#5893 - Vascular System And Foetal Circulation - Cardiorespiratory system

Vascular System and Foetal Circulation

Vascular system

1. The vascular tree

Blood is sent mostly ‘in parallel’ from the heart. This means each organ receives blood fresh from the heart and not from other tissues. However, a few organs are connected ‘in series’, and obtain their blood from the venous outflow of another organ. This is called a portal system. The main advantage of this system is it allows a solute to be transported from one place to another without dilution in the general circulation. For example, the hepatic portal vein in vertebrae carries blood from the gastrointestinal tract to the liver, which allows filtration of newly absorbed compounds.

The vascular tree is split into two parts – the pulmonary circulation and the systemic circulation.

In the systemic circulation, arteries extend from the aorta and transport blood to the individual organs. Each artery branches several times, increasing the number, whilst the diameter of the individual arteries gradually decreases. The smallest arteries are called arterioles. The pressure in arteries and arterioles is relatively high, and they contain around 15% of the blood volume.

Each of the arterioles branch into even smaller capillaries. Each capillary has a diameter similar to the size of a single erythrocyte. Exchange of nutrients and gases takes place across the capillary wall. The pressure in capillaries is intermediate, and they contain around 5% of the blood volume.

The capillaries empty into venules, which merge into larger veins, which then finally empty into the heart. The pressure in veins and venules is relatively low, and they contain around 80% of the blood volume.

Some organs receive an anastomosed blood supply. This is a collateral blood supply, which avoids the organ becoming necrotic if blood supply is blocked. Such systems are present in the brain (the circle of Willis) and the heart.

All blood vessels, except capillaries, are under neural and hormonal regulation.

2. Structure of blood vessels

The general structure of a blood vessel is as follows …

1. Arteries and arterioles

Arteries and arterioles contain relatively thicker walls, more smooth muscle and more elastin fibres than veins and venules.

The high level of elastin and collagen in the aorta dampens the oscillating cardiac output when the blood is pushed from the heart. This is achieved as the elastic properties of the aorta allow it to stretch to accommodate the sudden inrush of blood, decreasing the pressure a little.

Arterioles are have high amounts of smooth muscle in their walls. Contraction of the smooth muscle decreases the radius of the arterioles. This would result in a corresponding increase in blood pressure. In this way, contraction of the arterioles allows regulation of blood pressure. Resistance to blood flow is higher in arterioles than in arteries.

2. Capillaries

Capillaries do not contain any collagen or elastin fibres in their walls. Pressure in capillaries is relatively low. This is important as capillaries are very fragile, so high pressure would damage them. They are also very permeable, and high pressure would force a lot of fluid out through the capillary wall.

Individual capillaries have a very small radius, but there are a large number of capillaries. This results in a large surface area and small blood flow that allows efficient exchange. The slower flow of blood through capillaries allows time for diffusion of molecules.

There are three types of capillaries …

• Continuous – continuous lining of endothelial cells except for clefts between cells. This most common type present in the body.

• Fenestrated – fenestrations are points where the cell membrane is compressed to permit greater fluid transmission. An example is the glomerular capillaries in the kidney.

• Discontinuous/sinusoid – wider intercellular gaps permit increased exchange with surrounding tissues. This type of capillary is present in the liver, bone marrow, lymphoid tissues and some endocrine glands.

3. Veins and venules

Veins have a high amount of collagen fibres in their walls, but a low amount of elastin fibres. The collagen confers strength to the vessel.

The venous return depends on the pressure difference between venules and the right atrium. However, this is not the only method of venous return. Other methods of venous return include …

• Smooth muscle contraction in the tunic media

• Inspiration – this causes abdominal compression, and so compression of abdominal veins and venules.

• Existence of venous valves – this present backflow of blood

• Skeletal muscle contraction – this also results in compression of veins and venules.

• Gravitation – venous return from upper parts of the body.

3. Blood pressure

Blood pressure is the pressure exerted by the blood against the walls of the blood vessels. Blood pressure is therefore produced by cardiac output and stretching of blood vessels. Both systolic and diastolic pressure are greater than resting pressure (i.e. no cardiac activity).

The mean arterial pressure (MAP) is the result of the discharge of a volume of blood from the heart to the arterial system, which cannot all escape through to the venous system before the next beat occurs. There is a rise in MAP during systole, which is sustained due to the stretch of the arterial walls. Over time, the MAP would return to resting pressure, but before resting pressure is reached the heart contracts again. The MAP is therefore always maintained above resting pressure. The degree of arterioconstriction determines how quickly pressure decreases. If the arterioles are constricted, there is slower run off and pressure decreases more slowly.

The MAP can be calculated as …

[(2 x diastolic pressure) + systolic pressure]/3

Diastole is counted twice as much as 2/3 of the cardiac cycle is spent in diastole.

The usual range of MAP is 70-110mmHg. An MAP of 60mmHg is required to perfuse coronary arteries, brain and kidneys.

The MAP is dependent on cardiac output and the total peripheral resistance – this is the resistance to flow provided by the entire systemic circulation. An increase in sympathetic tone leads to arteriolar constriction, increasing MAP. The MAP is also dependent on where in the body it is measured.

MAP is also dependent on factors such as the viscosity of blood (the more viscous the blood, the higher the total peripheral resistance) and the elasticity of the blood vessels.

Pulse pressure is the difference between the systolic and diastolic pressures. A strong pulse does not therefore equal a high MAP and good tissue perfusion. However, this can usually be inferred from a strong pulse.

In response to a haemorrhage, the following effects would occur …

• Preload (volume of blood returning to the heart) – decreased.

• Stroke volume – decreased (as a result of decreased preload).

• MAP – decreased (as a result of decreased cardiac output).

• Viscosity – no change (PCV is the same, just a smaller volume of blood).

• Tissue oxygen delivery – decreased (as a result of above factors).

A decrease in blood pressure is noted by baroreceptors, which are stimulated by stretch. In the presence of a haemorrhage, there would be less stretching of arterial walls, so the baroreceptors would produce fewer action potentials and fewer nerve impulses. The response is therefore rapid and dynamic – the faster the change happens, the more it responds.

The autonomic nervous system would then respond by increasing sympathetic activity to increase heart rate and contractility, and decreasing parasympathetic activity (which slows heart rate and reduces contractility). The result is an increased stroke volume, so an increased cardiac output. This moves MAP back towards the normal level. Increased sympathetic tone also results in higher vascular resistance, helping to bring MAP back to normal.

Arterial blood pressure may be monitored using a peripheral pulse.

It may also be monitored directly by inserting a fluid filled catheter into an artery such as the carotid artery. The catheter is connected to a pressure transducer that converts the oscillations in arterial pressure into recordable electrical signals. This method is used in anaesthetised animals. It is a more accurate reading and gives a real time, beat by beat recording of arterial pressure. However, it is more costly and technically more difficult.

Arterial blood pressure may also be monitored using indirect methods. Doppler sphygomanometry uses a Doppler ultrasound probe and inflatable cuff to measure arterial pressure. The ultrasound probe is used to detect an audible pulse in a distal artery. The inflatable cuff is placed proximal to the flow detector. It is inflated to a pressure that exceeds systolic pressure, causing the blood flow to cease. Pressure within the cuff is then gradually decreased. The pressure at which flow is first heard to return is the systolic blood pressure.

Arteries of the hindlimb

The abdominal aorta terminates into the external and internal iliac arteries. It is these arteries that supply the hindlimb and pelvis.

The main arteries of the hindlimb are as follows …

• External iliac a. – terminal branch of aorta, continues as …

• Femoral a. lies in femoral triangle on medial side of thigh. Arterial pulse routinely taken in dog. Gives rise to saphenous...