Kidney Control of Blood Pressure - Anatomy & Physiology

Control of Blood Pressure

If a sudden change in blood pressure occurs it is controlled in the short term by the sympathetic nervous system which alters three things:

• Total peripheral resistance
• Capacitance
• Cardiac output

It is only in the long term in response to chronic changes in blood pressure that the kidney works to alter the balance between fluid intake and output in order to regulate blood pressure. It is this response which will be covered here.

Renal Regulation

• Increased pressure has a direct effect on the kidney

 Q = (PA - PE) ÷ R
 Q = Flow, PA = Pressure in afferent arteriole, PE = Pressure in efferent arteriole, R = Resistance

Three mechanisms of Renal Regulation

Pressure Diuresis

• As arteriolar blood pressure increases, so flow through the kidneys also increases - see above formula
• This increases filtration rate
• This increases urinary output

Pressure Natriuresis

• If renal perfusion pressure is increased then sodium excretion increases
• I.e. sodium excretion increases when blood pressure increases
• If more sodium is excreted less water is reabsorbed therefore the ECF volume decreases and blood pressure decreases.
• The actual mechanism is not clear but it is thought to involve a direct effect of the pressure on the renal interstitium.

Renin-Angiotensin-Aldosterone System

• Specialized cells in the distal tubule called the macula densa sense the concentration of sodium and chloride.
• If blood pressure falls there is a reduction in concentration of sodium and chloride in the distal tubule which is sensed by the macula densa.
• The macula densa releases prostaglandins which act on the juxtaglomerular apparatus which releases renin into the bloodstream.
• The drop in blood pressure is also detected by baroreceptors in the aortic arch, carotid sinus and the afferent renal arteriole which stimulates renin release by the juxtaglomerular apparatus.
• Renin cleaves angiotensinogen into angiotensin 1 which in turn is cleaved by Angiotensin Converting Enzyme (ACE) into angiotensin 2.
• Angiotensin 2 is a potent vasoconstrictor and also stimulates the adrenal cortex to release aldosterone.
• Aldosterone acts on the distal tubules and collecting ducts in the kidney causing retention of sodium and water.
• Blood pressure increases.

Regulation of Renal Blood Flow

It is essential that renal blood flow is maintained to ensure that adequate filtration of toxins from the blood takes place. Changes in pressure affect renal blood flow. Important auto-regulatory processes are responsible for this and are covered in detail here.

The Role of Salt

Increasing the salt intake of an animal increases blood pressure in the short term. It increases the osmolarity of the blood which therefore increases water movement from tissues to the blood causing an increased circulating volume. As a result of this increased osmolarity more ADH is released as the osmoreceptors in the hypothalamus are triggered. This results in increased water retention in the kidneys further increasing the circulating volume. Secondary to the increase in salt the thirst centre is stimulated to increase fluid intake to try and counter act the increased osmolarity. This would increase blood volume and therefore pressure temporarily until this was corrected by the compensatory mechanisms detailed above.

Kidney Control of Blood Pressure - Anatomy & Physiology Learning Resources
Flashcards Test your knowledge using flashcard type questions	Renal Blood Pressure Flashcards

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