B13.01 Circulation: Blood and Blood Vessels

Biological Transport and Diffusion Constraints

In single-celled organisms, such as protists, and small multicellular organisms, essential molecules move to where they are needed via the process of diffusion. Within a cell of approximately $100\,\mu m$ in size, molecules diffuse internally to where they are required. In small multicellular structures of approximately $200\,\mu m$, molecules diffuse from one cell to another. However, once an organism, such as a very early embryo, exceeds a certain size, the diffusion path becomes too large to move molecules efficiently over longer distances. For example, in an organism of $2\,metres$, which is equivalent to $2,000,000\,\mu m$, a transport system is required to deliver essential molecules all over the body.

Common features of these transport systems include they are made up of tubes or vessels that carry materials from one part of the organism to another. Additionally, these systems make close contact with cells, such as those found at exchange surfaces, to ensure efficient distribution. The circulatory system serves as this transport mechanism, utilizing the heart to pump blood throughout the body via a network of blood vessels.

Components and Functions of the Blood

Blood is a complex fluid composed of several distinct components, each with specific functions and adaptations. Plasma is the liquid component of blood responsible for transporting carbon dioxide ($CO_2$), digested food molecules, urea, and hormones. It also plays a critical role in distributing heat throughout the body.

Red blood cells are specialized for the transportation of oxygen. They contain haemoglobin, a protein that binds to oxygen. Their structure is bi-concave, which provides a large surface area for the absorption of oxygen. Furthermore, red blood cells are adapted to be flexible, allowing them to squeeze through small, narrow blood vessels. Three primary adaptations of red blood cells include the presence of haemoglobin, the bi-concave shape for increased surface area, and the ability to deform to fit through capillaries.

White blood cells are integral to the immune system. They protect the body from infection and illness by killing pathogens. This is achieved through two main methods: ingesting pathogens and producing antibodies or other substances to kill pathogens.

Platelets are small fragments involved in blood clotting. This process is essential for preventing excessive blood loss and protecting the body from the entry of pathogens through wounds. If an individual does not have any platelets, it becomes a significant medical problem because their blood will not clot, leading to prolonged bleeding from even minor injuries.

The Circulatory System and Blood Vessels

Blood moves around the body in a system of tubes called blood vessels. There are three main types of blood vessels: arteries, veins, and capillaries. Arteries carry oxygen-rich, or oxygenated, blood away from the heart to the rest of the body. Veins carry blood containing waste carbon dioxide, or deoxygenated blood, back to the heart. Capillaries are very thin tubes that join arteries to veins, facilitating the exchange of materials between the blood and the tissues.

Humans and other mammals possess a double circulatory system, which means blood passes through the heart twice on every full circuit of the body. This system is advantageous because it separates the blood with oxygen from the blood without oxygen. The double circulatory system is divided into two distinct circuits: the pulmonary circuit and the systemic circuit. The pulmonary circuit pumps blood to the lungs to pick up oxygen and release carbon dioxide. The systemic circuit pumps oxygenated blood to the body systems, including the head, limbs, and trunk.