Pressures and flow in systemic circulation

Question 1

Define systolic blood pressure

Answer 1

The pressure being exerted against the arterial wall during ventricular systole

Question 2

Define diastolic blood pressure

Answer 2

The pressure being exerted against the arterial wall during ventricular diastole

Question 3

How do you calculate mean arterial pressure (MAP)?

Answer 3

MAP = DBP + 2/3 SBP
MAP = 1/3 (SBP - DBP) + DBP

Question 4

How do you calculate pulse pressure (PP)?

Answer 4

PP = SBP - DBP

Question 5

What are the two types of baroreceptors?

Answer 5

1) High pressure - arterial

2) Low pressure - venous and right side of heart

Question 6

Where are the most important arterial baroreceptors located?

Answer 6

In the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch

Question 7

What is the mechanism of action of arterial baroreceptors?

Answer 7

1) Constantly fire with arterial wall stretch during systole
2) Vital role to compensate for sudden reduction in blood pressure
3) Decreased blood pressure causes decreased firing rate
4) Reflex via medulla acts to increase blood pressure

Question 8

What is the mechanism of action of low pressure/venous baroreceptors?

Answer 8

• Predominantly in atria, ventricles and pulmonary arteries
• Mainly responds to change in blood volume
• Reduction in blood volume increases release of antidiuretic hormone from the posterior pituitary gland and stimulates catecholamine production resulting in increased water and sodium retention

Question 9

What do the chemoreceptors detect?

Answer 9

• Stimulated by hypoxia, hypercarbia, pH change
• Primarily effect the respiratory system (ventilation) but can effect the cardiovascular system
• leads to increased sympathetic outflow to the heart and peripheral vasculature via medullary centres causing increased cardiac output and blood pressure

Question 10

How does pulse pressure change throughout the vessels?

Answer 10

Due to increased vessel wall rigidity, the pulse pressure increases as one moves peripherally

Question 11

How can vasoconstriction in arterioles affect resistance?

Answer 11

• Because of the fourth power flow reduction (Poiseuille equation) arterioles can influence blood flow with relatively minor radius change.
• Each vessel running in parallel controls its own flow but vasoconstriction in one will increase the combined resistance

Question 12

How can veins’ radius be altered?

Answer 12

• Increased sympathetic tone can sufficiently venoconstrict to maintain circulatory volume with 25% blood loss.
• Reduced tone and venodilation can accommodate rapid infusion of 500mls of blood

Question 13

What is the importance of lymphatics?

Answer 13

• About 10% of interstitial fluid drainage can be returned from tissues to circulation
• Enter by one way valves between endothelial cells
• Major route of nutrient transport from bowel
• Flow both passive and peristaltic, aided by skeletal muscle contraction. Valves ensure one way flow
• Bacteria removed when traversing lymph nodes

Question 14

What are the objectives of local perfusion?

Answer 14

1) Delivery of oxygen to the tissues.
2) Delivery of nutrients such as glucose, amino acids, and fatty acids.
3) Removal of carbon dioxide from the tissues.
4) Removal of hydrogen ions from the tissues.
5) Maintenance of proper concentrations of ions in the tissues.
6) Transport of various hormones and other regulatory substances to the different tissues.

Question 15

What are the two theories for acute local blood flow regulation?

Answer 15

1) Vasodilator substance theory

2) Oxygen demand theory

Question 16

What role does nitric oxide play?

Answer 16

• Nitric Oxide (NO) is released by endothelial cells in response to the sheer stress of extra blood flow. This results in activation of cGMP-dependent protein kinase which causes vasodilation by several mechanisms
• Sheer stress on small tissue arterioles causes release of NO in larger upstream larger arterioles to ensure adequate perfusion to match tissue metabolic demand.

Question 17

What role does endothelin play?

Answer 17

Endothelin is a potent vasoconstrictor that is released from damaged endothelium and acts locally to prevent excessive blood loss from traumatised blood vessels.

Question 18

How is perfusion increased chronically?

Answer 18

• The acute control mechanisms usually increase blood flow to a level which falls short of meeting the increased tissue metabolic demands.
• Lack of oxygen or other nutrients in a tissues causes release of small peptides termed angiogenic growth factors which stimulate increased tissue vascularity to a level determined by maximum tissue requirement.
• These changes occur over hours to weeks, complete the compensation process and are especially important if tissue metabolic demands are more than short term.
• If vessels are obstructed collaterals develop over days to weeks.

Question 19

Describe the sympathetic and parasympathetic innervation of vessels

Answer 19

• Noradrenergic (sympathetic) nerve endings on all vessels
  Vasoconstrictors - constant tone
• Cholinergic (parasympathetic) fibres travel with sympathetic nerves
  Vasodilators - no constant tone

Question 20

What are the actions of angiotensin II?

Answer 20

1) Potent vasoconstrictor at arteriolar level
2) Increases total peripheral resistance
3) Constricts renal afferent and efferent arterioles reducing renal blood flow and increasing sodium and water retention
4) Increases thirst and water intake
5) Stimulates ADH secretion
6) Directly inhibits Renin secretion : -ve feedback loop

Question 21

What are the actions of aldosterone?

Answer 21

1) Controls reabsorption of sodium in renal cortical collecting duct
2) Induces production of proteins in collecting duct including membrane channels for Na and K
3) Increases Na reabsorption from gut, sweat and salivary glands

Question 22

What are the actions of antidiuretic hormone (ADH, vasopressin)?

Answer 22

Synthesised in hypothalamus then secreted from posterior pituitary in response to reduction in osmoreceptors afferent stimulus

1) Increases the water permeability of the collecting duct luminal membrane by Inserting protein channels (aquaporins) for water reabsorption
2) A vasoconstrictor
3) Can reduce renal blood flow and GFR

Question 23

What are the actions of histamine and other ions?

Answer 23

Produced within Mast cells and released in response to allergic tissue inflammation.
1) Potent vasodilator, increases permeability.
These reactions can be sufficiently severe as to cause marked oedema.
2) Calcium > vasoconstriction due to action on smooth muscle
3) Magnesium causes marked vasodilation.
4) Hydrogen ions and CO2 cause vasodilation.

Question 24

What does bradykinin do?

Answer 24

Bradykinin causes arteriolar vasodilation and increases vascular permeability.
Important mediator in inflamed tissues

Question 25

What does atrial natriuretic hormone do?

Answer 25

Stretch receptors in cardiac atrial cells
Stimulation causes release of ANP
1) Causes increased renal excretion of sodium and water
2) Also increases GFR by dilating afferent and constricting efferent renal arterioles - increasing filtration pressure
3) Inhibits renin secretion and aldosterone release

Question 26

What are the cardiovascular changes in sepsis?

Answer 26

1) Hyperdynamic circulation
2) Local factor vasodilatation
3) Relative hypovolaemia
4) Increased tissue oxygen demand
5) Decreased cardiovascular supply

Question 27

What is the sepsis 6?

Answer 27

1) Administer high flow oxygen
2) Administer broad spectrum antibiotics
3) Administer intravenous fluid challenges
4) Obtain blood cultures
5) Obtain blood to measure serum lactate
6) Obtain measurements of hourly urine output