6.2 The blood system

Specialised structure of arteries

Narrow lumen to maintain a high blood pressure
Thick wall to prevent rupturing
Arterial wall contains an inner layer of muscle and elastic fibres to help maintain pulse flow

What do arteries branch into?

Arteries -\> arterioles -\> capillaries

Specialised features of capillaries

Small diameter
Capillary wall is made of a single layer of cells
Contain pores

Specialised structure veins

Wide lumen, thin wall, low pressure

What is the contraction of the heart called?

Myogenic - the signal for cardiac compression arises within the heart tissue itself

name for pacemaker of the heart

Sinoatrial node

Order of pacemakers

1. SA node (70-160 bpm)
2. AV node (40-60 bpm)
3. bundle branches (20-40 bpm)

nerve signalling

The pacemaker is under autonomic (involuntary) control from the brain, specifically the medulla oblongata (brain stem)

Two nerves connected to the medulla regulate heart rate by either speeding it up or slowing it down.

Hormonal signalling

Hormones are chemical messengers released into the bloodstream that act specifically on distant target sites (like the heart)

Heart rate can undergo a sustained increase in response to hormonal signalling in order to prepare for vigorous physical activity

The hormone adrenaline (a.k.a. epinephrine) is released from the adrenal glands (located above the kidneys)

Adrenaline increases heart rate by activating the same chemical pathways as the nerves

Function muscle fibres

Form a rigid arterial wall that is capable of withstanding the high blood pressure

function elastic fibres

Allow the arterial wall to stretch and expand upon the flow of a pulse through the lumen

flow of blood in capillary

Slow, low pressure

flow of blood veins

low pressure

What creates movement of valves in veins

Skeletal muscles

systole

contraction of the heart muscle

Blood returning to the heart will flow into atria and ventricles as there is a lower pressure.

When the atria is almost full it will contract.

As ventricles contract, ventricular pressure exceeds atrial pressure and AV valves close

When both valves are close. Pressure builds up

When ventricular pressure exceeds blood pressure the blood flushes into the aorta

Diastole

Relaxation of the heart

As blood exits the ventricle and travels down the aorta, ventricular pressure falls

When ventricular pressure
When ventricular pressure < atrial pressure the AV valve opens

Blood composition

NACHO-UH
Nutrients
Antibodies
Carbon dioxide
Hormones
Oxygen
Urea
Heat

Causes of coronary occlusion

Coronary heart disease is the hardening and narrowing of the arteries due to the deposition of cholesterol

Atheromas (fatty deposits) develop in the arteries and significantly reduce the diameter of the lumen (stenosis)

The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall (from shear stress)

The damaged region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall

As the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques

If the plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow

If the thrombus is dislodged it becomes an embolus and can cause a blockage in a smaller arteriole

Consequences of Coronary Occlusion

coronary heart disease can lead to blood clots which cause coronary heart disease when they occur in coronary arteries

Myocardial tissue requires the oxygen and nutrients transported via the coronary arteries in order to function

If a coronary artery becomes completely blocked, an acute myocardial infarction (heart attack) will result

Blockages of coronary arteries are typically treated by by-pass surgery or creating a stent (e.g. balloon angioplasty)