STUDIES ON BLOOD CAPILLARIES

The pathway by which intravenously injected ferritin molecules move from the blood plasma across the capillary wall has been investigated in the muscle of the rat diaphragm. At 2 min after administration, the ferritin molecules are evenly distributed in high concentration in the blood plasma of capillaries and occur within vesicles along the blood front of the endothelium. At the 10-min time point, a small number of molecules appear in the adventitia, and by 60 min they are relatively numerous in the adventitia and in phagocytic vesicles and vacuoles of adventitial macrophages. Thereafter, the amount of ferritin in the adventitia and pericapillary regions gradually increases so that at 1 day the concentration in the extracellular spaces approaches that in the blood plasma. Macrophages and, to a lesser extent, fibroblasts contain large amounts of ferritin. 4 days after administration, ferritin appears to be cleared from the blood and from the capillary walls, but it still persists in the adventitial macrophages and fibroblasts. At all time points examined, ferritin molecules within the endothelial tunic were restricted to vesicles or to occasional multivesicular or dense bodies; they were not found in intercellular junctions or within the cytoplasmic matrix. Ferritin molecules did not accumulate within or against the basement membranes. Over the time period studied, the concentration of ferritin in the blood decreased, first rapidly, then slowly, in two apparently exponential phases. Liver and spleen removed large amounts of ferritin from the blood. Diaphragms fixed at time points from 10 min to 1 day, stained for iron by the Prussian Blue method, and prepared as cleared whole mounts, showed a progressive and even accumulation of ferritin in adventitial macrophages along the entire capillary network. These findings indicate: (1) that endothelial cell vesicles are the structural equivalent of the large pore system postulated in the pore theory of capillary permeability; (2) that the basement membrane is not a structural restraint in the movement of ferritin molecules across the capillary wall; (3) that transport of ferritin occurs uniformly along the entire length of the capillary; and (4) that the adventitial macrophages monitor the capillary filtrate and partially clear it of the tracer.

What Are Blood Vessels?

Blood vessels are intricate networks of hollow tubes that transport blood throughout the entire body.
Types of Blood Vessels

Arteries
Arteries are elastic vessels that transport blood away from the heart.

Veins
Veins are elastic vessels that transport blood to the heart.
Capillaries
Capillaries are extremely small vessels located within the tissues of the body that transport blood from the arteries to the veins.

Sinusoids
Sinusoids are extremely small vessels located within the liver, spleen and bone marrow.

Circulation
Blood vessels carry blood from the heart to all areas of the body. The blood travels from the heart via arteries to smaller arterioles, then to capillaries or sinusoids, to venules, to veins and back to the heart.

Microcirculation deals with the flow of blood from arterioles to capillaries or sinusoids to venules. As the blood moves through the capillaries, substances such as oxygen, carbon dioxide, nutrients and wastes are exchanged between the blood and the fluid that surrounds cells.

The Blood And Circulation

This page tells you about the blood and circulation. There is information on

• What the blood does
• The circulation
• What is in the blood
• White blood cells
• Red blood cells
• Platelets
• How and where blood cells are made
• Your blood and chemotherapy

What is blood for?
The blood circulates throughout the body. It carries nutrients (food) and oxygen to all the cells of the body. And carries away waste products so that they can be removed from the body. Without access to the blood, cells and body tissues die.

The blood moves around the body inside the circulatory system. This is made up of blood vessels (tubes) called arteries, veins and capillaries. The blood keeps moving through these blood vessels because it is being pumped by the heart.

Arteries carry blood that is full of oxygen from the heart to all parts of the body. As the arteries get further and further away from the heart, they get smaller and smaller.

Eventually they turn into capillaries. These are the smallest blood vessels. They go right into the tissues. Here the blood in the capillaries gives oxygen to the cells and picks up the waste gas, carbon dioxide, from the cells.

The capillaries are connected to the smallest veins in the body. The veins get bigger and bigger as they carry the blood back towards the heart.
The blood passes through the right side of the heart and goes to the lungs where it gets rid of carbon dioxide and picks up more oxygen.

It then passes through the left side of the heart and is pumped back around the body.
The blood always circulates through the body in the same direction. As well as oxygen and carbon dioxide, many other substances are carried in the blood. The blood circulating through the digestive system picks up digested food products and carries them to the liver to be used or stored.

The circulation can help explain why some cancers nearly always spread to the same place. Cancers of the colon (large bowel) often spread to the liver. This is because blood circulates from the bowel through the liver on its way back to the heart. If there is a cancer in the large bowel, and some cancer cells have found their way into the circulation, they may stick in the liver as the blood passes through. They can then begin to grow into secondary cancers.

The Blood

Although blood looks like a red liquid, if some is left in a test tube it separates out into a pale liquid called plasma and a solid layer of blood cells.
The blood is about 55% plasma and 45% cells. Plasma is mostly water with some proteins and other chemicals dissolved in it. There are three main types of cells in the blood

• White blood cells
• Red blood cells
• Platelets

White blood cells
There are several different types of white cells in the blood in differing amounts. They all play a part in the immune response. This is the response of the body to infection, or anything else the body recognises as 'foreign'. These blood cells can be made very quickly and generally have a short life. Some only live for a few hours, others for days.

There isn't an exact 'normal' figure for blood counts. 'Normal' for a large man wouldn't be the same as for a small woman. But generally the normal white cell count is between about 4,000 and 11,000 per cubic millimeter of blood. If you have surgery or an infection, your white blood cell counts will go up within a day or two.

The most numerous of the white blood cells are the neutrophils. There are between 2,000 and 7,500 of these per cubic millimeter of blood. They are important for fighting infection. If you have chemotherapy, particularly high dose, your doctors will probably talk about your neutrophil count.

The next most numerous are the lymphocytes. A normal lymphocyte count is between 1,300 and 4,000 per cubic millimeter of blood. Lymphocytes are involved in making antibodies as part of the immune response.

There are other white blood cells that are present in smaller numbers in the circulating blood. There are between

• 40 and 400 eosinophils
• 0 and 100 basophils
• 200 and 800 monocytes

per cubic millimeter of blood. As we've said, the range quoted as normal for blood cell counts does vary. These figures have come from the Oxford Handbook of Clinical Medicine.

Red blood cells
Red blood cells give the blood its red colour. There are more than 4 or 5 million of them in every cubic millimeter of blood. A red blood cell can live for up to 120 days.

Red blood cells are able to attach to oxygen to carry it within the circulation to the tissues. When they get to an area where the oxygen is needed, they give it up and pick up carbon dioxide which they carry back to the lungs. A shortage of red blood cells is called anaemia. The role of the red blood cell in carrying oxygen explains why very anaemic people usually feel breathless.

Platelets
Platelets are really bits of much bigger cells called megakaryocytes. A normal platelet count is between 150,000 and 440,000 per cubic millimeter of blood.

Platelets are very important in blood clotting. They clump together to form a plug if bleeding occurs. Then they release other chemicals that help the blood to clot and the blood vessel to be repaired.

How and where blood cells are made
All the different types of blood cells develop from one type of cell called a 'blood stem cell'. In adults, blood stem cells are normally found in the red bone marrow inside the bones. Blood cells are made in the bone marrow in the skull, ribs, sternum (breast bone), spine and pelvis.

The stem cells divide and multiply to make the blood cells. These cells differentiate (develop and mature) as they grow into white cells, red cells or platelets. It is now possible to collect stem cells and freeze them. Then give them back to you after high dose chemotherapy treatment. There is information about stem cell and bone marrow transplants elsewhere in this section of CancerHelp UK.