The brain, heart and skeletal muscle exhibit autoregulatory control over their blood vessels (as opposed to blood vessels of organs under sympathetic nerve innervation). This control is based on the metabolism of surrounding tissues, and regulates blood flow locally; If there are factors such as carbon dioxide, lactic acid or certain ions, then the blood vessels will dilate. Conversely, excess blood flow will trigger vasoconstriction.
Regulation by the nervous system
Brain stem
The brain stem fires sympathetic nerve messages, and the sympathetic nerves release norepinephrine only. Sympathetic nerves also stimulate the adrenal medulla to secrete epinephrine and norepinephrine. The overall effects are raised blood pressure and increased heart rate in response to blood pressure changes.
| Receptor | Response to | Location | Effect |
| alpha-1 | epinephrine and norepinephrine | peripheral blood vessels | vasoconstriction |
| beta-1 | norepinephrine and sympathetic nerves | heart muscle | increase cardiac output |
| alpha-2 | norepinephrine | presynaptic terminals of CNS | negative feedback inhibiting norepinephrine release |
| beta-2 | epinephrine | blood vessels of the skeletal muscle and heart | vasodilation |
Hypothalamus
The hypothalamus releases ADH, which increases water reabsorption in kidneys and also constricts peripheral blood vessels.
How are changes in blood pressure detected?
- If blood pressure is too high, baroceptors in the carotid and aortic sinuses (vessels in the upper body) fire signals that inhibit sympathetic output.
- The hypothalamus responds to increased serum osmolality (mainly increases of sodium in the blood) by secreting more ADH.
- The brain stem responds to increased concentrations of blood CO2 and H+, which are associated with low blood pressure and poor blood flow, by stimulating sympathetic output.
Regulation by the kidney
Glomerular level
Water filtration at the glomerulus depends on the hydraulic pressure and osmotic pressure of blood. The higher the hydraulic pressure, the more easily blood will pass through filtration to be secreted. The higher the osmotic pressure, the more amount of water in the blood that can be filtered.
During filtration, plasma osmolality is higher in the vessels because blood is more concentrated with proteins that do not pass into the nephron. Water tends to be drawn back into the blood due to osmotic pressure (to reach equilibrium). Thus, one must consider osmotic pressure with hydraulic pressure to determine how much water is filtered.
ADH and angiotensin II (from the renin pathway) stimulate mesangial cells in the glomerular capsule to contract. This decreases surface area and membrane permeability and more water is retained.
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| Renal corpuscle by M.Komorniczak A: Renal corpuscle. B: Proximal tubule. C: Distal convoluted tubule. D: Juxtaglomerular apparatus. 5: Mesangium 6: Granular cells. 7: Macula densa. (Note the location in the nephron, upper left diagram). |
Juxtaglomerular apparatus and renin-angiotensin-aldosterone pathway
Three ways to stimulate renin secretion:
- Granular cells of the juxtaglomerular apparatus secrete renin in response to sympathetic stimulation.
- Granular cells secrete renin directly in response to low blood pressure.
- If renal tubular fluid is low in sodium, the macula densa stimulates granular cells to secrete renin. Low sodium in the renal tubules is typically the result low blood volume and slow fluid filtration, which allows too much sodium to be reabsorbed into the blood as filtrate moves too slowly.
Regulation by the heart
Atrial natriuretic factor (ANF)
Stretching of the atrial walls (by increased blood volume) releases ANF, which promotes sodium secretion and therefore loss of water. This results in lower blood volume and pressure.
Sterling's Law
Greater cardiac stretch or stroke volume = greater cardiac contraction, an automatic response to match heart input and output.
Autoregulation
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