Kidneys
The kidneys perform more vital functions for you than you might have previously thought. In addition to obvious functions of producing the urine and maintaining acid-base balance, they also play critical roles in the maintenance of blood pressure, the production of red blood cells, and the synthesis of active forms of vitamin D.

Overview of Kidney Anatomy
The kidneys are retroperitoneal structures about 4 inches long and 2-3 inches wide that are wrapped within a fibrous pouch. The fascia of this pouch is what anchors the kidneys to the abdominal wall and to the peritoneum. The superior poles of each kidney are capped with the adrenal glands. The right kidney sits a scooch lower than the left simply because it pushed inferiorly by the presence of the liver. These para-vertebral organs are located between T12 and L3. Their medial borders, called hilum, are sites of entry and exit for the renal arteries and veins, respectively. Also seen along the medial border of each kidney is the renal pelvis, where urine collects en route to the ureter.

Internally, the kidney is divided into an outer cortex and an inner medulla. The cortex is the area where nephrons begin (see section on nephrons below), though the structures of nephrons continue deeper into the medulla, traveling within multiple triangular-shaped structures known as the renal pyramids. Also note that between the pyramids are extensions of the cortical tissue called the renal columns. The apex of each medullary pyramid is called the renal papilla. The renal papilla leads into minor calyces. Multiple minor calyces unite to form major calyces and the major calyces in turn empty into the larger renal pelvis, which then becomes the ureter.

The Nephron and Collecting Duct
The nephron is the functional unit of the kidney that creates and processes a filtrate from blood plasma. The structures of the nephron begin in the cortex of the kidney and then continue into the medulla. The medulla contains pyramids that collect the filtrate produced by the nephrons while it is being processed. The volume of the filtrate is reduced through the processes of reabsorption, which return water, electrolytes, glucose, vitamins, and amino acids back to the systemic circulation. Once the finished filtrate moves from the renal papilla and into the minor calyces it is referred to as urine. It then continues through the major calyces, into the renal pelvis, and onward to the ureter.
The first structure of the nephron is the renal corpuscle, composed of capillaries called the glomerulus and a surrounding capsule called the glomerular capsule (Bowman’s capsule). Blood to be filtered is carried to the capillaries that compose the glomerulus through afferent glomerular arterioles. The process of filtration begins as plasma passes through small holes in the glomerulus called fenestrae. Then, around the outer surface of the glomerulus the filtrate passes through small interlacing projections (pedicels) of cells called podocytes. These podocytes around the outer surface of the glomerulus make up the inner (visceral) layer of the glomerular (Bowman’s) capsule. As filtrate passes through tiny channels between the pedicels of the podocytes, called slit pores, it empties into the capsular space. This filtering system through the fenestrae, through the pedicels of the podocytes, and into the capsular space, is structured so that vital elements such as blood cells and protein, which cannot fit between the pedicels, are not normally passed into the filtrate. From the capsular space, filtrate moves into the proximal convoluted tubule, where a great deal of reabsorption occurs. Reabsorption then continues in the the descending and ascending limbs of the loop of Henle and the distal convoluted tubule. Multiple convoluted tubules then pass filtrate into collecting ducts, where a smaller amount of water is reabsorbed. The collecting duct (papillary tube) carries the filtrate through the renal papilla and into the calyces as urine.


Blood Supply and Innervation of the Kidneys

The renal arteries serving each kidney branch into segmental arteries (superior, anterior superior, anterior inferior, inferior, and posterior). These then branch into interlobar arteries that will give themselves to arteries called arcuate arteries that ring the pyramids. The next, and smallest, arterial branches are the interlobular arteries that pass into the cortex and give off the afferent arterioles. Remember that the afferent arterioles feed blood into the capillaries constituting the glomerulus. Blood that has been filtered leaves the glomerulus through an efferent arteriole and then moves into the venous system via peritubular capillaries or capillaries of the vasa recta. Unlike the capillaries of the glomerulus that are needed to filter blood, the vasa recta surround the Loop of Henle and serve to reabsorb substances that may have further value to the body. Venous blood is removed through a system that parallels the arterial system already outlined, and empties into the inferior vena cava.

The nerve supply from the renal plexus to the kidney is autonomic, and originates from thoracic spinal nerves 10-12.

Ureters
The ureters are retroperitoneal tubes that travel from each renal pelvis to the posterolateral portion of the base of the bladder. Collection of urine in the renal pelvis stimulates peristaltic waves as the muscles of the ureters contract. This forces urine into the bladder below. The ureters are composed of three layers: a mucosa (composed of transitional epithelium), a muscularis, and the adventitia (loose connective tissue). In its proximal 1/3 portion, the muscularis consists of a middle circular layer with a longitudinal layer both inside and outside of it. Along the next 2/.3 of the ureters, there is only an outer circular and an inner longitudinal layer. Three arteries supply the ureters: the renal artery in its upper portion, the testicular/ovarian artery in the middle portion, and the superior vesicular artery in the inferior portion. Innervation is sympathetic through the renal, gonadal, and hypogastric plexi.

Renal stones may block the ureter and cause obstruction of urinary flow or be small enough to be passed through the urinary tract (although women who have experienced both childbirth and the passing of kidney stones tell me that passing a stone is actually more painful). Stones are precipitates of one or several substances, including: calcium oxalate, calcium phosphate, cystine, or uric acid. Once a person forms a stone, they are more likely to form subsequent stones.

Urinary Bladder and Urethra
The bladder is a distensible reservoir that holds urine and is composed of four layers: a mucosa of transitional epithelium, a supportive submucosa, a muscularis ( aka detrusor muscle that is made of three, interwoven layers of smooth muscle), and the adventitia, which is peritoneum. The mucosa contains rugae that allow for distension, except in the area of the trigone that exists between the two ureters and the neck of the opening to the urethra. The mucosa also contains flaps that cover the openings of the ureters to prevent retrograde urine flow from the bladder into the ureters.

An infection of the bladder is termed cystitis and is characterized by frequency of urination, urinary urgency, and burning pain during urination. These infections are more common in women than in men because women have a straight and short urethra, about 4cm, whereas in men the urethra is about 20cm and is s-shaped. The urethra in females exits the urethral orifice, between the labia minora. In men, the urethra has three sections, the proximal prostatic section, the middle membranous section, and the distal spongy section. In males, the urethra receives both urine and the secretions from the reproductive system, as we will discuss later.

Micturition (urination)
Micturition involves activation of the micturition reflex, though superimposed upon this reflex is voluntary input from the cerebral cortex and midbrain that makes the exact timing of micturition voluntary (mostly). Stretch receptors in the bladder sense distension of the bladder wall and stimulate parasympathetic fibers going to the micturtion reflex center located in segments S2-S4 of the sacrum. Nerve impulses then move through parasympathetic neurons back to the bladder wall, causing contraction of the detrusor and opening of the internal urethral sphincter. At this time, nerve impulses coming to the brain give a sense of urinary urgency, a sense that can be reduced as inhibitory impulses are sent from the midbrain and cerebrum to the micturition center. When a decision is made to begin urination, then the skeletal muscles of the external sphincter are relaxed and urination begins.