Transport systems in animals

Multicellular organisms, particularly larger and more active animals like mammals, require specialized transport systems because diffusion alone is too slow to supply all their cells with essential substances like glucose and oxygen, and to remove waste products like carbon dioxide. This is due to the large distances substances would have to travel to reach cells deep within the body and the low surface area to volume ratio of larger animals, which makes it difficult to exchange enough substances through a relatively small outer surface. Highly active animals also have increased metabolic rates, demanding greater oxygen supplies.

The primary transport system in mammals is the circulatory system, which is a mass transport system. Mass transport refers to the bulk movement of gases or liquids in one direction, usually through a system of vessels and tubes. This system works together with specialized exchange systems like the lungs and digestive system to maintain concentration gradients.

Key features and components of animal transport systems, especially the mammalian circulatory system, include:

• Closed and Double Circulation:

  • In a closed circulatory system, blood is always contained within a network of blood vessels. This is characteristic of all vertebrates and many invertebrates.

  • A double circulation means blood passes through the heart twice for each complete circuit of the body.

    • The pulmonary circulation takes blood from the heart to the lungs and back to the heart.

    • The systemic circulation takes blood from the heart around the rest of the body and back to the heart.

  • This double circulation system allows oxygenated blood to be pumped around the body at a higher pressure and faster, providing a more efficient oxygen supply to cells compared to single circulatory systems found in fish where pressure is lost in gill capillaries.

• The Heart:

  • The heart acts as the pump that drives blood through the circulatory system.

  • Mammalian hearts have four chambers (two atria and two ventricles).

  • The walls of the ventricles are thicker and more muscular than the atria, with the left ventricle having a thicker wall than the right to pump blood at higher pressure to the whole body.

  • The heart's muscle is myogenic, meaning it contracts and relaxes automatically without nerve impulses. The sinoatrial node (SAN) acts as a natural pacemaker, initiating waves of electrical activity that spread through the atria and then to the ventricles via the atrioventricular node (AVN) and Purkyne tissue.

• Blood Vessels:

  • Arteries carry blood away from the heart at high pressure. They have thick, muscular, and elastic walls to withstand high pressure and maintain blood flow. Their inner lining is folded to allow stretching.

  • Arterioles are smaller arteries that branch throughout the body. Their muscles contract or relax to direct blood flow to different areas.

  • Capillaries are the smallest vessels, forming networks (capillary beds) near cells. They are adapted for efficient exchange: their walls are only one cell thick and are permeable, providing a short diffusion pathway and a large surface area.

  • Veins carry blood back to the heart under low pressure. They have wider lumens, less elastic and muscle tissue, and contain valves to prevent backflow. Blood flow in veins is aided by surrounding body muscle contractions.

• Blood:

  • Blood is a complex fluid composed of plasma (the liquid component), red blood cells (erythrocytes), white blood cells (leucocytes), and platelets.

  • Plasma is mostly water (95%) and acts as a solvent for transporting nutrients (glucose, amino acids, lipids), waste products (urea, carbon dioxide), hormones, and heat. It also contains plasma proteins like albumin that regulate water potential and antibodies for immunity.

  • Red blood cells are biconcave and enucleated (no nucleus) to maximize space for haemoglobin, the oxygen-carrying compound. Each haemoglobin molecule can bind with four oxygen molecules (eight oxygen atoms).

  • Oxygen loading and unloading is described by the oxyhaemoglobin dissociation curve, with oxygen loading at high partial pressure of oxygen (pO2) in the lungs and unloading at low pO2 in respiring tissues. The Bohr shift explains how increased carbon dioxide concentration and lower pH (more acidic conditions from respiration) reduce haemoglobin's affinity for oxygen, aiding unloading in tissues.

  • Carbon dioxide is transported mainly in the plasma as hydrogencarbonate ions, with some bound to haemoglobin as carbaminohaemoglobin. The enzyme carbonic anhydrase in red blood cells catalyzes the reaction between CO2 and water.

  • White blood cells are involved in the immune system and platelets in blood clotting.

• Tissue Fluid and Lymph:

  • Tissue fluid is formed from blood plasma that is forced out of capillaries (by pressure filtration at the arteriole end) and surrounds cells in tissues. It contains oxygen, water, and nutrients, but lacks red blood cells and large proteins due to their size. Cells take in substances from tissue fluid and release waste into it.

  • Most water re-enters capillaries at the venule end by osmosis due to lower water potential in the capillaries.

  • Any excess tissue fluid drains into the lymphatic system, a network of tubes that transports it back to the circulatory system. Lymph nodes filter the lymph and are sites for immune cells.

• Energy for Transport:

  • Active transport is a crucial mechanism for moving substances across cell membranes against their concentration gradient, requiring energy from ATP hydrolysis. Examples include absorbing glucose from the ileum, transporting solutes in plants, and maintaining ion gradients in nerve cells (e.g., sodium-potassium pump).

  • Other movements like muscle contraction, cell division, and protein synthesis also require ATP.

• Comparison with Plant Transport Systems:

  • Unlike animals, plants do not need a transport system for oxygen and carbon dioxide, as these gases diffuse directly into their cells.

  • Plants have two distinct transport systems: xylem (transports water and inorganic ions from roots to leaves) and phloem (transports organic substances like sucrose and amino acids from sources to sinks).

  • Plant transport relies more on passive processes like transpiration for water movement in xylem (cohesion-tension theory) and active transport mechanisms in companion cells for phloem loading (mass flow hypothesis), rather than a central pumping organ like the heart in animals.