Arteries, Veins & Capillaries

Arteries

• Transport blood away from the heart at high pressure.
• Arterial walls are thicker than venous walls, due to muscle tissue.
• Blood spurts at high pressure when an artery is cut.

Veins

• Returns blood at low pressure to the heart.
• Venous walls are thinner, having less muscle and elastic tissue.
• Some veins have valves that prevent backflow, particularly in leg veins.

Capillaries

• Capillaries connect arteries and veins in the tissues.
• They consist of a single layer of endothelial cells through which water and small molecules pass.
• They serve as the site of exchange of substances between blood and tissue fluid that bathes cells.

Fluid Dynamics

• Blood pressure encourages fluid to leave blood vessels, while osmotic pressure pulls fluid back in.
• Osmotic pressure is promoted by plasma proteins, mainly albumin.
• The lymphatic system removes excess fluid if it remains outside of the blood vessels, eventually returning it to the circulatory system.

Oedema

• Oedema is an obvious sign of extra fluid in pregnancy.
• Typically it occurs because there is extra fluid in the body, venous stasis occurs in the legs, and the blood has decreased concentration of plasma proteins.
• Oedema in the lower legs is not unusual in later pregnancy.
• Oedema can be a sign of pre-eclampsia in the fingers and face.

The Heart

• The A and P book of Ross and Wilson is relevant to learning about the anatomy and control of the heart
• There are two circulatory systems, pulmonary and systemic, within the heart
• Understanding blood flow through the heart is key.

Pulmonary Circulation

• Blood is pumped to the lungs to be oxygenated.
• De-oxygenated blood enters the right atrium via the superior and inferior vena cava in a continuous process.
• Blood passes from the right atrium to the right ventricle, aided by the contraction of the atrium's muscle.
• Ventricles contract, pushing blood into the pulmonary artery, which takes the blood to the lungs.
• In the lungs, carbon dioxide is removed from the blood for excretion, and oxygen attaches to haemoglobin.

Systemic Circulation

• Oxygenated blood returns continuously to the left atrium via the pulmonary artery.
• Blood then flows through the mitral valve to the left ventricle, which receives further aid from the contraction of the muscle walls.
• The ventricle contracts, moving blood into the aorta for circulation around the body.
• Both sides of the heart operate simultaneously.

Heart Valves

• Four valves prevent blood from flowing in the wrong direction: the tricuspid, pulmonary, mitral, and aortic valves.

Terminology

• Stroke volume is the amount of blood pumped from the ventricles with each contraction, affected by factors like blood pressure; it decreases if BP is high.
• Cardiac output is the amount of blood pumped from the ventricles per minute and is affected by stroke volume and heart rate.
• Increased or decreased heart rates lead to corresponding increases or decreases in cardiac output.
• Venous return is the blood returned to the heart, affecting cardiac output.
• Peripheral resistance is the pressure the heart pumps against into the blood vessels due to the pressure of the blood already in those vessels.
• The diameter of smaller arteries determines resistance.
• Vasoconstriction raises resistance, while vasodilation lowers it.

Blood pressure

• Blood pressure measures the force of blood on artery walls.
• Influencing factors include cardiac output (stroke volume x heart rate) and peripheral resistance.
• High blood pressure can damage blood vessels and bleeding if vessels rupture.
• Low blood pressure can lead to inadequate blood flow, depriving organs and tissues of oxygen and nutrients.

Controls of BP

• Peripheral resistance has elastic tissue in the artery walls so they expand when blood flows in and contracts to push the blood along.
• Baroreceptors in major arteries maintain BP when moving from sitting to standing.
• Hormones like adrenaline constrict blood vessels.
• Hypoxia and metabolic activity dilate vessels, increasing blood flow.
• Kidney regulation of body fluid impacts BP.

Recording Blood Pressure

• Blood pressure is recorded with two measurements: systolic and diastolic.
• Systolic is the measure of when the left ventricle contracts and pushes blood into the aorta and around the body.
• Diastolic is the measure of when the aortic valve closes, and the heart is resting.
• A normal BP during pregnancy is between 100/60 mmHg and 140/90 mmHg.

Changes Due to Pregnancy

• Maternal plasma volume increases during pregnancy to support the placental circulation and the fetus, increasing by 50% by term.

Changes to BP in Pregnancy

• An increased circulating blood volume would normally increase cardiac output and BP.
• However, progesterone counteracts this by relaxing smooth muscle in artery walls, reducing peripheral resistance.
• BP tends to drop slightly in the first trimester and rises gradually for the remainder of the pregnancy.

Other Cardiovascular System Changes

• As the uterus grows in pregnancy, so too does the diaphragm, moving the heart upwards and laterally
• The volume of the heart will also increase due to the increased blood flow.
• The pulse rate increases by 25-30%.

Supine Hypotension

• From 20 weeks gestation, when pregnant women lie flat, the inferior vena cava is compressed, reducing deoxygenated blood flow returning to the heart
• Compression of the vena cava can reduce venous return and cardiac output.
• Blood pressure can fall, reducing blood flow to major organs.
• If severe, compression can result in fainting and loss of consciousness.
• Supine hypotension has a negative effect on circulation to the placenta.

How to Relieve Supine Hypotension

• When examining a woman, ensure she is semi-recumbent, not flat on her back.
• If she is lying flat, turn her to the left side, or put a wedge under her left side, tilting the uterus away from the inferior vena cava.