Plasma and formed elements

Blood as a connective tissue

Blood is classified as connective tissue because it consists of cells scattered in a matrix, even though its matrix is liquid rather than solid or gel-like. The matrix is plasma, a pale yellow fluid that makes up a little more than half of blood volume. The cells and cell fragments are the formed elements, and they make up the rest. Blood has three broad functions. Transport: it carries oxygen from the lungs, nutrients from the gut, hormones from glands and waste products to the kidneys and lungs. Regulation: it distributes heat, helps control pH through buffers and influences water balance. Protection: white blood cells and plasma proteins defend against infection, and platelets and clotting proteins prevent blood loss. Recognising these three functions helps organise everything else you learn about blood.

For example: When you exercise, blood carries heat from the working muscles to the skin, where it is lost — a regulation function in action.

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Plasma and its proteins

Plasma is about 92% water, with dissolved substances making up the rest. It carries nutrients such as glucose, amino acids and lipids; ions such as sodium, potassium and calcium; waste products such as urea; gases in small amounts; and hormones. Its most distinctive solutes are the plasma proteins, which fall into three main groups. Albumins are the most abundant and are the main contributors to the osmotic pressure that holds water inside the vessels. Globulins include the antibodies that fight infection and the carrier proteins that transport lipids, hormones and iron. Fibrinogen is a clotting protein that is converted into fibrin when bleeding occurs. Plasma from which the clotting proteins have been removed is called serum, which is why a clotted blood sample yields serum rather than plasma. Because plasma proteins hold water in the vessels, losing them causes fluid to move into the tissues and produce oedema.

For example: In severe malnutrition or liver disease, albumin production falls, plasma osmotic pressure drops, and fluid leaks into the tissues, producing swelling.

Red blood cells

Red blood cells, also called erythrocytes, are the most numerous formed elements. A mature human red blood cell is a biconcave disc: thinner in the middle than at the edges. This shape gives a large surface area for exchanging gases and makes the cell flexible enough to pass through narrow capillaries. It has no nucleus and few organelles, which leaves more room for haemoglobin, the iron-containing protein that binds oxygen. Each red cell also carries antigens on its surface that determine blood group. A typical count is about five million cells per microlitre, and the cells live roughly 120 days before being removed by the spleen and liver. Their main job is to transport oxygen, and they also carry a share of the carbon dioxide produced by tissues.

For example: The biconcave shape is easy to remember from its consequence: a flat disc folds in the middle so it can bend as it squeezes through a capillary narrower than itself.

Common misconception: A mature human red blood cell has no nucleus, but it is not dead. It remains metabolically active, and it is a cell, not a fragment.

White blood cells

White blood cells, or leukocytes, defend the body. They are far fewer than red cells — normally about 5,000 to 10,000 per microlitre — and they contain a nucleus. They are grouped into five main types. Neutrophils are the most abundant and are the first responders to bacterial infection; they engulf microbes by phagocytosis. Eosinophils are involved in fighting parasites and in allergic responses. Basophils are the rarest; they release histamine and heparin and contribute to inflammation and allergic reactions. Lymphocytes include the T cells and B cells of immunity; B cells produce antibodies, and T cells help coordinate and carry out immune responses. Monocytes are the largest; they circulate, enter tissues and become macrophages that engulf debris and microbes. White cells can leave the bloodstream by squeezing between capillary cells, a process called diapedesis, so they can reach the site of infection.

For example: In acute appendicitis, the white cell count often rises because neutrophils are being mobilised to the inflamed tissue.

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Platelets

Platelets, also called thrombocytes, are not whole cells. They are small fragments pinched off from very large cells called megakaryocytes in the bone marrow. They have no nucleus and circulate for about a week to ten days. Their job is haemostasis: when a vessel is damaged, platelets stick to the exposed tissue, change shape, clump together to form a plug, and release substances that attract more platelets and help the clotting process. A normal platelet count is roughly 150,000 to 400,000 per microlitre. Too few platelets causes easy bruising and prolonged bleeding; too many increases the risk of unwanted clotting. Platelets are therefore essential for the first response to injury, even though they are not complete cells.

For example: When you cut yourself, the bleeding usually stops within minutes because platelets form a plug while clotting proteins build a firmer fibrin mesh.