Renal blood flow

Introduction to blood circulation in the kidneys
The blood flow through the kidneys is 20-25% of the cardiac output, at rest this value reaches about 1.2 l / min. Of this, the flow through the cortex is about 90% (5 ml / min per gram of tissue) and the flow towards the inner cortex is reduced to 0.5 ml / min. If the blood flow to the marrow were higher, ions would wash out and osmotic marrow stratification would not be possible. High flow rates are necessary to maintain the glomerular filtration rate (value around 120 ml / min) and at the same time ensure the supply of oxygen and nutrients to cover the high energy requirements of the tissue (especially the cortex). Consumed O 2(about 18 ml / min) is used mainly for aerobic metabolism of the renal cortex, where ATP consumption is high mainly for the needs of active transport processes. The renal marrow, on the other hand, uses energy obtained mainly by the anaerobic route.

The vascular architecture in the kidneys shows a portal arrangement. The blood is carried sequentially by two consecutive capillary systems before being drained into the veins. The first is formed by a capillary network of glomeruli. It is classified between two arterioles, and therefore we observe an unusually high blood pressure (about 55 mm Hg) necessary for glomerular filtration. The second capillary system consists of peritubular capillary braids of the cortex and medulla, in which the pressure, on the other hand, is very low (15 mm Hg), which aids in reabsorption.

Renal arteries
''Aa. renales are branches of the abdominal aorta, they withdraw in most cases just below the beginning of the superior mesenteric artery''. The right renal artery is longer, passing behind the inferior vena cava and the right renal vein. The left renal artery is shorter, often located slightly lower, and runs behind the left renal vein. A. renalis accessoria is present in 30% of cases. Near the renal hilum, each of the renal arteries divides into posterior and anterior compartments, from which ''aa. segmentales'' supplying the vascular segments of the renal parenchyma.

Segmental arteries
''Aa. segmentales'' supply the vascular renal segments. According to today's terminology, we distinguish five segments on each kidney supplied by five segmental arteries. In contrast to arterial supply, intrarenal veins do not have a segmental arrangement and often anastomosis with each other. Knowledge of vascular segmentation of the renal parenchyma is necessary when performing partial nephrectomy (most often to excise the tumor).

Lobed arteries, interlobs, arcuates, interlobulas
These are muscle-type arteries. ''Aa. lobares'' are branches of segmental arteries, originally supplying the developmental lobes of the kidneys. It is then divided into two to three ''aa. interlobares, which rise through the crust between the pyramids and at the interface between the crust and the marrow, dichotomously divide in aa. arcuatae'' receding at right angles. During their course, they issue branches - ''aa. interlobulares ( aa. corticales radiatae ), which advance radially to the superficial layers of the cortex, leaving the arteriolae afferentes'' as lateral branches.

Afferent glomerular arterioles (vas afferentia)
They supply blood to the capillary nodules in the renal bodies. They show a typical vascular wall structure with 2-3 layers of smooth muscle cells. Towards the capillary nodules, the wall thins to a thickness of about 30-50 μm, at this level the endothelium begins to show modifications typical of the cells of the juxtaglomerular apparatus. These play an important role in tubuloglomerular feedback and regulation of glomerular filtration.

Regulation of renal blood flow
Like other organs, blood flow in the kidneys has its own regulatory mechanisms, which can be divided into local and central. They are used especially in changes in blood pressure, thus in maintaining constant glomerular filtration. In general, vasoconstriction reduces glomerular filtration, while vasodilation increases it.

Local regulation
Under physiological circumstances, local regulation plays a major role in maintaining a constant flow through the kidneys.


 * Myogenic autoregulation - maintains a constant blood flow. With increased blood pressure, the tension on the capillary wall increases. As a result of baroreceptor stimulation, vasoconstriction vas afferens occurs . The blood flow through the vessel is thus normalized. In the opposite situation, reduced blood pressure causes vasodilation with a consequent increase in flow;


 * Tubuloglomerular feedback - macula densa cells produce signaling molecules to which the smooth muscle in vas afferens responds and, thanks to their receptors, are able to detect the amount of flow in the tubular fluid. The feedback mechanism consists in the detection of reduced flow through macula densa cells (decrease in glomerular filtration) and in the production of signaling substances that have a paracrine vasodilatory effect on vas afferens receptors . When increasing glomerular filtration, the principle is the opposite, the secreted substance is a molecule with a vasoconstrictive effect.

Local regulatory mechanisms can maintain constant glomerular filtration even with changes in blood pressure from 80 to 180 mm Hg.

Central regulation
This type of regulation is used especially in pathological conditions (ex. loss of a larger volume of fluids). When arterial pressure changes, local regulatory mechanisms fail and central regulatory mechanisms with nervous and hormonal action apply.


 * The sympathetic nervous system mainly acts on vasa afferens and, by means of its α 1 -adrenergic receptors, causes constriction after binding of adrenaline, noradrenaline ;


 * Angiotensin II has receptors located in both vasa afferens and vasa efferens . It is involved in the stimulation of catecholamines from the adrenal medulla and sympathetic endings. It acts contractively on mesangial cells and reduces glomerular filtration;


 * Prostaglandins are not used in physiological situations. In pathological conditions, they are produced locally in the kidneys and their vasodilating effect on the vas afferens and vas efferens seeks to prevent ischemia, which is threatened by the action of catecholamines and angiotensin II, whose increased activity is a stimulus for prostaglandin flushing.

Other substances that potentiate blood flow in the glomeruli include: atrial natriuretic peptide, glucocorticoids, nitric oxide, kinins. Other vasoconstrictors are, for example:  antidiuretic hormone, ATP, endothelin.