Chapter 8: Transport in animals

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8.1 Transport systems in multicellular animals

In single-celled organisms, processes such as diffusion, osmosis, active transport, endocytosis and exocytosis can supply everything the cell needs to import or export. These processes arc also important in multicellular organisms, transporting substances within and between individual cells. However. as organisms get bigger, the distances between the cells and the outside of the body get greater. Diffusion would transport substances into and out of the inner core of the body, but it would be so slow that the organism would not survive.

Specialised transport systems are needed because:

• the metabolic demands of most multicellular animals arc high (they need lots of oxygen and food, they produce l01s of waste products) so diffusion over the long distances is not enough to supply the quantities needed

• the surface area to volume (SA: V) ratio gets smaller as multicellular organisms get bigger so not only do the diffusion distances get bigger but the amount of surface area available to absorb or remove substances becomes relatively smaller

• molecules such as hormones or enzymes may be made in one place but needed in another

• food will be digested in one organ system, but needs robe transported to every cell for use in respiration and other aspects of cell metabolism

• waste products of metabolism need to be removed from the cells and transported to excretory organs.

Types of circulatory systems

Most large, multicellular animals have specialised circulatory systems (transport systems) which carry gases such as oxygen and carbon dioxide, nutrients, waste products and hormones around the body. Most circulatory systems have features in common: Most large, multicellular animals have specialised circulatory systems (transport systems) which carry gases such as oxygen and carbon dioxide, nutrients, waste products and hormones around the body. Most circulatory systems have features in common:

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8.2 Blood vessels

Circulation in humans is typical of a mammalian circulatory system. It is estimated that if all the blood vessels of an average adult human were laid end to end they would stretch to I 00000 miles- that is the equivalent of about four times around the circumference of the Earth. There are several different types of blood vessels in the body and their structural composition is closely related to their function.

Some examples of different components utilised in some blood vessels are:

• Elastic fibres- these are composed of elastin and can stretch and recoil, providing vessel walls with flexibility.

• Smooth muscle- contracts or relaxes, which changes the size of the lumen (the channel within the blood vessel).

• Collagen- provides structural support to maintain the shape and volume of the vessel. The sections below detail which components are found in each of the blood vessel types.

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8.3 Blood, tissue fluid, and lymph

Blood is a body fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.

In vertebrates, blood is composed of blood cells suspended in blood plasma. Plasma, which makes up 55% of blood fluid, is mostly water and contains proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), and blood cells themselves. Albumin, the main protein in plasma, functions to regulate the colloidal osmotic pressure of blood.

The blood cells are mainly red blood cells (also called erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates oxygen transport by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is mostly transported extracellularly as bicarbonate ion transported in plasma.

Vertebrate blood is bright red when oxygenated and dark red when deoxygenated. The image below shows the difference in venous blood (darker) and arterial blood (brighter). This variation is due to the difference in colour between oxyhemoglobin and deoxyhemoglobin.

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8.4 Transport of oxygen and carbon dioxide in the blood

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8.5 The heart