Blood pressure

Factors maintaing blood pressure Central factors Cardiac out put Heart Rate Peripheral factors Peripheral resistance Blood volume venous return Elasticity of blood vessels Diameter of blood vessel Viscosity of blood

Cardiac out put To Systolic blood pressure depend upon CO&stroke volume –directly proportional Dystolic BP is directly Peripheral resistance when PR decreased dystolic pressure is decreased BP is directly proportion to blood volume BP is directly pro to venous return BP is inversly proport to elasticity of vessels& diameter of vessels

Control of blood pressure Outline Short term control (baroreceptors) Location Types of baroreceptor Baroreceptor reflex Other stretch receptors Long-term control Renin/ angiotensin/ aldosterone system Vasopressin Atrial natiuretic peptide Response to blood loss (shock)

Regulation of blood pressure Short term Baroreceptors Long term Kidney via renin angiotensin system

. Baro-receptors Nerve endings in all large thoracic and neck arteries 2 major populations: Carotid sinus and Arch of the aorta Activation on stretch Carotid: Hering’s nerve to Glossopharyngeal nerve to tractus solitarius in brainstem Aortic: Vagus nerve to tractus solitarius If baro-receptors sense increased BP Secondary signals from tractus solitarius: Inhibition of vasoconstrictor centre and excitation of vagal parasympathetic centre

Baro-receptors Important in maintaining postural blood pressure (When standing from lying down  strong sympathetic discharge) Long term changes in blood pressure result in resetting of baroreflexes (i.e. not influential)

Location of baroreceptors Baroreceptors sense stretch and rate of stretch by generating action potentials (voltage spikes) Located in highly distensible regions of the circulation to maximise sensitivity

Overview of short-term control mechanisms

. Anatomy of the autonomic nervous control of blood pressure: Sympathetic vasomotor nerve fibres leave cord through (T+L) spinal nerves Sympathetic chain  1)Sympathetic nerves (viscera) 2)Spinal nerves (vasculature) Vagus nerve (PNS) .

. Vasomotor centre: Vasoconstrictor area origin of excitatory pre-ganglionic vasoconstrictor neurones Vasodilator area internal inhibition of vasoconstrictor area Sensory area input from vagus and glossopharyngeal nerves modulate vasoconstrictor/dilator area activity

. To raise the arterial pressure: SNS release NA from nerve terminals NA acts on the α adrenergic receptors of the VSMC All arterioles constricted Veins strongly constricted Heart directly stimulated

….It’s all about intracellular calcium VSMC Stimulates formation of IP3 IP3 causes release of calcium from sarcoplasmic reticulum in the heart and increasing inotropy Cardiovascular Physiology Concepts, Richard E Klabunde. www.cvphysiology.com 13

Beta-adrenoceptors Heart: Increased contractility Blood vessel: reduced contractility Usually sympathetic stimulation of VSMC leads to vasoconstriction. The alpha vasoconstrictor effects are far dominant over the beta vasodilator effects, BUT if given an alpha blocker then the beta function will dominate G-proteins are linked to an enzyme, adenylyl cyclase, that dephosphorylates ATP to form cyclic AMP (cAMP). Gs-protein (stimulatory G-protein) activation (e.g., via β-adrenoceptors) increases cAMP. This activates PK-A (cAMP stimulated protein kinase) and causes increased influx of Ca++ by phosphorylation and activation of L-type calcium channels, and enhanced release of Ca++ by the sarcoplasmic reticulum in the heart. These and other intracellular events increase inotropy, chronotropy, dromotropy and lusitropy. Cardiovascular Physiology Concepts, Richard E Klabunde. www.cvphysiology.com 14

Long term control of blood pressure Involves control of blood volume/sodium balance by the kidneys Hormonal control Renin-angiotensin-aldosterone system Antidiuretic hormone (vasopressin) Atrial natiuretic peptide Pressure natriuresis

Macula densa is the collective name for a specialised group of epithelial cells in the initial portion of the distal tubules Juxtaglomerular cells (granular cells) produce Renin Mesangial cell: provide anchor, may be involved in response to hypotension as contain actin + myosin and may act under sympathetic stimulation 16

Afferent arteriole Resistance Glomerular Hydrostatic pressure GFR Arterial Pressure Renin Macula Densa NaCL Na Cl resorption Flow rate in loop of Henle Angiotensin II Efferent Arteriole Resistance Glomerular Filtration Pressure Afferent arteriole Resistance

Vasopressin

Atrial natiuretic peptide Increases salt excretion via kidneys By reducing water reabsorption in the collecting ducts relaxes renal arterioles inhibits sodium reabsorption in the distal tubule Released in response to stimulation of atrial receptors

Atrial volume reflex Reduced secretion of anti-diuretic hormone from hypothalamus Atrial stretch due to pressure Reflex dilation of renal afferent arteriole Reduction in water resorbtion from renal tubule Fluid loss by kidneys Increased glomerular capillary pressure Increased filtration of fluid into renal tubule 21

Summary of long term BP control Cardiac output and BP depend on renal control of extra- cellular fluid volume via: Pressure natriuresis, (increased renal filtration) Changes in: Vasopressin Aldosterone Atrial natiuretic peptide