole, and then pumped into the rest of the body through the aorta.

Strucure of the Heart

The frequency of the beats of the heart is controlled by the heart's own conducting system - the Conducting system. The pacemaker, also known as the sino-atrial node (SAN), is the source of the electrical impulse which causes the heart to beat, located in the top of the right atrium. This spontaneous stimulation causes atrial systole, this impulse travels down the middle of the heart and through the atrio-ventricular node (AVN). This impulse then travels down through the apex of the heart via conducting fibres, causing ventricular systole. This process allows the heart to either speed up or slow down, this depends on the stimulation of either the sympathetic or parasympathetic nerves.

Blood Pressure:
The pressure of blood as it flows through the circulatory system fluctuates. This is caused by the ventricular systole and diastole. Blood pressure can be measured with an instrument called a sphygmomanometer. The pressure of blood decreases in progressive order: (highest to lowest) arteries, arterioles, capillaries, venules then veins.

Peripheral Resistance is the resistance of blood flow caused by the friction between the blood and walls of the vessels. The arterioles present the greatest resistance to blood flow and are the cause for the greatest overall resistance.

Lymphatic System

The walls of the minuscule lymphatic vessels are porous, allowing them to absorb the excess tissue fluid filtered out of the bloodstream in capillary beds. This fluid, called lymph, is collected by a network of lymph capillaries which unite to form larger lymphatic vessels. These vessels eventually return their content back to the bloodstream via two lymphatic ducts which enter the veins coming from the arms.
The Lymphatic System

The flow of lymph through these vessels is brought about by the movement of the limbs which pushes the lymph around, the back-flow of lymph is prevented by the presence of valves.

Each finger like villus in the small intestine has its own channel for absorbing lipids (fats) called a lacteal. The lipids is then a constituent of lymph, which will be transported into the bloodstream to use as an energy source during respiration (or stored for the future).

Lymph nodes are bean like structures found throughout the lymphatic system. They mostly occur in groups found around the neck, groin and armpits. Each node consists of a layer of lymph nodules surrounding a Central Medulla. In the middle of each lymph nodule there is a germinal centre which is the site of lymphocyte production. As the lymph enters the nodes they pass through a channel covered with macrophage cells which are able to engulf any pathogens which may have entered the body.
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