The urinary system - the renal system, produces, stores and eliminates urine, the fluid waste excreted by the kidneys. The kidneys make urine by filtering wastes and extra water from blood. Urine travels from the kidneys through two thin tubes called ureters and fills the bladder.

They are essential in homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and the regulation of blood pressure. They serve the body as a natural filter of the blood and remove wastes that are excreted through the urine. 

The kidneys produce hormones including calcitriol, erythropoietin, and the enzyme renin, which are involved in renal and hematological physiological processes.

Key points:

1.      Human urinary system consists of two kidneys, two ureters, a urinary bladder, a urethra and sphincter muscles:

1. Kidneys are the major organs of urinary system. Formation of urine takes place in kidneys which are two bean shaped organs lying close to the lumbar spine, one on each side of the body.
2. Ureters are muscular tubes extending from the kidneys to the urinary bladder. Urine flows in these tubes from kidney to the urinary bladder.
3. Urinary Bladder collects urine before it is excreted from the body. Urinary bladder is a hollow muscular and elastic organ siting on the pelvic floor
4. Urethra is a tube that connects the urinary bladder to the external genitalia for elimination from the body.
5. Sphincter muscles are two sphincter muscles to control the elimination of urine from human body. The external of the two muscles is striated and is under voluntary control of the body.

Kidneys lie behind the peritoneum high up on the posterior abdominal wall on either side of the vertebral column. In their common location, they are largely under cover of the costal margin. The left kidney lies approximately at vertebral level T12 to L3 and the right kidney slightly lower.

The right kidney lies slightly lower than the left kidney, because of the larger size of the right lobe of liver. Both kidneys move up and down with the movements of the diaphragm during respiration.

The kidneys are located at the wall of the abdominal cavity just above the waistline and are protected by the ribcage. They are considered retroperitoneal, which means that they lie behind the peritoneum, the membrane lining of the abdominal cavity. 

Each kidney is reddish-brown, bean shaped organ and is about 4 inches long and 2 inches wide. It has a medial concave surface and a lateral convex surface.

On the medial concave border of each kidney, there is a vertical slit called hilus renalis, which is bounded by thick lips of the renal substance. Inside the kidney, the hilum extends into a large cavity called sinus renalis. All structures that enter or exit the kidney pass through the hilum. These structures are:

•      Renal vein

•      Two branches of renal artery

•      Ureter

•      Lymph vessels

•      Sympathetic fibers

Each kidney has the following four coverings on its exterior:

Fibrous capsule surrounds the kidney and is closely applied to its outer surface.

Perirenal fat surrounds the fibrous capsule.

Renal fascia is a condensation of connective tissue and lies outside the perirenal fat.

Pararenal fat is found often in large quantity and lies external to the renal fascia. If forms part of the retroperitoneal fat.

When viewed in transverse section, each kidney appears to consist of two layers. The dark brown outer layer is called cortex renalis and the light brown inner layer is called medulla renalis, which is composed of about one dozen renal pyramids (pyramis renalis).

The layers are not discrete and there are projections form one to the other. The cortex extends into the medulla as renal columns (columnae renales). Similarly there are striations, known as medullary rays (pyramis renalis), which extend from the renal pyramids into the cortex.

The renal sinus, the space within the hilum, contains the renal pelvis (pelvis renalis) that is the upper expanded end of the ureter. The pelvis is divided into 2 or 3 major calyces (calix renalis), each of which divides into 2 or 3 minor calyces. Each minor calyx is indented by the apex of the renal pyramid, called the renal papilla (papilla renalis).

Blood supply to the kidney comes from the renal artery, which arises from the abdominal aorta. According to the most common arrangement, each renal artery divides into 5 segmental arteries. These segmental arteries enter the hilum and are distributed to different areas of the organ.

For each renal pyramid, a lobar artery arises from the segmental artery. Each of these lobar arteries gives rise to 2 or 3 interlobar arteries, which run towards the cortex.

At the junction of the cortex and medulla, the interlobular arteries give off the arcuate arteries, which arch over the bases of the pyramids. Interlobular arteries, which ascend into the cortex, arise from the arcuate arteries and give off the afferent glomerular arterioles.

The rather complex arterial system of the kidney can be summarized as: Abdominal aorta -> Renal arteries -> Segmental arteries -> Lobar arteries -> Interlobar arteries -> Arcuate arteries -> Interlobular arteries -> Afferent glomerular arterioles.

The venous blood from the kidney is drained by the renal vein, which itself drains into the inferior vena cava.

Lymphatics from the kidney drain into the lateral aortic lymph nodes, which are present around the origin of the renal artery.

Each kidney receives its nerve supply from the corresponding renal sympathetic plexus. The afferent fibers from the kidney travel through the renal plexus and enter the spinal cord in the thoracic segments.

A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.

Its function is vital for homeostasis of blood volume, blood pressure, and plasma osmolality. It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.

The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place. Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na⁺/K⁺/ATPase pumps that actively transports sodium across the basolateral membrane. 

Water and glucose follow sodium through the basolateral membrane via an osmotic gradient, in a process called co-transport.

The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed, which increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle’s loop is impermeable to water but highly permeable to ions, which causes a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L.

The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH. 

Normally, it is impermeable to water and permeable to ions, driving the osmolarity of fluid even lower. However, anti-diuretic hormone (secreted from the pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water to increase water reabsorption. This example results in increased blood volume and increased blood pressure. Many other hormones will induce other important changes in the distal convoluted tubule that fulfill the other homeostatic functions of the kidney.

The collecting duct is similar in function to the distal convoluted tubule and generally responds the same way to the same hormone stimuli. It is, however, different in terms of histology. The osmolality of fluid through the distal tubule and collecting duct is highly variable depending on hormone stimulus. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.

Nephron is the basic structural and functional unit of the kidney. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine.

Nephron eliminates wastes from the body, regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates blood pH.

Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone. In humans, a normal kidney contains 800,000 to 1.5 million nephrons.

Two general classes of nephrons are cortical nephrons and juxtamedullary nephrons, both of which are classified according to the length of their associated Loop of Henle and location of their renal corpuscle.

All nephrons have their renal corpuscles in the cortex. Cortical nephrons have their Loop of Henle in the renal medulla near its junction with the renal cortex,

while

the Loop of Henle of juxtamedullary nephrons is located deep in the renal medulla; they are called juxtamedullary because their renal corpuscle is located near the medulla (but still in the cortex).

The majority of nephrons are cortical. Cortical nephrons have a shorter loop of Henle compared to juxtamedullary nephrons. The longer loop of Henle in juxtamedullary nephrons create a hyperosmolar gradient that allows for the creation of concentrated urine.

The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman’s capsule. 

A group of specialized cells known as juxtaglomerular apparatus (JGA) are located around the afferent arteriole where it enters the renal corpuscle. The JGA secretes an enzyme called renin, due to a variety of stimuli, and it is involved in the process of blood volume homeostasis.

The Bowman’s capsule surrounds the glomerulus. It is composed of visceral and parietal layers. The visceral layer lies just beneath the thickened glomerular basement membrane and only allows fluid and small molecules like glucose and ions like sodium to pass through into the nephron.

Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances.

The juxtaglomerular apparatus (JGA) is located between the afferent arteriole and the returning distal convoluted tubule of the same nephron. It is responsible for regulating both intrarenal (tubuloglomerular feedback) and extrarenal (renin-angiotensin-aldosterone) mechanisms necessary to maintain both renal and entire body volume status.

The three components of the JGA are the following:

(1) the juxtaglomerular cells of the afferent arteriole, synthesize and store renin, which is secreted in response to specific stimuli (e.g., low blood flow, decreased NaCl delivery). The juxtaglomerular cells could be considered the “effector arm” of the renin-angiotensin-aldosterone axis.

(2) the macula densa, a region of the distal convoluted tubule characterized by tubular epithelial cells which are more densely-packed than in other regions of the nephron (and thereby leading to its characteristic appearance on light microscopy). The macula densa can be considered the “sensory arm” of the renin-angiotensin-aldosterone axis in that these are the cells which sense decreased Na Cl delivery which determines downstream function. They are also involved in the mechanism of tubuloglomerular feedback.

(3) mesangial cells, which form connections via actin and microtubules which allow for selective vasoconstriction/vasodilation of the renal afferent and efferent arterioles with mesangial cell contraction.

The renin-angiotensin-aldosterone system (RAAS) is a signaling pathway responsible for regulating the body's blood pressure.

Stimulated by low blood pressure or certain nerve impulses (e.g. in stressful situations), the kidneys release an enzyme called renin. This triggers a signal transduction pathway: renin splits the protein angiotensinogen, producing angiotensin I. This is converted by another enzyme, the angiotensin-converting enzyme (ACE), into angiotensin II.

Angiotensin II not only causes vasoconstriction, it also simultaneously stimulates the secretion of the water-retaining hormone vasopressin in the pituitary gland as well as the release of adrenaline, noradrenaline and aldosterone in the adrenal gland.

Whereas adrenaline and noradrenaline enhance vasoconstriction, aldosterone influences the filtration function of the kidneys. The kidneys retain more sodium and water in the body and excrete more potassium. The vasopressin from the pituitary gland prevents the excretion of water without affecting the electrolytes sodium and potassium.

In this way, the overall volume of blood in the body is increased: more blood is pumped through constricted arteries, which increases the pressure exerted on the artery walls – the blood pressure.

Angiotensin, aldosterone and vasopressin can also have a direct effect on the heart. Particularly in certain remodeling processes, for example after a heart attack, these hormones are involved in the abnormal enlargement of the heart or development of scar tissue, which can ultimately lead to heart failure.

Several cardiovascular therapies – e.g. for high blood pressure – target the renin-angiotensin-aldosterone system. For example, diuretics increase the discharge of water and thus reduce the volume of blood; ACE inhibitors block the enzyme that is needed for the formation of angiotensin II – thus interrupting the signaling pathway.

Human body has two ureters, each of which is a muscular tube that extends from the corresponding kidney to the posterior surface of the urinary bladder. Urine is propelled along the ureters by peristaltic contractions of their muscular wall, aided by the filtration pressure of the glomeruli.

The ureter is roughly 25-30 cm long in adults and courses down the retroperitoneum in an S curve. At the proximal end of the ureter is the renal pelvis; at the distal end is the bladder.

Both ureters have 3 constrictions (similar to the esophagus).

The bladder is an extraperitoneal muscular urine reservoir that lies behind the pubic symphysis in the pelvis. A normal bladder functions through a complex coordination of musculoskeletal, neurologic, and psychological functions that allow filling and emptying of the bladder contents. The prime effector of continence is the synergic relaxation of detrusor muscles and contraction of the bladder neck and pelvic floor muscles. This occurs during bladder filling and urine storage.

The normal adult bladder accommodates 300-600 mL of urine. A central nervous system response is usually triggered when the volume reaches 400 mL and is perceived as the sensation of bladder fullness and the need to void. However, urination can be prevented by cortical suppression of the peripheral nervous system or by voluntary contraction of the external urethral sphincter.

The adult bladder is located in the anterior pelvis and is enveloped by extraperitoneal fat and connective tissue. It is separated from the pubic symphysis by an anterior prevesical space known as the space of Retzius or retropubic space. The dome of the bladder is covered by peritoneum, and the bladder neck is fixed to neighboring structures by reflections of the pelvic fascia and by true ligaments of the pelvis.

The body of the bladder receives inferior support from the pelvic diaphragm in females or prostate in males and lateral support from the obturator internus and levator and muscles.

Specific "landmarks" are used to describe the location of any irregularities in the bladder.

These are:

Trigone: a triangle-shaped region near the junction of the urethra and the bladder

Right and left lateral walls: walls on either side of the trigone

Posterior wall: back wall

Dome: roof of the bladder

The bladder is pear shaped, becoming more oval as it fills with urine. The kidneys form the urine, which passes through the ureters to the bladder for storage prior to elimination.

The bladder opens into the urethra at the neck of the bladder. The wall of the bladder is composed of 3 layers:

•      Outer layer of loose connective tissue, containing blood and lymphatic vessels and nerves

•      Middle layer, consisting of a mass of interlacing smooth muscle fibres and elastic tissue – this is called the detrusor muscle

•      Inner layer composed of transitional epithelium

When empty, the bladder inside is arranged in folds, which disappear as the bladder fills with urine.  The bladder can stretch and distend to a normal capacity of 500-600mls. The bladder acts as a storage reservoir for urine. Micturition is the process by which urine is expelled from the bladder.