1. Describe the layout of the circulation in general terms.
  2. Describe the structure and functions of arteries.
  3. Define shock and outline possible causes.
  4. Explain what has happened to cause David’s symptoms (i.e., symptoms of haemorrhagic shock).
  5. Describe the first aid management of arterial bleeding.

Blood Pressure Homeostasis

When blood pressure drops, three systems intervene to attempt to maintain normal blood pressure levels:


The baroceceptor system can be divided into two parts, the arterial (high pressure) and cardiopulmonary (low pressure.)

Primarily in the aortic arch and the carotid sinuses (the area at the bifurcation of the common carotid sinuses.) They are mechanoreceptors which respond to a stretch in the walls of the artery, and respond via the vagus and glossopharyngeal nerves. These nerves react to increased arterial wall stretch by parasympathetic stimulation of the heart to decrease
contractility and frequency
of heart beats.

Found in large veins and atria of the heart. The low pressure baroreceptors have a role in maintaining blood volume. They can have effect on the renal and circulatory system by release of hormones which have profound effect on the storage of salt and water.

Systemic circulation, its function & pathways and its description

· It transports oxygenated blood from the lungs towards the brain, bowel and this circuit starts in the heart and ends in the heart.
· Blood is carried by arteries (efferent vessels), and returned by veins (afferent vessels). Capillaries are the connection between arteries and veins, and they allow exchange of nutrients and dissolved gases and waste products between the blood into the surrounding tissue.
· The heart has four chambers, and the left ventricle pumps the blood into the systemic circuit and flows back into the right atrium.

Structure & Functions of Arteries


· Thicker, contain more muscle and elastic fibres than veins; this resists the pressure generated by the heart.
· Lumen constriction due to recoiling of artery walls, lumen of arteries seems smaller than that of veins, however its only due to the thickness of the arteries.

Typical Artery
General Appearance
Usually round with relatively thick walls.
Tunica Intima:
Internal Elastic membrane
Rippled, due to vessel constriction present, it is the innermost layer and includes endothelial lining and connective tissue underneath, and a number of elastic fibres, the outer part of elastic fibres called the internal elastic fibre membrane.
Tunica Media:
External Elastic membrane
Thick dominated by smooth muscle cells with loose connective tissue and elastic fibres, is the middle layer of vessels, collagen connects them to the other tissues. This is the thickest layer in small arteries, the smooth muscle cells are the reason for dilation of contraction of arteries. Large muscles contain longitudinal layers of smooth muscle cells.
Tunica Externa:
Tunica Adventitia
The outer layer on vessels, made of scattered collagen and elastic fibres, generally thicker than tunica media and is networked of elastic fibers and bundles of smooth muscle cells.

Vasoconstriction and vasodilation occurs in the arteries if they are stimulated they affect the afterload of the heart, peripheral pressure as well as the capillary blood flow.

Elastic Arteries

Elastic arteries can have a diameter of up to 2.5 cm and transport large amounts of blood away from the heart. They are highly resilent (able to spring back to their initial shape), as they contain many fibres however only few smooth muscle cells which enables them to adapt to the pressure changes in the vascular system.
Their elasticity enables them to cushion the effect of too high blood pressure, however there is a threshold. When the pressure decreases the artery springs back to its initial size, which is the driving force for blood pressure, as near the heart the blood flows, in pulses, however near the capillaries the blood flows rather continuously.

Muscular Arteries
Median sized arteries are distributed throughout the body and their characteristics is that they have a increased tunica media with more smooth muscle cells. Their size is usually about 0.4 cm lumen diameter, however some are as small as 0.5 mm. The external carotid sinus of the neck, brachial arteries in the arm, the meenteric arteries in the abdomen and the femoral artiers of thighs are all muscular arteries.
Most of the superficial arteries are important for pressure points, however deep muscular arteries are to prevent severe blood loss, and reduce blood flow.


The have a diameter of 30 μm or less and they are considerable smaller than muscular arteries. Arterioles have a small tunica externa, the tunica media in the larger arterioles consists of one or two layers of smooth muscle cells, however the tunica media of the smallest arterioles do not form a complete layer of smooth muscle cells.
Their diameter changes according to the local requirement e.g. endocrine stimulation or sympathic stimulation, however changes in the diameter affect the blood flow directly as well as the pressure, as more pressure is required to push blood through a constricted hence they are resistance vessels.
Arteries can vary in sizes and are adapted to handle pressure, however if pressure exceeds local requirement it can produce local aneurysm, or a bulge in the weakened wall of the artery. If the bulge causes blowouts then it might be fatal as it depends where it occurs, the most dangerous can occur in the brain, however a rupture in the aorta can be fatal as well.


The capillaries do the real work of the cardiovascular system, they surround muscle fibres and radiate through connective tissue and branch beneath the basal laminae of epithelial cells.
They are the only blood vessels which allow the exchange nutrients and gases between the blood and interstitial fluids, as the walls are thin, diffusion distances are small, so exchange can occur quickly. The blood flow in capillaries is quite slow, so diffusion can occur as the blood moves in two way directions.
A capillary is made of endothelial tubes that is surrounded by delicate basal lamina only and the average diameter is 8 μm.

Continuous Capillaries
Fenestrated Capillaries
Capillary Beds
· Complete lining of endothelium cells
· Are present everywhere except epithelial and cartilage
· Diffusion of water, small solutes and lipid-soluble materials except blood and plasma proteins
· Bulk transportation of vesicles
· There are specialized capillaries in the brain
· They contain more pores and this permits rapid exchange of water
· They are in intestinal tract and filtration sites in kidney

· They are a network made by arterioles e.g. one arteriole makes dozens of capillaries


Definition: “it is a failure of Perfusion”, which means a inadequate supply of blood to the required tissue, the reasons can be loos of blood and hence inadequate cardiac output is the outcome.
The different types of shock will be explained and their name is as follows: Hypovolemic shock, vasogenic or low-resistance shock, due to inadequate pumping of the heart (cardiogenic shock) or because of an obstruction in the lungs (obstructive shock).

Hypovolemic shock
Also known as cold shock, characteristics can be as follows, rapid, thready pulse; a cold, pale, clammy skin; intense thirst; rapid respiration and relentlessness or torpor. A hypovolemic shock can be subdivided into the following five categories:
  • Haemorrhage
  • Traumatic
  • Surgical
  • Burn
  • Fluid Loss


Usually happens when about 1 litre of blood is lost, and this causes the shock in the body. Decline in blood has direct effect on the blood pumped into the heart, as the heart rate increases and the blood pressure decreases as there is not sufficient blood to pump inside the body.
Pulse pressure will be reduced however mean arterial pressure may be the same. Skin is cool and pale and the patient has a immediate intense thirst.
As not enough Oxygen is supplied due to inadequacy of blood, anaerobic glycolysis produces large amounts of lactic acid and the blood lactate level will rise from 1 mmol/L to 9 mmol/L. The increase in lactic acid can have a serious outcome and might lead to coma.

Compensatory reactions

Activation of Renin – Angiotensin – Aldosterone system
There is a decrease in the blow to the kidney which causes them to secrete renin. Renin initiates the renin-angiotensin-aldosterone system. As angiotensin II causes vasoconstriction to increases blood pressure, and the release of aldosterone to increase absorption of NA+ and water by the kidneys. This causes a overall increase in blood pressure.

Secretion of Antidiuretic Hormones
ADH is secreted and this increases water absorption and hence conserves human blood. In addition it causes vasoconstriction and increases in systemic vascular pressure.

Activation of sympathic divisions of the ANS
Aortic and carotid baroreceptors initiate powerful sympathic responses throughout the body, as a result vasoconstriction occurs at all the veins and arteries except the heart or the brain ones. This increases the systemic vascular resistance in the arterioles and increases venous return in the veins. Sympathetic stimulation increases heart rate and contractility as well as secretion of heart rate and pressure increasing hormones such as epinephrine and norephrine through the adrenal medulla.

Release of Vasodilators
In response to inadequate oxygen cells release K+, H+, lactic acid, adenosine and nitric acid. These dilate the blood so local blood flow can increase, as wel as oxygenated blood, but it can have a decreasing effect on systemic vascular resistance.

If blood volume drops more than 10-20% or the blood pressure has decreased a lot it could cause death due to anaerobic respiration of the cells and increased production of lactic acid.

Long term

Circulating plasma is increased and plasma volume is restored in 12-72 hours. Performed albumins enter the plasma as well however most of the tissue is protein-free. The overall protein loss is replaced in the next 3-4 days. The red cells are restored in 4-8 weeks time, but a low level in red blood cells is still tolerable.

What is the first air treatment for arterial shock?

The patient should be inspected if he is bleeding or burns, then the bleeding should take priority and should be stopped as fast as possible. The casualty should be laid down and insulated against cold, and they should be reassured constantly. If blood loss is too much, then the legs should be raised so the blood is supplied to the vital organs improves, however special care should be taken if breaks are suspected.
Tight clothes should be loosened or removed completely especially if they around the chest or waist to reduce constriction in these areas. The casualty should be kept warm and the ambulance should be called. Important vital signs should be monitored like pulse, response and breathing. If patient becomes unconscious airways should be opened and the breathing should be checked and chest compressions should rescue breaths should be carried out if necessary.