Chapter 19 blood vessels

Blood vessels

• Are a closed delivery system (Begins and ends at the heart)

• There are three main types

  1. Arteries 

  2. Capillaries

  3. Veins

The three main types of vessels 

1. Arteries 

2. Capillaries

3. Veins 

Arteries 

• Carry blood away from the heart 

• Are said to branch, diverge or fork 

• Always carry oxygenated blood 

Veins 

• Carry blood toward the heart 

• Said to join, merge or converge 

• Always carry deoxygenated blood 

2 Exceptions to arteries and veins 

1. In the pulmonary circulation (arteries carry oxygen-poor blood to the lungs and veins carry oxygen-rich blood to the heart 

2. In special umbilical vessels of a fetus, the roles of veins and arteries differ 

Blood vessel structure

• Most have three layers 

  1. Tunica intima 

  2. Tunica media 

  3. Tunica externa 

• Exception is capillaries 

  • Just have a basement membrane and endothelial cells 

Types of arteries 

• Elastic arteries 

• Muscular arteries 

• Arterioles 

Elastic arteries 

• Thick walled 

• Near the heart 

• Are large diameter, therefore have low resistance 

• Conduct blood from the heart to smaller vessels 

• They act as pressure reservoirs 

  • When ventricles contract the elastic arteries expand

  • When contraction stops, the elastic constricts back, keeping blood flowing

  • If blood vessels harden (atherosclerosis) then blood flows less continuously

  • Also without pressure reducing ability of hardened arteries, higher pressure is felt in the periphery

Muscular arteries

• Found distal to elastic arteries

• Deliver blood to specific body organs (distributing arteries)

• The majority of arteries in the body are muscular

• As the name suggests they have the thickest Tunica Media of all vessels

  • Because of this they are active in vasoconstriction and less capable of stretching

Arterioles

• Smallest of the arteries

• Have three layers, but Tunica Media is only one layer of smooth muscle and a few scattered elastic fibres

• They control the minute-to-minute blood flow into the capillary bed

  • Are called resistance vessels because when they constrict the tissues they serve receive very little blood. When they are dilated the tissues they serve receive a is rich blood supply

  • Diameter of these vessels determined by neural, hormonal and local chemical influences

Capillaries 

• Are the smallest blood vessels

• Walls consist of just Tunica intima and a basement membrane

• Average length is 1 mm and the diameter is just large enough for red blood cells to slip through in single file

• Located next to the tissue they are supplying

• Their very thin walls and location near most tissues of the body make them very well-suited for the exchange of materials between blood and interstitial fluid

Types of capillaries 

• Continuous 

• Fenestrated 

• Sinusoid 

Types of capillaries: Continuous 

• Most abundant

Types of capillaries: Fenestrated 

• Occur in areas of active filtration (kidneys) or absorption (small intestine) 

Types of capillaries: Sinusoid 

• Most permeable and only in the liver, spleen, bone marrow and Adrenal medulla

Capillary beds

• Formed by an interweaving network of capillaries 

• Blood flow from arteriole to venule is called microcirculation 

• Most arterioles branch into 10-20 capillaries that form the capillary bed 

• Blood flow through the capillary bed is controlled by the diameter of the arteriole that feeds the capillary bed 

• Carry blood from the capillary bed toward the heart 

• Along the route the diameter of the vessels increases (ending in the vena cava)

• Veins 

Venules

• Range in size from 8-100um

• The smallest ones are the postcapillary venules - just endothelium with pericytes around them 

• Larger venules have tunica media (one or two layers of smooth muscle) and a thin tunica externa 

Veins characteristics

• Usually have 3 distinct layers in their walls 

• Walls are always thinner and lumens larger than corresponding arteries 

• Relatively little smooth muscle and elastic tissue, tunica externa is the most well-developed 

• Thin walls and large lumen mean that can accommodate a fairly large blood volume 

• Are called capacitance vessels and blood reservoirs because they can hold up to 65% of the body's blood supply at any time 

• Even tho walls are much thinner than arteries they are not in danger of bursting because their pressures are much lower 

Why doesn't backflow occur 

• Because veins have valves

Venous valves

• Resemble semilunar valves of the heart 

• Prevent backflow of blood 

• Are most abundant in veins of the libs 

Vascular anastomoses 

• Take an organ like the liver. It does not just receive blood from one artery 

• Instead arterial anastomoses form that provide alternate paths (collateral channels) that enable blood to reach the given body region 

• Are common in the heart, abdominal organs, joints and brain

Venous anastomoses 

• Are much more common - as a result, blockages in veins rarely result in tissue damage because blood flow can generally continue

Blood flow 

• The volume of blood  flowing through a vessel and organ, or the entire circulation in a given period (ml/min)

Blood pressure

• Force per unit area exerted on a vessel wall by the contained blood

• Is expressed in millimetres of mercury (mmHg)

• Unless stated otherwise it refers to the systemic arterial pressure

• Blood always flows from areas of high pressure to areas of low pressure ( referred to as hydrostatic pressure gradient)

Resistance

• The opposition to blood flow. 

• It is a measure of the friction blood encounters as it passes through the vessels. 

• Usually referred to as total peripheral resistance

Blood viscosity 

• Resistance to flow of liquids, which is related to the thickness of the liquid

• Blood is more viscous than water because of its formed elements and plasma

• Blood viscosity is fairly constant

• Anything that impacts viscosity will impact resistance to blood flow

Total vessel length 

• The longer the blood vessel the greater the resistance

Blood vessel diameter 

• The smaller the diameter the greater the resistance

  • Larger arteries, close to the heart contribute little to total peripheral resistance

  • Small diameter arterioles ( that constrict and dilate) are major determinants of total peripheral resistance

How are flow, pressure and resistance related 

• Blood flow = Difference in pressure between 2 points / Total peripheral resistance 

• Total peripheral resistance is much more influential and can more readily be changed

• Even a small change in peripheral resistance can result in large change in blood flow to tissues 

Blood pressure 

• As we have seen the pumping of the heart creates blood flow

• Pressure is created as this blood meets resistance in the periphery 

• Remember that blood will always flow from high to low pressure 

• The nearer the blood is to the pump (the heart) the greater the resistance will be 

Arterial blood pressure 

• This is where the highest pressure exists. When we talk about (or take) blood pressure, this is the pressure we are referring to

Arterial BP reflects to things:

• How much the arteries close to the heart can stretch when blood is ejected into them (compliance and dispensability)

• The volume of blood forced into them at any given time 

Mean arterial pressure (MAP)

• Is the pressure that propels the blood to the tissues 

• It is used clinically as an important indicator oh how effectively the heart is sending blood to tissues 

• It is determined by systolic and diastolic pressure 

• This is what you feels as your pulse pressure 

• It is increased temporarily by increased stroke volume and faster ejection of blood from the heart 

• It is increased chronically by atherosclerosis (which will raise systolic and decrease diastolic pressure)

Vital signs

• Used to monitor patient status 

  • Blood pressure 

  • Heart rate 

  • Respiratory rate 

  • Body temperature 

Pulse 

• You can stake pulse in any artery that lies close to the surface 

• Also caused pressure points because pressure here will stop distal blood flow (important in control of bleeding)

Pressure in capillaries 

• Capillary pressure is low (helps to protect more fragile vessels)

• Because capillaries are so permeable, fluids can still be forced out of the bloodstream into tissues, despite this low-pressure 

Venous return 

• The venous pressure is too low to ensure blood gets returned to the heart efficiently

• Three adaptations that ensure venous return occurs are:

  • The muscular pump

  • The respiratory pump

  • Sympathetic vasoconstriction 

Three adaptations that ensure venous return occurs are

• The muscular pump

• The respiratory pump

• Sympathetic vasoconstriction 

Blood pressure regulation 

• Involves 

  • Cardiac output 

  • Total peripheral resistance 

  • Blood volume 

Short term BP regulation 

• Minute-to-minute BP is regulated through both neural and hormonal mechanisms 

• Neural control can impact both cardiac output and also total peripheral resistance 

• The two main goals of the nervous system when controlling TPR are:

  1. Maintain adequate MAP by altering blood vessel diameter 

  2. Alter blood distribution to respond to specific demands 

The two main goals of the nervous system when controlling TPR are

1. Maintain adequate MAP by altering blood vessel diameter 

2. Alter blood distribution to respond to specific demands 

Baroreceptors and neural control of BP

• Baroreceptors are mechanoreceptors that respond to changes in arterial pressure and stretch 

• They are located in the carotid sinus, aortic arch and walls of large arteries in the neck and thorax 

Baroreceptor reflex 

• Very effective against short-term changes in blood pressure 

• They are relatively ineffective at responding to sustained changes in blood pressure (this is the reason why hypertension becomes chronic)

• Baroreceptors adapt to a new ‘higher’ normal pressure 

• When the baroreceptor reflex no longer functions normally will develop orthostatic hypotension 

Chemoreceptors and control of BP

• There are chemoreceptors in the aortic arch and large arteries of the neck that detect carbon dioxide, oxygen and pH levels in the blood 

• They transmit impulses to the cardiovascular control center which can in turn change heart rate and vessel diameter as required 

• As they are more important in the regulation of respiratory function they are discussed in detail in Chapter 22

Short-term hormonal controls of BP

• Hormones regulate BP in the short term via changes in peripheral resistance (or long term via changes in blood volume but this is discussed later)

• Adrenal medulla hormones:

  • Epinephrine and norepinephrine

• Angiotensin II  

• Antidiuretic hormone (ADH)

• Atrial natriuretic peptide

Adrenal medulla hormones

• Epinephrine and norepinephrine from the adrenal gland increase CO and vasoconstriction (increase BP)

Angiotensin II 

• Stimulates vasoconstriction 

Antidiuretic hormone (ADH)

• High levels can cause vasoconstriction 

Atrial natriuretic peptide

• Decreases BP by causing generalized vasodilation 

Long-term BP regulation 

• Blood volume is a main determinant of cardiac output. Anything that alters blood volume will alter blood pressure 

  • Higher blood volume - Higher BP

  • Lower blood volume - Lower BP

Direct Renal control 

• Increased BP or blood volume causes the elimination of more urine, thus reducing blood volume and BP

• Decreased BP or blood volume causes kidneys to conserve water, and blood volume and BP rises 

Indirect renal control 

• The kidneys also regulate blood pressure indirectly via the Renin-angiotensin-aldosterone mechanism 

  • This leads to the release of renin from the kidneys 

  • Renin ultimately leads to the creation of angiotensin II

  • Angiotensin II does 4 things to help increase BP:

    1. Stimulates aldosterone secretion 

    2. Causes ADH release from the posterior pituitary 

    3. Triggers hypothalamic thirst center to drink more water 

    4. Act as a potent vasoconstrictor, directly increasing blood pressure 

Angiotensin II does 4 things to help increase BP

1. Stimulates aldosterone secretion 

2. Causes ADH release from posterior pituitary 

3. Triggers hypothalamic thirst center to drink more water 

4. Act as a potent vasoconstrictor, directly increasing blood pressure 

Summary of blood pressure regulation

• The goal of blood pressure regulation is to keep blood pressure high enough to provide adequate tissue perfusion, but not so high that blood vessels are damaged 

  • Example: If BP to the brain is too low, perfusion is inadequate, and the the person loses consciousness 

  • If BP to the brain is too high, person could have a stroke 

Homeostatic imbalance in BP

• Hypertension 

  • Chronically elevated BP (systolic > 140mmHg or diastolic > 90mmHg

  • 30% of people over 50 have hypertension 

  • It is known as the ‘Silent killer’ because there are basically no symptoms, yet prolonged high BP increases the risk  for heart failure, renal failure, stroke and vascular disease 

Hypertension 

• Chronically elevated BP (systolic > 140mmHg or diastolic > 90mmHg

• 30% of people over 50 have hypertension 

• It is known as the ‘Silent killer’ because there are basically no symptoms, yet prolonged high BP increases risk  for heart failure

Hypertension types

• Primary 

• Secondary 

Primary hypertension 

• 90% of cases 

• No direct cause, multiple factors:

  • Hereditary 

  • Diet 

  • Obesity 

  • Age 

  • Diabetes

  • Stress

  • Smoking 

• Treatment involves addressing risk factors and anti-hypertensive drugs 

Secondary hypertension 

• 10% of cases 

• Hypertension is due to an underlying condition 

  • Obstructed renal arteries 

  • Kidney disease 

  • Endocrine disorders 

• Treatment focuses on correcting problems that led to hypertension 

Circulatory shock 

• Blood flow is inadequate to meet tissue needs 

• 3 types 

  1. Hypovolemic shock 

  2. Vascular shock 

  3. Cardio genic shock (pump failure)

Hypovolemic shock 

• The most common form of circulatory shock 

• Results from large-scale blood or fluid loss

• Heart rate will increase to correct the problem 

• Intense vasoconstriction will also occur to shunt blood to the heart and brain 

  • Treatment is to replace fluid volume as soon as possible 

Vascular shock 

• Blood volume is normal but circulation is poor because of vasodilation (results in a large drop in total peripheral resistance)

• Can happen due to:

  • Anaphylactic shock 

  • Neurogenic shock 

  • Septic shock 

• Treatment is to address underlying cause of vasodilation in an effort to improve circulation and give fluids to support circulation 

Cardio genic shock (pump failure)

• Occurs when heart is so inefficient it cannot sustain adequate circulation 

• Usually caused by damage to the myocardium, as would happen following a myocardial infarction the

Arteries and veins naming

• In most of the body arteries and veins travel together and are named similarly

  • Subclavian artery and subclavian vein 

• In many cases the name of the vessel tells you where it is or the organ it supplies 

  • Subclavian artery 

  • Thoracic aorta 

  • Femoral artery 

  • Posterior tibial artery 

  • Renal artery 

Some differences between systemic arteries and veins 

• While arteries are always deep, some veins are just below the skin (limbs, neck, and face)

• Venous pathways are very interconnected - this makes venous pathways harder to follow and more confusing to name 

• The brain and digestive system have unique venous drainage

  • Brain has dural venous sinus rather than typical veins 

  • Digestive system has a hepatic portal system The digestive

Systemic circulation 

• Aorta 

• Vena cava 

• Abdominal aorta 

• Thoracic aorta 

• Common carotid 

• Subclavian 

• Femoral artery 

Major arteries 

• Aorta 

• Abdominal aorta 

• Thoracic aorta 

• Common carotid 

• Subclavian 

• Femoral artery