BSI: Circulation

• Arteries: Transport of oxygenated blood
• Arterioles: Act as valves for entry into capillaries
• Capillaries: Gas and nutrient exchange
• Venules: Collects blood from capillary
• Veins: Carry deoxygenated blood back to heart

Pulmonary Veins

The capillaries are only one endothelial layer of cells thick. Therefore, they are thin enough to allow gas exchange to occur.

A tunica, in laymen's terms, is a covering or a layer. Each tunica (or layer) in an vessel has different components to it.

• Tunica interna (intima)
• - Contains lining of simple squamous epithelium (endothelium), basement membrane, and layer of elastic tissue (internal elastic lamina)
• - The endothelium lines the entire cardiovascular system
• - Normally, endothelium is the only tissue that contacts blood

• Tunica media
• - Thickest layer
• - Middle layer
• - Consists of elastic fibers and smooth muscle that extend circularly around the lumen

• Tunica externa
• - Outermost layer
• - Composed mainly of elastic and collagen fibers
• - In muscular arteries, an external elastic lamina composed of elastic tissue separates the tunica externa form the tunica media

Endothelium, which is found in tunica interna of the arteries.

Endothelium, which is found in Tunica interna of the arteries.

• Elastic arteries
• - Arteries with largest diameter
• - Tunica media contains large amounts of elastic fibers
• - Help propel blood onward (by recoiling) while ventricles are relaxing

• Muscular arteries
• - More smooth muscle in tunica media (which is innervated by SNS)
• - Fewer elastic fibers
• - Capable of greater vasoconstriction and vasodilation to adjust blood flow

Muscular arteries have smooth muscle in the tunica media layer which is innervated by the SNS. This allows vasoconstriction and vasodilation.

Arterioles carry oxygenated blood and empties into capillaries.

Metarterioles are the region of arterioles that have control over blood flow into capillary bed.

Capillaries are the site of gas and fluid exchange with the tissues.

Venules receive deoxygenated blood from capillaries.

Veins consist of the same three tunics as arteries.

• Thinner tunica interna & tunica media
• - Less elastic tissue and smooth muscle than arteries

• Thicker tunica externa
• 

Compliance, in laymen's terms, is the ability to be able to stretch, and then recoil back to its original shape.

Veins much more compliant than arteries.

Veins are ~24x more compliant compared to an artery

• Veins can hold more blood: they act as a reservoir
• Amount of blood that a vessel can hold for each mm Hg pressure rise (Ask Naylor what this sentence means)

As you decrease compliance, that can increase Pulse Pressure.

Compliance can decrease as you age because vessels will get stiffer.

Veins contain valves to prevent backflow of blood.

The purpose of the cardiovascular system is to maintain blood flow to allow for the delivery of oxygen and nutrients to the tissues.

• CO = MAP / TPR
• Cardiac Output = (Mean Arterial Pressure) / (Total Peripheral Resistance)
• These factors are intimately related: you can't change one without affecting the others.

• MAP = CO x TPR
• Mean Arterial Pressure = (Cardiac Output) x (Total Peripheral Resistance)
• These factors are intimately related: you can't change one without affecting the others.

MAP = Mean Arterial Pressure

Blood pressure is the force exerted on the walls of the blood vessels by the blood.

Blood flows due to a pressure gradient.

Yes! Pressure gradients not only differ depending on the vessel type, but the pressure gradients also differ depending on whether you are talking about the systemic circulation versus the pulmonary (lung) ciruculation.

• Arteries = 80-120 mmHg
• - Diastolic = 80
• - Systolic = 120
• - Mean (MAP) = DBP + 1/3(SBP-DBP) = 80 + 1/3(120-80) = 93 mmHg

Capillaries = 17 mmHg (mean)

Veins and right atrium = 0 mmHg

Arteries rang in their pressure from 80-120 mm Hg.

• In a normal healthy individual, the different arterial pressures are as follows:
• - Diastolic = 80 mm Hg
• - Systolic = 120 mm Hg

• Therefore, the MAP can be calculated as follows:
• Mean (MAP) = DBP + 1/3(SBP-DBP)
• MAP = 80 + 1/3(120-80)
• MAP = 93 mmHg

Note that the pulmonary circulation is a lower pressure system than the systemic circualation.

• - Pulmonary artery = 8-25 mm Hg
• - Capillaries = 7 mm Hg

Pulse Pressure is simply the difference between systolic and diastolic blood pressure.

• PP = SBP – DBP
• Pulse Pressure = Systolic Blood Pressure - Diastolic Blood Pressure

• In a normal healthy individual, the pulse pressure could be calculated as follows:
• PP = SBP - DBP
• PP = 120 - 80
• PP = 40 mm Hg

• Stroke volume
• - Greater stroke volume results in greater pressure rise with each heart beat

• Compliance
• - Lower compliance of artery results in a greater pressure rise with every heart beat

Pulsations are dampened further down the vascular tree—arterioles and capillaries do not experience significant pulsations.

TPR is the resistance to blood flow through all of the ARTERIAL vasculature in the body.

TPR refers to ARTERIAL vasculature only...not venous vasculature.

TPR is also referred to as SVR (systemic vascular resistance).

Most of the resistance is in the arterial vasculature compared to the venous vasculature since the venous vasculature is so compliant.

• 1) vessel diameter
• 2) venous pump
• 3) respiratory pump
• 4) Renin-angiotensin-aldosterone system (RAAS)
• 5) ADH: Anti-diuretic hormone (vasopressin)
• 6) ANP: Atrial natriuretic peptide
• 7) Autonomic nervous system (SNS & PNS)

TPR increases as vessels vasoconstrict (as vessel diameter decreases)

TPR decreases as vessels vasodilate (as vessel diameter increases)

• The vessel diameter can be found by the following formula:
• Resistance = 1/diameter⁴

The vessel diameter can be found by the following formula:

Resistance = 1/diameter⁴

Vasodilation will increase vessel diameter and therefore decrease Total Peripheral Resistance (TPR).

Vasoconstriction will decrease vessel diameter and therefore increase Total Peripheral Resistance (TPR).

The venous pump is the rhythmic contraction of skeletal muscle which helps to push blood forward towards the heart.

Note that valves prevent backflow during relaxation.

Skeletal muscle

During inspiration, abdominal pressure increases and intrathoracic pressure decreases.

This change in pressure leads to increased venous return due to pressure gradient.

Activation of RAAS increases blood pressure.

• Renin (enzyme)
• Angiotensin II (hormone)
• Aldosterone (hormone)

Two things will stimulate Renin and therefore the RAAS system:

• 1) decreased stretching of blood vessels (specifically arterioles)
• 2) activation of the SNS (which makes sense because the SNS will be activated when BP drops)

Renin, which is released from the kidney, cleaves Angiotensinogen to Angiotensin I.

Renin is released from the kidney.

Angiotensin Converting Enzyme (ACE) cleaves Angiotensin I to Angiotensin II.

ACE comes from the lungs.

• Angiotensin II will effectively have several consequences to increase arterial pressure:
• - increase further stimulation of the SNS
• - increase contractility
• - directly affects the kidney to increase renal retention of salt and water
• - promote vasoconstriction
• - allow for release of Aldosterone which is released from the cortex in the adrenal glands

• 

Aldosterone (like Angiotesin II) directly affects the kidneys for renal retention of salt and water for the purpose of increasing overall blood pressure.

Aldosterone is released from the cortex of the adrenal glands.

• 1) Two things stimulate the release of Renin from the kidneys:
• - decreased stretching of blood vessels (specifically arterioles)
• - activation of the SNS (due to decreased BP)

2) Renin will cleave Angiotensinogen into Angiotensin I.

3) Angiotensin I will be converted by the Angiotensin Converting Enzyme (ACE) found in the lung to Angiotensin II.

• 4) Angiotensin II will then effectively have several consequences to increase arterial pressure:
• - increase further stimulation of the SNS
• - increase contractility
• - directly affects the kidney to increase renal retention of salt and water
• - promote vasoconstriction
• - allow for release of Aldosterone which is released from the cortex in the adrenal glands

5) Aldosterone also directly affects the kidney for renal retention of salt and water.

6) Angiotensin II can be inactivated by the Angiotensinase enzyme.

• AT1: brain, kidney, myocardium, peripheral vasculature, adrenal glands
• AT2: adrenal medullary tissue, uterus, brain (does not influence blood pressure)

AT2 does not influence blood pressure.

Angiotensin II causes the kidneys to retain salt & water.

Angiotensin II causes the kidneys to retain salt & water in two ways:

• 1) Acts directly on kidneys to cause salt and water retention.
• 2) Angiotensin II causes adrenal glands to secrete aldosterone. The aldosterone in turn increases salt and water reabsorption by kidneys.

• Angiotensin II causes vasoconstriction
• Angiotensin II increases contractility of the heart
• Angiotensin II stimulates the SNS

Antidiuretic Hormone (also known as vasopressin)

ADH is released from the posterior pituitary gland in response to decreased blood volume and/or changes in osmolarity.

Recall ADH is released in response to decreased blood volume or changes in osmolarity for the purpose of promoting water retention.

Therefore, an increase in osmolarity (higher salt content, less fluid) would increase ADH because the blood would be more concentrated.

For example, think of it in terms of exercising. When you exercise, you sweat and therefore you lose overall body fluid. Therefore, there will be less blood volume which makes what blood you have more concentrated (ie an increase in osmolarity). Therefore, ADH would be stimulated to act on kidneys to promote the retention of water.

ADH is released from the posterior pituitary gland.

ADH will act directly on the kidneys to promote the retention of water.

• ADH increases blood pressure by:
• - Causing vasoconstriction
• - Acting on kidneys to promote retention of water

Atrial natriuretic peptide

ANP is released by cells of atria in response to stretch.

• ANP decreases blood pressure by:
• - Causing vasodilation
• - Acting on kidneys to promote loss of salt and water in urine

• ADH: increases overall blood pressure
• ANP : decreases overall blood pressure

• SNS: Symapthetic Nervous System
• ANS: Autonomic Nervous System

The SNS plays a role in controlling circulation.

• The SNS innervates the heart and all vessels except:
• - capillaries
• - precapillary sphincters
• - metarterioles

• No. The SNS innervates the heart and all vessels except:
• - capillaries
• - precapillary sphincters
• - metarterioles

The SNS increases heart rate and contractility.

• The ANS plays very small role in controlling blood flow and circulation since majority of blood vessels are not innervated by PNS.

The PNS innervates the heart.

The PNS decreases heart rate and decreases contractility slightly.

The vasomotor center regulates the ANS.

• Baroreceptors
• Chemoreceptors
• Atrial and Pulmonary Stretch Receptors

Baroreceptors are stretch receptors located in walls of large arteries (carotid artery and aortic arch).

Baroreceptors function optimally in the range of 60-180 mm Hg.

If arterial pressure increases, stretching of baroreceptors causes vasodilation and decreases heart rate and contractility.

If arterial pressure decreases, stretching of baroreceptors is reduced and therefore causes vasoconstriction and increases heart rate and contractility.

Baroreceptors reflexes are important for rapid changes in BP and reducing variation in BP throughout the day.

Baroreceptor reflexes are not important in long-term regulation because they reset in 1-2 days.

• Chemoreceptors are found in carotid bodies (a small "body" of tissue rich in capillaries) and aortic bodies and sense:
• 1) a decrease in O₂ content
• 2) an increase in CO₂ and H⁺ content.

Chemoreceptors are similar to baroreceptors. However, chemoreceptors work at a much lower pressure than baroreceptors. Also, baroreceptors can increase or decrease BP, whereas chemoreceptors only function to increase BP.

Chemoreceptors will regulate breathing, ventilation and respiration, but some information is also sent to increase blood pressure when a decrease in blood pressure is detected.

Chemoreceptors function at lower blood pressures (<80 mmHg)

If blood pressure decreases, blood flow decreases through chemoreceptors. They become stimulated and excite the vasomotor center resulting in increased blood pressure.

Atria and pulmonary artery contain stretch receptors that are low pressure receptors optimal for detecting changes in pressure in low pressure areas.

Low pressure receptors

Atria and pulmonary artery stretch receptors are important in minimizing changes in pressure in response to volume changes.

Example: if infuse 300 ml of blood into a dog, the blood pressure only rises ~15 mm Hg. However, if denervate baroreceptors, blood pressure rises 40 mm Hg. If denervate baroreceptors and low pressure receptors, blood pressure rises 100 mm Hg.

Baroreceptors

With all of the reflexes active, the body is sensitive to pressure changes in a full range of pressures.