Faculty of Biological Sciences, University of Leeds

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Introductory Anatomy: Circulatory System & Blood

Dr. D.R.Johnson

Perhaps the most important thing to remember about the circulatory system is its variability. This is because of the way the circulatory system develops: the early embryo is essentially a blood filled sponge - intricate network. Most of this is removed during development, leaving remnants governed by highest flow and pressure. Its plasticity is also shown by migratory organs: if an organ moves during development eg the kidney it will take its nerve supply with it, but acquire a new local blood supply - successive segmental arteries of the kidney as it rises may persist as 'abnormal' renal arteries. Sensible place to start is the heart, complicated at first glance so let's reduce it to a diagram: four chambered, atria and ventricles in one way communication. Right half circulates blood from body to lungs, left half circulates blood from lungs around body. This has immediate implications: the pulmonary circulation is rather small, little peripheral resistance, so low pressures needed and the walls of the right ventricle are rather thinner. The left side pumps the same volume against greater peripheral resistance, so the left ventricular walls are more muscular.
There is functional specialisation in the vessels too.

1. Distribution system
Low volume, high pressure system of large arteries which leave the heart: we speak of the arterial tree, trunk, branches, twigs. The branches are always smaller than the trunk and so on. Several branches may come off at a single level, but more often a main trunk gives off branches and continues. Structure is also graded as you pass away from the heart. Large trunks have much elastic tissue in their walls. Repeated distention and contraction of this tends to even out heart contractions into a steady, though pulsatile flow. Smaller branches have less elastic tissue and more smooth muscle offering better control of flow to suit temperature, activity etc.
2. Resistance vessels
Control blood flow at more intimate level - muscular arterioles and precapillary sphincters provide principle resistance to blood flow which governs pressure in arterial tree.
3. Exchange vessels
Capillaries are often composed of a single wrapped cell at any given level. Across their walls occurs exchange between blood and tissue fluids, oxygen, CO2, nutrients, water, inorganic ions, vitamins, hormones, metabolic products, immune substances, even immune competent cells. Capillaries may be plain, fenestrated or sinusoidal - to slow blood flow.
4. Capacitance vessels.
After capillary beds blood is collected in venules which are tributaries of veins. These vessels provide a low pressure blood reservoir through which blood returns to the heart. Veins have the same basic histological structure as arteries, but tend to be greater in cross sectional area at any given level, because of slower flow rate. For the same reason arteries are often accompanied by paired veins, vena combatants.
Veins often have a dead space around them to allow for dilation, so not sheathed as arteries are, but run in loose connective tissue. Because only a little external pressure would stop flow veins are confined to the dorsum of the foot and the back of the hand, and often run on the flexor aspects of joints.
Valves
The tendency for gravity to stop or reverse the flow of blood in veins is countered by valves - pockets in the walls, usually in twos or threes. Reflex blood pours into these pockets, filling them and stopping the flow. They are found where a tributary joins a larger vein, and at intervals along main veins. Most frequent in lower limb, where the effect of gravity is greatest, diminishing as we move superiorly and virtually absent above the heart, where gravity acts with venous return, not against it. Return of blood is ensured by several factors. Smaller veins are continually filled by capillaries, larger veins (especially in the lower limb) are continually squeezed by muscular action, valves controlling the direction of flow.

Exceptions to the rule
1. Portal circulations

These differ from those already described in having two capillary beds. The largest portal system covers the spleen, pancreas, stomach, small intestine. Blood supplying these, having passed through their capillary network ends up in the hepatic portal vein, which drains into the liver and through a hepatic capillary bed, where products of digestion are removed, processed and stored. A second, smaller but equally important portal system is found connecting the brain and pituitary gland.
2. Anastomoses
Arteries do not always end up as capillaries. They often anastomose with other arteries, either of equal (in the brain) or unequal size (elsewhere). In limbs anastomoses are most common around joints: quite clearly they form an alternative route when the main road is obstructed by, say, flexion of the elbow: they will also allow equalisation of pressure. They become more frequent as you get further from the heart i.e. as the arteries get smaller, so that arterioles tend to form a network. If you cut an artery which forms part of an anastomosis it will bleed from both ends. Anastomoses form the basis of collateral circulation. This is important if an artery is gradually furring up: alternative arteries will enlarge to restore an adequate supply. Sudden blockage may lead to the death of the area supplied (avascular necrosis) if collaterals are absent or inadequate. This is seen in the head of the femur, scaphoid after fracture and central artery of the retina, and in coronary arteries supplying heart muscle - end arteries.
3. Vascular shunts.
These are important shortcuts from artery to vein cutting out capillary bed. These are seen as
a. preferential thoroughfares - areas of a capillary bed are deferentially supplied with blood under different demands. Areas are then restricted by closing precapillary sphincters.
b. artero-venous anastomoses - direct connections between arteries and veins, by coiled vessels with a thick muscular coat, which may allow complete closure. These, found in skin of nose, ear, tongue, erectile sexual tissue, hands and feet are temperature control mechanisms, cutting off blood supply to a capillary bed to reduce heat loss, or encouraging it to encourage it (panting dog). Regulation of blood pressure? Maybe.
Absent in newborn, atrophy in old age - so wrap up newborns and grannies.

Lymphatic circulation As well as blood vessels we can also find a system of lymphatic capillaries and larger vessels. Lymph capillaries coexist with blood capillaries in capillary beds, have fenestrated walls and can thus exchange anything from liquids to cells. Most of the fluid which leaks from blood capillaries into tissues returns, but 10-20% doesn't, and would therefore gradually flood the tissues if left (oedema). This is mopped up by the lymphatics,, which shadow the veins and eventually dump lymph, usually via one or more lymph nodes into the blood stream, via the thoracic duct and right lymphatic duct which open into veins in the neck.
As well as fluid this system contains cells, notably phagocytic dustman cells which circulate in the blood, exit via the capillaries, spend time in the tissues collecting anything from dead cells to bacteria, then return via the lymphatic system. Because of this behaviour the phagocytic cells are often laden with bacteria, viruses or diseased cells. Lymph nodes are thus prone to infection and swelling and the lymphatic system is important in the spread of cancerous cells - metastasis.

Blood
While we are considering the circulatory system it is logical to look at blood. Blood is a liquid with suspended cells 30-50% by volume. These cells are of three basic types, erythrocytes or rbcs, leucocytes or wbcs and thrombocytes or platelets. Two of these three cell types are odd, in being anucleate. All originate in the bone marrow, although the number of primary stem cell types is unresolved.

Erythrocytes
By far the commonest blood cells (4-6m/mm3) erythrocytes are classically biconcave enucleate discs 7-8* in diameter. They are red because the cytoplasm is packed with haemoglobin which transports oxygen. They also transport carbon dioxide. Their shape is variable: they are able to deform to squeeze through capillaries. Because they lack a nucleus they have a short lifespan (c 120d) after which their components are recycled (iron) or excreted (bilirubin). Young erythrocytes - reticulocytes (1%) contain a net made up of the remains of the RNA used in haemoglobin synthesis.
Leucocytes
Less common than rbcs (5-10,000/mm3), two main types distinguished according to the presence or absence of cytoplasmic granules.
Granulocytes.
The nuclei of granulocytes are multilobed and cells also called polymorphonuclear leucocytes or polymorphs. The cytoplasm contains two sorts of granules: primary granules - lysosomes - present in all polymorphs and associated with their phagocytic nature secondary or specific granules: which allow identification because they take up specific stains.
We thus distinguish
neutrophils - commonest (50-70%) 10-12* diameter, lobed nucleus, secondary granules unstained. Phagocytic against micro-organisms in blood
eosinophils - (1-4%), 10-12* diameter, lobed nucleus, secondary granules stained pink by eosin. Eat antigen/antibody complexes
basophils - (<1%), 9-10* diameter, lobed nucleus usually hidden by blue staining secondary granules. Not so actively phagocytic -blue granules are heparin and histamine which are released in inflammation and immune responses.
Agranulocytes.
Lack granules, and have a rounded nucleus. Lymphocytes are the commonest (40%), 6-8* diameter, with a large nucleus all but obscuring the cytoplasm . Monocytes (4-8%) larger, 12-16*m diameter with a relatively smaller nucleus.
Platelets
Small (2-3*m) purple staining cell components, non nucleated (In background, monocyte slide). They tend to aggregate in clumps. Involved in wound repair and blood clotting. Formed by the disintegration of huge (150*m) megakaryocytes in the bone marrow.


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