Adrenaline may cause vasodilation

Blood pressure regulation

Table of Contents



Capillary system

The capillary system is the terminal flow path of the arterial vessels. There the exchange of substances takes place in both directions, from the capillaries to the environment and vice versa. The arterioles are upstream of the capillaries and the venules downstream. The lymphatic vessels also start blindly in the surrounding tissue.

Due to the large number of capillaries, despite their small diameter, 300m² are available for replacement processes. This area can increase to 1000m² with load.

The exchange is made possible by the construction of the capillaries, namely the single-layer endothelium with intercellular gaps and pores. Water, small water-soluble molecules and ions pass through the cracks into the environment practically unhindered, lipid-soluble substances get into the tissue transcellularly.

The pressure at the beginning of the capillary is about 30mmHg, then drops to about 22mmHg in the venules as it progresses. In contrast, the oncotic pressure mainly caused by albumin remains constant at around 25mmHg. This means that the pressure conditions are reversed in the course of the capillary.

The filtration or resorption rate is determined by the above properties and is calculated as follows:

Qf = Kf x Peff

Filtration rate = filtration coefficient x effective filtration pressure

The filtration coefficient is determined by the water permeability K and the exchange area f.

In one day, 20 liters of fluid reach the interstitium, of which 18 liters are reabsorbed into the venules and 2 liters enter the lymphatic system.

If vasodilation of the precapillary arterioles increases the pressure in the capillaries, this leads to increased filtration out of the vessels and thus to the formation of edema.

Local regulation of blood flow

The regulation of blood pressure at the level of individual organs can have the following causes:

Maintaining a constant blood pressure: For example, in the kidney with increased blood pressure, vasoconstriction of the renal arteries leads to a reduction in pressure in the downstream capillary systems.

Adaptation of blood pressure to metabolic requirements: In the case of increased stress, increased blood flow to the myocardium is necessary, which is ensured by vasodilation of the coronary arteries.

The pressure-regulating factors can be divided into vasoconstrictive and vasodilative factors.

Have vasoconstrictive effects:

Bayliss effect (myogenic tone): The smooth muscle cells of the vessels react to increased transmural pressure in arteries and arterioles and thus to a vasodilatation with a contraction and thus control the organ blood flow in the sense of auto-regulation. The exact mechanism of action is not known; it is suspected that voltage-dependent calcium channels open due to the stretching of the cells.

The Bayliss effect can be seen in most organs, but not in the skin and lungs.

Endotheline: They are the strongest known vasoconstrictive substances, of which endothelin-1 is to be emphasized because of its effect specifically on the cardiovascular system.

Contrary to their name, endothelins are not only formed in the endothelium but also in other cells.

Thromboxane: Thromboxane activates platelet aggregation, but causes smooth muscle cells to contract. In this way, blood loss should be minimized in the event of vascular injuries.

Have a vasodilatory effect:

NO (nitrogen monoxide): It is released by binding acetylcholine, ATP, or histamine to endothelial cells. NO then diffuses into the neighboring vascular muscle cells and causes them to dilate.

Increase in CO2Partial pressure

Increase in K+-Concentration in the interstitium

Reduction of the O2Partial pressure causes dilation throughout the body with the exception of the lungs.

Adenosine leads to dilation in the heart, skeletal muscles and brain.

Systemic blood pressure regulation: neural control

The nervous control of the blood pressure takes place via the sympathetic nervous system, which acts on the arteries and arterioles with a vasoconstriction. It also leads to increased heart activity.

The sympathetic activity is controlled by baroreceptors in the aorta, the carotid artery, the vena cava, the atria and the left ventricle. These receptors reach the elongated spinal cord via cranial nerves, where they inhibit or activate the sympathetic nervous system.

With decreased sympathetic activity, blood pressure drops.

When activated, the parasympathetic nervous system leads to vasodilation in the gastrointestinal tract and genitals.

Systemic blood pressure regulation: humoral control

Catecholamines (adrenaline and noradrenaline): They lead to vasoconstriction in all vessels. They bind to α1- and α2- Smooth muscle cell adrenergic receptors.

An exception to this is the binding of adrenaline to β-adrenoreceptors. This leads to vasodilation at low concentrations of adrenaline. Vasoconstriction only occurs after the β-receptors have been blocked, i.e. at high adrenaline concentrations. There are a particularly large number of β-receptors in the skeletal muscles, the myocardium and the liver. This effect does not occur with noradrenaline because of its higher affinity for α-receptors.

Renin-Angiotensin System: If the blood pressure drops below 90mmHg in the kidney, renin is released. In the liver, this leads to a breakdown of angiotensinogen to angiotensin I and finally to angiotensin II. Antiotensin II leads to a release of aldosterone in the adrenal cortex, increases thirst, increases the appetite for salt and also leads to general vasoconstriction. The result is that blood pressure rises and further renin release is blocked.

Atrial Natriuretic Peptide ANP: The volume stress of the atria causes a stretch and muscle cells in the atria release ANP. Increased plasma levels of ANP lead to a dilatation of peripheral veins and thus a shift of blood from central to peripheral. In the kidney, the vasodilation lowers the concentrations of the osmotically active substances and lowers the diuresis.



Vegetative Physiology - Understanding the Basics

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✔ Physiology of breathing, digestion and kidney

✔ Physiology of the blood and heart

✔ Energy and heat balance

✔ Circulatory regulation

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Human Physiology, 31st edition; Schmidt, Lang, Heckmann; Pages 590-604

Pocket Atlas of Physiology, 6th edition; Silbernagl, Despopolous; Pages 188, 189, 208-217

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