GA The Kidneys

Excretory Pathway cont'd

A minor calyx is a double-walled funnel-like structure that fits over the tip or papilla of a pyramid and collects urine as it passes out of the collecting ducts (from the area cribosa). The minor calyx is the first structure in the excretory pathway leading away from the kidney. Urine then drains into a major calyx; a major calyx is formed by the fusion of 2 or more minor calices. The renal pelvis is the upper, proximal end of the ureter, formed by the fusion of 2-3 major calyces. It is located in the renal sinus, an empty space deep to the hilus that also contains vessels, nerves, fat and connective tissue. The uretero-pelvic junction is the site where the renal pelvis narrows, as it passes out of the hilus, to form the ureter proper. The uretero-pelvic junction is a common site of entrapment for renal calculi

Hydronephrosis

Another source of ureter compression may be presence of accessory renal arteries. These arteries enter the lower pole of the kidney can compress the ureter and cause urine to back up into the renal pelvis and kidney calyces (hydronephrosis). These accessory vessels occur in approximately 25% of individuals and typically result from failure of the inferior renal vessels to degenerate during development.

Symptomatic Nephroptosis

Nephroptosis refers to the inferior displacement of the kidney, such as that which occurs when perirenal fat is lost (e.g. in cases of extreme emaciation). Perirenal fat surrounds each kidney, extends into the hilum and fills most of the renal sinus; it surrounds the renal vessels, renal pelvis and calyces, thus providing support to keep the kidneys in place. If this fat is lost, the kidneys drop. Nephroptosis can also occur from a kidney punch or a blow to the lower lumbar region of the back, or jarring of the body while riding trail bikes, horses or operating certain types of construction vehicles. "Kinks" in the ureter associated with nephroptosis can lead to hydronephrosis (distension of the pelvis and calyces of the kidney with urine, followed by atrophy of the kidney parenchyma). A mass in the pelvis may be a kidney that failed to ascend to its adult position, or a kidney that was displaced inferiorly due to loss of perirenal fat, renal fascia and/or pararenal fat.

Kidney Blood Supply

Renal arteries (paired) arise from the aorta, immediately inferior to the origin of the Superior Mesenteric Artery at L1-L2 vertebral level; they course on laterally to the hilus. The Left renal artery arises slightly higher on the aorta and is shorter than the right renal artery; it lies posterior to the renal vein, body of pancreas and splenic vein. It is crossed by the Inferior Mesenteric Vein. The Right renal artery arises lower and is longer (because of the location of the aorta) than the left renal artery. It passes deep to the Inferior Vena Cava, head and uncinate process of the pancreas and descending duodenum. Before entering the hilum, each artery typically gives rise to ureteric branches and a single, inferior suprarenal artery. At the hilum each artery divides into anterior and posterior divisions. The anterior division lays dorsal to the renal vein and ventral to the renal pelvis. The posterior division lays dorsal to the renal pelvis (Vein-Artery-Ureter/Pelvis-Artery)

Nephrolithiasis/Ureterolithiasis

Renal calculi (kidney stones) may obstruct the renal pelvis. Depending on the level of obstruction, pain is referred to the lumbar or inguinal regions ("loin to groin"), the proximal anterior thigh or the external genitalia/testis (skin supplied by T11-L2).

Renal Fascia

Renal fascia: membranous layer that surrounds the perirenal fat; it separates the perirenal fat from the pararenal fat and adipose tissue of the posterior abdominal wall. Superiorly the renal fascia creates a closed space. It surrounds and fuses with the fascia of the diaphragm. Often, it sends a septum between the kidney and the adrenal gland so that each organ is contained in its own compartment. Laterally, the anterior and posterior layers of the renal fascia usually fuse and become continuous with the transversalis fascia. Medially, the anterior layer passes in front of the great vessels and the posterior layer passes behind them. In the midline, these layers join and are continuous with their counterparts from the opposite side. Inferiorly, the renal fascia DOES NOT create a closed compartment. The anterior layer merges with the retroperitoneal areolar tissue and the posterior layer becomes continuous with the deep fascia of the muscles of the posterior abdominal wall. Because the renal fascia does not fuse inferiorly around the ureters, kidney infections may spread down into the pelvis and/or pelvic infections may spread up to the kidneys. In cases of extreme emaciation, where perirenal fat is lost, the kidney may be displaced inferiorly (nephroptosis).

Kidney Venous Drainage

Renal veins (drain into Inferior Vena Cava). The Left renal vein is longer and higher than the right one; it receives blood from the left gonadal vein and left suprarenal vein, as well as other vessels; it is crossed by the Superior Mesenteric Artery. The Right renal vein is shorter and lower than left one; courses dorsal to the descending portion of the duodenum. Lymphatic Drainage: Renal lymphatics follow the renal veins and drain into the lumbar lymph nodes, near the origin of the renal artery.

Excretory Pathway

The collecting ducts are part of the kidney nephrons - the actual functional units of the kidney. Many nephrons empty into a single collecting duct. Many collecting ducts unite to drain urine obtained by the kidney into papillary ducts, and then into a minor calyx.

Kidney Structure

The kidney has an anterior and a posterior surface, a medial and a lateral border, and a superior and an inferior pole. It is divided into a renal capsule, a renal cortex and the innermost renal medulla. The renal capsule constitutes the outer layer of the kidney; it is characterized by a thin yet tough, smooth fibrous membrane. The renal cortex is a dark, granular parenchymal layer of tissue located immediately below the renal capsule. It projects between the medullary pyramids as renal columns. These columns contain glomeruli and portions of the renal tubules. The renal medulla makes up the innermost part of the kidney. This inner parenchymal tissue forms 10-15 renal or medullary pyramids whose apices point toward the concave medial border of the kidney called the renal hilus (a longitudinal fissure that transmits the renal vein, the renal artery and the renal pelvis/ureter). The medullary pyramids also contain portions of the renal tubules, as well as collecting ducts; they do not contain glomeruli. The tips of the pyramids form papillae. Urine leaves the kidneys via the collecting ducts that become papillary ducts before opening onto these papillae, in the area cribosa. A renal lobe is formed by a medullary pyramid and its associated cortical tissue. There are 10+ lobes in each kidney.

Kidney Location

The kidneys are retroperitoneal organs. They are encased in three layers of fat and fascia and typically found in the paravertebral gutters on either side of the upper lumbar vertebral column (T12-L2). The right kidney is slightly lower than the left due to the presence of the right lobe of the liver. The position of both kidneys varies with movements of the diaphragm (particularly during deep breaths) and individuals' posture and body build (especially in terms of amount and distribution of body fat).

Renal Fat and Fascia

The main functions of the perirenal (perinephric) fat, renal fascia and pararenal (paranephric) fat are to support the kidneys, hold them in place on the posterior abdominal wall and circumscribe the route for spread of infections/abscesses. Perirenal (perinephric) fat comprises the adipose capsule of the kidney; it envelops each kidney and extends into the hilus, becoming continuous with the fat in the renal sinus. Pararenal (paranephric) fat lays posterior to the renal fascia.

Kidney Function

The major functions of the kidneys include: •Regulation of water, extracellular fluid, electrolyte and acid-base balance of the blood. •Urine excretion (water, solutes, waste products). •Synthesis of prostaglandins and kinins. •Synthesis of three hormones: Renin, Erythropoietin and 1,25-dihydroxycholecalciferol.

Kidney Blood Supply cont'd

The renal arteries give rise to multiple branches inside the kidney. At the cortico-medullary junction the arteries supply specific segments of the organ. As such, they are end-arteries, with NO intercommunication/anastomotic connections. Because there is minimal, if any, communication between segmental renal arteries, occlusion of one of these vessels will result in infarction of the area of renal tissue it supplies.

Urinary System

The urinary system (or renal system) produces, stores and excretes urine. It is comprised of the paired kidneys, the paired ureters, a urinary bladder and a urethra. During this unit we will be exploring the anatomy of the kidney.

Nerves Posterior to Kidney

Three nerves pass posterior to the kidney. The subcostal nerve is the most superior of these nerves. The ilioinguinal and iliohypogastric nerves also pass posteriorly.

Renal Transplants

When considering whether a renal transplant is safe to perform there are several anatomical considerations: 1.First and foremost - does the donor patient have two kidneys? About 1 in 750 people go through life with only 1 functional kidney. Another factor to consider is whether one kidney is smaller than the other. The donor should always keep the better kidney. 2.Is there a history of bilateral kidney stones that may have damaged the kidneys? 3.Is there abnormal renal vasculature? Too many vessels or too small a vessel may complicate any transplant.

Horseshoe Kidney

•Lower poles of the kidneys may become fused during development (1 in 800) •Increased risk of blockage, stones and infections •May be asymptomatic in 1/3 of patients A horseshoe kidney occurs if the lower poles of the two kidneys fuse across the aorta and the vena cava during development. This type of kidney will remain in the lower lumbar region since the root of the Superior Mesenteric Artery blocks its ascent.