Module 1 Lecture 4

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the blood pressure gradient keeps blood flowing from high to low

- the volume of blood and the compliance of vessel, Fluctuates throughout the systemic circulation & during the cardiac cycle

– Systolic pressure is the max pressure in arteries when blood is pumped into them

Diastolic pressure is the min pressure in arteries when there is no heart contraction & blood continues to flow into arterioles

Pulse pressure is the difference between systolic & diastolic pressures. It is this pressure difference we can feel when taking our pulse

Mean arterial pressure is the average pressure during each cardiac cycle. This is the pressure that the body monitors & regulates

MAP = diastolic pressure + 1/3 (systolic – diastolic) MAP is the average pressure during each cardiac cycle

The main purpose of MAP is to drive blood into tissues

Blood flow to any given organ depends on: 1. resistance of local arterioles 2. vascularisation/open capillaries MAP is the driving pressure head

MAP must be closely monitored & regulated to ensure: 1. high enough for sufficient drive, despite local adjustments 2. low enough to avoid damage to heart & blood vessels

Regulation occurs through the integration of elaborate mechanisms, involving various aspects of the body. The two determinants of MAP are: MAP = CO x TPR

A change in any factor will change MAP unless another factor changes to compensate for it

Short-term: autonomic nervous system to heart, veins, & arterioles as well as hormones -> CO & TPR - Long-term: kidney & thirst responses to regulate total blood volume -> CO

Short-term: autonomic nervous system to heart, veins, & arterioles as well as hormones à CO & TPR - Long-term: kidney & thirst responses to regulate total blood volume à CO

-Baroreceptors fire action potentials in response to pressure in these arteries – Impulses are delivered via afferent neurons to the cardiovascular control centre in medulla of brain stem, where information is integrated – Sympathetic & parasympathetic outflow via efferent neurons are adjusted – Heart & blood vessels change their status to restore MAP

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Left atrial volume receptors and Hypothalamic osmorecepters

Left atrial volume receptors: detect a profound fall in blood volume/pressure. Stimulates hypothalamus to release ADH from posterior pituitary -> targets kidneys to reabsorb water (↑ blood volume) & arterioles to cause vasoconstriction (↑ TPR)

Hypothalamic osmoreceptors: detect a rise is ECF concentration (e.g. dehydration) à stimulates thirst responses (↑ fluid intake, ↑ blood volume)

Marked changes occur in anticipation & at onset of exercise: • CO ↑ ~3-5-fold with moderate to heavy exercise • CO is re-distributed compared to rest: - Skeletal muscle, cardiac muscle & skin receive a ↑ proportion - Other organs receive ↓ blood flow, but brain is maintained • TPR reduced due to widespread vasodilation in skeletal muscle • MAP modestly increased

Hypertension > 140/90 mmHg • Two broad types: - Secondary: known cause & accounts for 10% of cases, e.g. kidney disease - Primary/Essential: unknown cause, accounts for 90% of cases, e.g. genetic, exacerbated by environmental factors • Vicious cycle: damages vessel walls, atherosclerosis - increased TPR - further elevates BP

Baroreceptors do not return blood pressure back to normal levels à new set-point? • Increased workload on heart as it is now pumping against an increased pressure Complications: - Left ventricular hypertrophy -> systolic heart failure - Stroke (rupture of cerebral vessels) - Myocardial infarction (rupture of coronary vessels) - Kidney failure - Vision loss