Blood is fun! Blood is to body as the Thames is to London, although I sure hope slightly cleaner…
Blood is roughly split into the plasma and blood cells including erythrocytes and leucocytes (neutrophils, eosinophils, monocytes, lymphocytes). Plasma is the solution that blood cells are found in, and as such acts as their extracellular matrix. For skin cells for example, the extracellular matrix is formed of collagen, so it’s different to have it essentially a liquid like plasma. Plasma is a water solution containing proteins, sugars, clotting factors (as well as platelets involved in clotting), hormones, electrolytes, carbon dioxide and oxygen.
Erythrocytes are red blood cells/RBC (and also the most common blood cells) carrying haemoglobin around the body. Haemoglobin can bind and release oxygen and is central to aerobic respiration.
Leucocytes of varying types are white blood cells/WBC, colourless, and act in defence against infection and disease.
There are many types of white blood cell. Neutrophils are the most common and, alongside monocytes, digest invading cells of bacteria and fungi by engulfing them in a process called phagocytosis. The invader is engulfed, isolated in a lysosome that contains digestive enzymes, and its remains disposed off and recycled or excreted.
Monocytes also live longer and present antigens of invaders to a type of lymphocyte called a T cell for later reference should the same invader come back later in the future. Moreover, monocytes eventually leave the bloodstream to settle in a different tissue and become macrophages in charge of clearing up cell debris and further immune function.
Eosinophils are the least common in blood, and attack larger parasites as well as modulate allergic inflammatory responses.
These types of white blood cells are collectively called polymorphs (polymorphonuclear granulocytes) because of an old microscopic technique used to identify them in which they had multiple “lobes” and granules as opposed to lymphocytes which didn’t.
Finally, lymphocytes are relatively abundant in blood but much more prevalent in the lymphatic system and take part in the adaptive immune response. Phagocytosis as carried out by neutrophils and monocytes is part of the innate immune response and as such is more generic. Lymphocytes include B cells, T cells and natural killer cells (NK cells are part of the innate immune response however).
They can make antibodies against various pathogens or abnormal cells in the body such as those in tumours, present them on their surface and find target cells, and destroy any cells which do not present the expected antigens on their surface.
All in all, the function of blood covers defence, transport and formation of lymph and tissue fluid.
Lymph is formed from the interstitial fluid found throughout tissue and is similar in composition to blood plasma, lacking red blood cells. The lymphatic system plays a role in circulating blood back to the heart as well as passing it though the lymph nodes to clear it of any infectious agents.
Tissue fluid is found between cells and its role is to transport oxygen and nutrients from blood to cells, and waste products and carbon dioxide back to blood. It is similar to blood plasma, lacking blood cells and proteins.
Hydrostatic pressure determines the flow of these fluids between the different systems in a tissue. The oxygenated blood travelling through capillaries from arterioles has a high hydrostatic pressure, pushing the liquid outwards through small pores. This contains all the nutrients and gases cells need, which cross plasma membranes via diffusion or facilitated diffusion. Big components of blood such as cells and plasma proteins cannot escape the capillary, so stay in the blood.
The tissue fluid is then collected back via a water potential difference between itself and blood. Water moves by osmosis back into the blood in capillaries due to blood having more solutes and hence a lower water potential. This is a special subtype of osmosis called oncotic pressure and is derived from the presence of proteins such as albumin in blood that has the effect of pulling water into the capillary.
Some tissue fluid, such as excess tissue fluid, doesn’t return to blood and instead is drained into the lymphatic system. This forms lymph which is poorer in oxygen and nutrients, and returns to blood later on.
Blood carries nutrients such as glucose, hormones such as antidiuretic hormone, excretory products such as urea, and heat around the body.