Pressures and flow in systemic circulation Flashcards
Define systolic blood pressure
The pressure being exerted against the arterial wall during ventricular systole
Define diastolic blood pressure
The pressure being exerted against the arterial wall during ventricular diastole
How do you calculate mean arterial pressure (MAP)?
MAP = DBP + 2/3 SBP MAP = 1/3 (SBP - DBP) + DBP
How do you calculate pulse pressure (PP)?
PP = SBP - DBP
What are the two types of baroreceptors?
1) High pressure - arterial
2) Low pressure - venous and right side of heart
Where are the most important arterial baroreceptors located?
In the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch
What is the mechanism of action of arterial baroreceptors?
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
What is the mechanism of action of low pressure/venous baroreceptors?
- 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
What do the chemoreceptors detect?
- 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
How does pulse pressure change throughout the vessels?
Due to increased vessel wall rigidity, the pulse pressure increases as one moves peripherally
How can vasoconstriction in arterioles affect resistance?
- 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
How can veins’ radius be altered?
- 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
What is the importance of lymphatics?
- 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
What are the objectives of local perfusion?
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.
What are the two theories for acute local blood flow regulation?
1) Vasodilator substance theory
2) Oxygen demand theory
What role does nitric oxide play?
- 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.
What role does endothelin play?
Endothelin is a potent vasoconstrictor that is released from damaged endothelium and acts locally to prevent excessive blood loss from traumatised blood vessels.
How is perfusion increased chronically?
- 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.
Describe the sympathetic and parasympathetic innervation of vessels
- Noradrenergic (sympathetic) nerve endings on all vessels
Vasoconstrictors - constant tone - Cholinergic (parasympathetic) fibres travel with sympathetic nerves
Vasodilators - no constant tone
What are the actions of angiotensin II?
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
What are the actions of aldosterone?
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
What are the actions of antidiuretic hormone (ADH, vasopressin)?
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
What are the actions of histamine and other ions?
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.
What does bradykinin do?
Bradykinin causes arteriolar vasodilation and increases vascular permeability.
Important mediator in inflamed tissues
What does atrial natriuretic hormone do?
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
What are the cardiovascular changes in sepsis?
1) Hyperdynamic circulation
2) Local factor vasodilatation
3) Relative hypovolaemia
4) Increased tissue oxygen demand
5) Decreased cardiovascular supply
What is the sepsis 6?
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
