RENAL CIRCULATION-, counter current mechanism, counter current multiplier, counter current exchanger,

Renal circulation, vasarecta, macula dens, renal blood flow, glomerular filtrqtion rate, abdominal aorta, segmental artery, Intra lobar artery, counter current mechanism, counter current multiplier, counter current exchange, vasa recta, hyper osmolarity

1. D r . E . M u r a l i n a t h , D r . P . M a n j a r i , D r . C . K a l y a n a n d D r . K . S . P r a g n a RENAL CIRCULATION

2. INTRODUCTION Blood vessels of kidneys are highly specialized to favour the functions of nephrons regarding the formation of urine. In the adults, during resting conditions both the kidneys receive 1,300 mL of blood per minute or about 26% of the cardiac output. Maximum blood supply to kidneys exhibits the functional significance. Renal arteries supply blood to the kidneys.

3. RENAL BLOOD VESSELS Renal Artery Renal artery arises directly from abdominal aorta and gain an entry into the kidney through the hilus. While passing through renal sinus, the renal artery categorizes into many segmental arteries. Segmental Artery Segmental artery subdivides into interlobar arteries . Interlobar Artery Interlobar artery passes in between the medullary pyramids. At the base of the pyramid, it turns and runs parallel particularly to the base of pyramid leads to the formation of arcuate artery. Peritubular capillaries and vasa recta drain into the venoous system Venous system begins with peritubular arcuate veins , interlobar veins, segmental veins and finally the renal vein. Renal vein leaves the kidney through the hilus through the hilus and joins inferior vena cava.

4. MEASUREMENT OF RENAL BLOOD FLOW Blood flow to kidneys is measured by using plasma clearance of para-aminohippuric acid .

5. REGULATION OF RENAL BLOOD FLOW Renal blood flow is controlled primarily by aut oregulation.

6. Glomerular Capillaries Each large capillary categorizes into small glomerular capillaries, which form the loop s a nd the capillary loops lead to the formation of the efferent arteriole, which leaves the Bowman capsule. Efferent Arteriole Efferent arterioles form a second capillary network termed as peritubular capillaries, which surround the tubular portions of the nephrons. Thus, the renal circulation plays an important role in portal l system because of the presence of two sets of capillaries such as glomerular capillaries and peritubular capillaries.

7. Peritubular Capillaries and Vasa Recta Peritubular capillaries are observed around the tubular portion of cortical nephrons only. The tubular portion of juxtamedullary nephrons is supplied by some specialized capillaries termed as vasa recta. These capillaries are straight blood vessels hence the name vasa recta. Vasa recta arise in a direct manner from the efferent arteriole of the juxtamedullary nephrons and run parallel to the renal tubule into the medulla and ascend up towards the cortex .

8. AUTOREGULATION Autoregulation is the intrinsic capa bility of an organ to control its own blood flow . 2) Autoregulation i s observed in some vital organs in the body such as brain, heart and kidneys. It is highly significant and more efficient in kidneys.

9. Renal Autoregulation Renal autoregulation is important to regulate the glomerular filtration rate (GFR). Blood flow to kidneys remains normal even when the mean arterial blood pressure vary widely particularly between 60 mm Hg and 180 mm Hg. This is useful regarding maintainance of normal GFR.

10. 1. Myogenic response 2. Tubuloglomerular feedback. 1. Myogenic Response A) Whenever the blood flow to kidneys enhances , it stretches the elastic wall of the afferent arteriole. B) Stretching of the vessel wall enhances the flow of calcium ions from extracellular fluid into the cells. C) The influx of calcium ions resu;ts in the contraction of smooth muscles in afferent arteriole, which leads to the constriction of afferent arteriole. So, the blood flow is reducced .

11. 2. Tubuloglomerular Feedback Macula densa plays an important role in tubuloglomerular feedback, which regulates the renal blood flow as well as GFR.

12. SPECIAL FEATURES OF RENAL CIRCULATION Renal circulation exhibits some special features to cope up with the functions of the kidneys. Such special features include 1. Renal arteries arise directly from the aorta. So, the high pressure in aorta enhances the high blood flow to the kidneys. 2. Both the kidneys receive about 1,300 mL of blood per minute, i.e. about 26% of cardiac output. Kidneys are the second organs to receive maximum blood flow, the first organ being the liver, which receives 1,500 mL per minute, i.e. about 30% of cardiac output. Whole amount of blood, which flows to kidney has to pass through the glomerular capillaries particularly before entering the venous system. Due to this, the filteration of blood occurs in a complete manner filtered at the renal glomeruli.

13. Renal circulation has a portal system, i.e. a double network of capillaries, the glomerular capillaries and peritubular capillaries. Renal glomerular capillaries lead to the formation of high pressure bed with a pressure of 60 mm Hg to 70 mm Hg. It is much greater compare to the capillary pressure elsewhere in the body, which is only about 25 mm Hg to 30 mm Hg. 5) High pressure is regulated in the glomerular capillaries due to the diameter of afferent arteriole is more than that of efferent arteriole. The high capillary pressure enhances glomerular filtration. 6. Peritubular capillaries form a low pressure bed with a pressure of 8 mm Hg to 10 mm Hg. This low pressure is associated with tubular reabsorption. 7. Autoregulation of renal blood flow is well established.

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