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Introduction to Transport Systems

Transport systems are required in multicellular organisms because their surface area to volume ratios are too small. Simple diffusion is too slow to supply each cell of the organism with its needs. Transport systems transport respiratory gases, food, waste products and hormones.

Transport in Humans

Components of the Circulatory System

Component

Function

Blood

Transports substances

Blood vessel

Carry blood

Heart

Pumps blood

Blood Vessels

Arteries and Arterioles

Arteries transport blood away from the heart. The blood is at high pressure. The artery walls are thick and contain elastic fibres to stretch and recoil as the heart beats, which helps to even out the pressure. Arterioles are smaller arteries that control blood flow to the capillaries.

Veins and Venules

Veins transport blood towards the heart at low pressure. They have thinner walls than arteries, wider lumen and contain valves to prevent backflow of blood. Venules are small veins that connect capillaries to veins.

Capillaries

Capillaries are tiny vessels that link arterioles to venules. They have a very narrow lumen and thin walls that are one cell thick, so that the diffusion distance is short.

Blood

Red Blood Cells (Erythrocytes)

Red blood cells transport oxygen from the lungs to the body cells. They have no nucleus and are packed with haemoglobin. They are small and biconcave in shape to maximise their surface area.

White Blood Cells (Leucocytes)

White blood cells are part of the immune system and defend the body against pathogens. Some white blood cells are phagocytes that engulf and destroy pathogens. Others are lymphocytes. There are two types of lymphocytes:

  1. B-lymphocytes produce antibodies which destroy pathogens.

  2. T-lymphocytes can be used to kill infected cells.

Platelets (Thrombocytes)

Platelets are small fragments of cells that help the blood to clot. They have no nucleus.

Plasma

Plasma is the liquid part of the blood. It transports blood cells, carbon dioxide, digested food, urea, hormones and heat around the body.

The Heart

The heart is a muscular organ that pumps blood around the body. It has four chambers: two atria and two ventricles.

The Cardiac Cycle

The cardiac cycle is the sequence of events that occurs during one heartbeat. There are three stages:

  1. Atrial systole: The atria contract, forcing blood into the ventricles.

  2. Ventricular systole: The ventricles contract, forcing blood into the arteries.

  3. Diastole: The heart relaxes, and the atria fill with blood.

Control of Heart Rate

The heart rate is controlled by the sinoatrial node (SAN), which is a group of cells in the right atrium. The SAN sends out electrical impulses that cause the heart to contract. The heart rate can be affected by:

  • Exercise

  • Stress

  • Hormones

Cardiovascular Disease

Cardiovascular disease (CVD) is a general term for diseases of the heart and blood vessels. CVD includes:

  • Cor