Transport in animals

• Circulatory systems can either be open, for instance in insects or closed, like in fish and mammals where the blood is confined to blood vessels only
• Closed circulatory systems come in two forms, either a single form which consists of a heart with two chambers
• Arteries – adapted to carrying blood away from the heart to the rest of the body, thick walled to withstand high blood pressure, contain elastic tissue which allows them to stretch and recoil thus smoothing blood flow, contain smooth muscle which enables them to vary blood flow, lined with smooth endothelium to reduce friction and ease flow of blood
• Arterioles – branch off arteries, have thinner and less muscular walls, their role is to feed blood into capillaries
• Capillaries – smallest blood vessels, site of metabolic exchange, only one cell thick for fast exchange of substances
• Venules – larger than capillaries but smaller than veins
• Veins – carry blood from the body to the heart, contain a wide lumen to maximise volume of blood carried to the heart, thin walled as blood is under low pressure, contain valves to prevent backflow of blood, no pulse of blood meaning there’s little elastic tissue or smooth muscle as there is no need for stretching and recoiling
• Tissue fluid is a liquid containing dissolved oxygen and nutrients which serves as a means of supplying the tissues with the essential solutes in exchange for waste products such as carbon dioxide. Therefore, it enables exchange of substances between blood and cells
• Hydrostatic pressure:

  1. when blood is pumped along the arteries, into arterioles and then capillaries. This pressure forces blood fluid out of the capillaries
  2. Only substances which are small enough to escape through the gaps in the capillary wall are components of the tissue fluid – this includes dissolved nutrients and oxygen. The fluid is referred to as tissue fluid,
• The fluid is also acted on by osmotic pressure which pushes some of the fluid back into the capillaries
• As both the tissue fluid and blood contain solutes, they have a negative water potential. Although the potential of the tissue fluid is negative, it is less negative in comparison to the blood (the blood contains more solutes).
• Therefore, the tissue fluid is positive in comparison to the blood. This causes water to move down the water potential gradient from the tissue fluid to the blood by osmosis.
• The remaining tissue fluid which is not pushed back into the capillaries is carried back via the lymphatic system
• The lymphatic system contains lymph fluid, similar in content to tissue fluid
• However, lymph fluid contains less oxygen and nutrients compared to tissue fluid, as its main purpose is to carry waste products
• The lymph system also contains lymph nodes which filter out bacteria and foreign material from the fluid with the help of lymphocytes which destroy the invaders as part of the immune system defences.