Lecture 7.2: The Peripheral Circulation Flashcards
What percentage of blood is found in the Heart?
10%
What percentage of blood is found in Systemic Arteries?
10%
What percentage of blood is found in Capillaries?
5%
What percentage of blood is found in Large Systemic Veins?
20%
What percentage of blood is found in Small Veins and Venules?
43%
What percentage of blood is found in Pulmonary Circulation?
12%
What are the 3 layers of arteries and veins walls?
- Tunica Intima
- Tunica Media
- Tunica Adventitia
What are the 3 different types of arteries?
- Elastic Artery
- Muscular Artery
- Arteriole
Types of Arteries: Elastic Artery Structure
- High proportion of elastic fibres
- Internal elastic lamina separates
tunic intima from tunica media - Tunica media contains significant
amounts of elastin - Elastin allows expansion and recoil
- Also known as conducting arteries
- Generally>10mm diameter
Types of Arteries: Muscular Artery Structure
- High proportion of smooth muscle
(connected via gap junctions) - Innervated by sympathetic ANS
- Around 0.1 to 10.0 mm diameter
- Also called a distributing artery
- Blood supply to the artery itself via
vasa vasorum
Types of Arteries: Arteriole Structure
- Arterioles are arteries that lead to a
capillary bed - Only a few layers of smooth muscle
- Resistance vessels
- About 30μm in diameter
- Display tone (degree of contraction
of the smooth muscle) - A number of factors determine the
degree of openness
What is a Metarteriole?
A short vessel linking arterioles and capillaries
What is a Pre-Capillary Sphincter?
The junction between a metarteriole and a capillary is called a pre-capillary sphincter and is characterised by smooth muscle
encircling it
What is Vasomotor Tone?
The end result of a complex set of interactions that control relaxation and contraction of blood vessels
Capillary Structure
- Consist of a single layer of
endothelium and basement
membrane - Typically, less than 1 mm long
What are the 3 Types of Capillaries?
- Continuous
- Fenestrated
- Sinusoid
Types of Capillaries: Continuous
- Most common
- Tight junctions between endothelial
cells - However, tight junctions are
incomplete (intercellular clefts)
allowing for movement of material
between blood and tissues - Contribute to BBB (blood-brain-
barrier)
Types of Capillaries: Fenestrated
- Has pores
- Permeable to larger molecules
- Found in small intestine and in the
choroid plexus as well as elsewhere
Types of Capillaries: Sinusoid
- Least common
- Flattened and incomplete
basement membrane - Intercellular gaps
- Facilitate movement of large
molecules and cells between
blood and interstitium eg bone
marrow
What types of molecules pass through the endothelium? What is this route called?
- Gases
- Small Lipophilic Molecules
- Transcellular Route
What types of molecules pass through pores (intercellular junctions, fenestrae)? What is this route called?
- Water
- Glucose
- Ions
- Paracellular Route
What is the role of the Lymphatic System?
Maintains fluid levels in our body tissues by removing all fluids that leak out of our blood vessels, returns excess fluid back to the circulation
What is interstitial fluid that enters lymph capillaries known as…?
Afferent Lymph
What is the fluid called after passing through lymph nodes?
Efferent Lymph
Where is efferent lymph returned to the circulation?
- Via the thoracic duct and the right
lymphatic duct - These enter the subclavian veins
What is Diapedesis?
The passage of blood cells through the intact walls of the capillaries, typically accompanying inflammation
Central Venous Pressure (CVP)
- Veins act as a variable reservoir of
blood - This means they can ‘add to’ larger
veins - This increases CVP, stroke volume
and arterial pressure - Volume sitting in the venous side of
the circulation depends on venous
blood pressure and the degree of
tone in tunica media
Smooth muscle in arterioles displays a tonic contraction (tone): what happens when there is an increase in tone?
Vasoconstriction
Smooth muscle in arterioles displays a tonic contraction (tone): what happens when there is a decrease in tone?
Vasodilation
Vasomotor tone principally produced by activity of the…?
- The sympathetic ANS (α=receptor
mediated) - NB Sympathetic cholinergic
vasodilators in sweat gland (M3
mediated) - NB Parasympathetic vasodilators eg
salivary glands, erectile tissue (M2
mediated) - BUT, also local control – a whole host
of vasoactive metabolites (local
chemicals for local vessels)
Reactive Hyperaemia
Increase in blood flow following arterial occlusion
Net fluid movement is driven by pressure gradient:
- Capillary pressure (outward Pc)
- Interstitial pressure (inward Pi)
- Oncotic pressure of plasma proteins
(inward πp) - Oncotic pressure of interstitium
(outward πi) - Pressure = (Pc-Pi) – (πp-πi)
- ~+28mmHg at the arterial end,
~+5mmHg at venous end - In total about 4-8 L exits the capillaries
– how is balance maintained?
