Control of blood volume

Question 1

What feedback system is key in control of body fluid volume?

Answer 1

Control of body fluid volume by the kidneys
– Renal-body fluid feedback system
• When arterial pressure increases, urine production increases
• When arterial pressure decreases, urine production decreases

Question 2

What are the primary determinants in long term regulation of blood pressure?

Answer 2

–The renal output curve for salt and water
–The level of salt and water intake
Impossible to change long-term mean arterial blood pressure without changing one or both of these!

Question 3

What is the impact of arterial pressure on urinary volume?

Answer 3

As arterial pressure increases, so does urinary volume in a not quite linear manner

Question 4

Discuss the release of antidiuretic hormone

Answer 4

Released by the pituitary gland in response to
– ↑ osmotic pressure (increased extra cellular osmolarity/osmolality)
• Hypothalmic osmoreceptors

– Hypovolemia (10% loss or greater)
• Atrial baroreceptors normally inhibit ADH release
• ↓ volume leads to ↓ firing rate therefore ↑ ADH release

– Hypotension
• ↓ arterial baroreceptor firing
• ↑ sympathetic activity and ↑ ADH release

– Angiotensin II

Question 5

What is antidiuretic hormone also known as?

Answer 5

ADH, arginine vasopressin

Question 6

What does antidiuretic hormone (a.k.a. ADH, arginine vasopressin) do?

Answer 6

Increases blood volume by
– ↑ water permeability in renal collecting ducts
• therefore ↓ urine production

In severe hypovolemic shock
– ADH release is high
– Causes vasoconstriction
• ↑ total peripheral resistance

Question 7

What the fuck is RAAS

Answer 7

Not a frogsoc phrase apparently

-Renin-angiotensin- aldosterone system

Question 8

What is Renin and when is it released?

Answer 8

• Proteolytic enzyme released from the kidneys in response to:
– Sympathetic nerve activation
• Mediated by baroreceptor feedback

– Renal artery hypotension
• Independent of baroreceptor feedback

– Decreased sodium in kidney distal tubules

Question 9

How does RAAS impact blood volume?

Answer 9

Renin released from kidney juxtoglomerular cells acts on substrate to produce Angiotensin I which is converted to Angiotensin II in the lungs

Angiotensin II acts on resistance vessels
– ↑total peripheral resistance

Angiotensin II acts directly on the kidneys
– Constricts renal arteries therefore ↓ blood flow via kidneys

Angiotensin II causes release of aldosterone from the adrenal glands
– ↑ Na+ and water reabsorption

Angiotensin II stimulates release of ADH from pituitary which then increases blood volume and therefore arterial pressure

Question 10

What is Atrial-natriuretic hormone (a.k.a. Atrial-natriuretic peptide)?

Answer 10

28-amino acid peptide synthesised and stored in muscle cells of the atria
– Released in response to stretch of the atria
– Helps oppose the effects of the RAAS system
May help counteract volume overload

Question 11

Discuss excessive loss of blood volume

Answer 11

Hypovolaemia
– Loss of blood volume
• ↓ whole blood, e.g. hemorrhage 
• ↓ plasma, e.g. burns
• ↓ sodium, e.g. vomitting
– ↓↓ in blood pressure

Classification of shock
– Class 1, 10-15% blood loss
– Class 2, 15-30% blood loss
– Class 3, 30-40% blood loss
– Class 4, >40% blood loss

Question 12

What is the immediate (reflex) response to hypovolemia?

Answer 12

Baroreceptor reflex (change in HR in response to change in BP, so in this case heart rate increases)
– Degree of volume loss affects how successful

Question 13

What are some later response(s) to hypovolemia

Answer 13

Arteriolar constriction
– ↓ hydrostatic pressure in the capillaries
– Favours fluid reabsorption
– Temporary redistribution

Decreased renal blood flow
Baroreceptors plus thirst

Question 14

Discuss severe hypovolemia (and treatment)

Answer 14

If volume of fluid lost can’t be compensated for
– Damage to tissues and organs can occur – Heart fails

Fluid replacement required:
– Resuscitation fluids
• Colloid (gel/starch/albumin) or Hartmann’s
• Blood

– Fluid challenge algorithm
• Whilst monitoring central venous pressure

Question 15

What other factors affect blood pressure control?

Answer 15

Cortex
– Conscious effects of emotions
• Nerves from cortex to medullary CVC centre

Time of day
– Diurnal variations due to hormones and cortical input

Respiration
– Via mechanical movements – Via chemoreceptors
• Aortic and carotid bodies detect changes in pO2
• If ↓pO2 then rate of firing↑

Question 16

Summary points

Answer 16

Long term control of blood pressure is through control of blood volume
Blood volume and blood osmotic pressure are detected
Hormonal regulation of kidney function, combined with direct effects of flow, mediate blood volume

Question 17

Learning outcomes

Answer 17

To describe the hormonal mechanisms involved in long-term regulation of blood pressure (i.e. vasopressin, the renin-angiotensin-aldosterone system and atrial- natriuretic peptide) and how their involvement is regulated.
To predict the short-term, intermediate and long-term physiological changes in the cardiovascular system in response to increased/decreased plasma volume, or changes in plasma osmolarity.
To identify other systems that integrate with the cardiovascular system and can influence blood pressure.