Urinary And Renal System: Anatomy, Physiology, Structure and Function

The renal system consists of the kidneys, ureters, bladder, and urethra. The kidney contains the nephron, the functional unit of the renal system. The nephron consists of the glomerular and peritubular capillaries and the associated tubular segments. The glomerular tuft (glomerulus) contains capillaries and the beginning of the tubule system, Bowman’s capsule.

Tubule fluid, an ultrafiltrate of plasma, is formed at the renal glomerulus and passes through the tubules. The composition of the filtrate is modified by secretion and reabsorption as it passes through the tubules of the renal cortex and medulla, ending with the collecting ducts. A second capillary bed, the peritubular capillaries, carries the reabsorbed water and solute back toward the vena cava. Filtrate from the tubules collects at the renal calyx and is transported by the peristaltic action of the ureter to the bladder. The bladder stores urine until elimination from the body through the urethra.

Kidneys

Renal Cortex And Medulla

Each kidney can be visually and functionally divided into an outer cortex and an inner medulla. The renal cortex contains all the glomeruli, a large portion of the peritubular capillaries, as well as the proximal tubule, distal tubule, and cortical portion of the collecting duct. The renal medulla contains the vasa recta, the loop of Henle, and the medullary portion of the collecting duct. The renal medulla has a pyramidal structure, with the collecting ducts emptying into the renal calyces.

Blood Vessels And Renal Tubules

The kidneys have an extensive vascular supply and receive about 20% of the cardiac output. The renal vascular pattern is unusual in that blood flows through two capillary beds, one with high pressure (glomerular) and the second with low pressure (peritubular), connected in series. Blood enters the kidney via the renal artery and, after a series of divisions, arrives at the glomerulus. Blood entering the glomerular capillaries must first pass through an afferent arteriole. Blood exiting the glomerular capillaries passes through a second arteriole, the efferent arteriole. Blood then flows through the peritubular capillaries, which include the vasa recta that extend into the renal medulla. Blood leaves the peritubular capillaries, collects in progressively larger venules and veins, and then exits the kidney via the renal vein.

Filtrate formed in Bowman’s capsule remains separated from the body fluid spaces by a layer of epithelial cells that extends through the remainder of the urinary system.

Consequently, renal filtrate and urine are functionally outside the body, similarly to the fluids of the GI tract. Renal tubules consist of a single layer of epithelial cells that selectively secrete or reabsorb compounds. Tubular transport represents a mechanism to reabsorb water and solutes filtered at the glomerulus before they are excreted from the body in the urine. The ureter, bladder, and urethra also have an epithelial lining, but the epithelial cells do not allow transport of water or solutes. Consequently, filtrate that exits the renal collecting duct and collects in the renal pelvis is identical to the final urine.

The tubular segments originate at the glomerulus. The glomerular filtrate travels progressively through Bowman’s capsule, the proximal tubule, loop of Henle, distal tubule, connecting segment, and collecting duct. Upon exiting the tubules, the tubular fluid passes into the renal papilla and exits the kidney via the ureter.

Tubule segments are anatomically adjacent to the vascular supply for that nephron. The junction of glomerulus and the macula densa of the distal tubule that originated from that glomerulus forms the juxtaglomerular apparatus. This arrangement allows negative feedback control of glomerular filtrate formation at the individual nephron level.

Ureters, Bladder, And Urethra

Ureters

The ureters originate at the renal hilus and conduct urine from the kidney to the bladder. Anatomically, the ureters consist of an epithelium-lined lumen surrounded by smooth muscle, nerves, blood vessels, and connective tissue. Peristalsis, originating in the renal calyx, propels urine toward the bladder. The wall of a ureter is formed of three layers. The external fibrous layer is composed of dense irregular connective tissue. The middle layer consists of smooth muscle cells that produce peristaltic waves for urine transport. The internal layer is a mucosa that is continuous with that of the renal pelvis and the urinary bladder. A flap like fold of mucosa in the urinary bladder covers the opening of the ureter, and it functions as a valve that prevents backflow of urine into the ureter.

Urinary Bladder

The bladder is a highly distensible organ lying behind the symphysis pubis. The wall of the bladder consists of an (detrusor) layer, and a thin connective layer containing nerves and blood vessels.This anatomic arrangement allows the wall of the bladder to distend to a large volume without generating much tension. Inflow to the bladder comes from the ureters, which connect with the bladder at the ureterovesical junction. Urine passing from the bladder into the urethra must pass through the smooth muscular internal bladder sphincter.

The internal floor of the urinary bladder contains the trigone (trf-gon), a smooth, triangular area that contains an opening at each of its angles. The openings of the ureters are located at the two laterally located posterior angles, and the opening of the urethra is located at the anterior angle.

Four layers compose the wall of the urinary bladder. The most internal layer is the mucosa, which is composed of transitional epithelium that is adapted to the repeated stretching of the urinary bladder wall. The epithelium stretches, and its thickness decreases as the urinary bladder fills with urine.

The mucosa is supported by the underlying submucosa formed of areolar connective tissue containing an abundance of elastic fibers. Blood vessels and nerves supplying the urinary bladder are present in the submucosa.

Smooth muscle cells compose the third, and thickest, layer. These cells form a muscle called the detrusor (de-tru t-sor). The detrusor is relaxed as the urinary bladder fills with urine, and it contracts as urine is expelled. Cells of the detrusor form an i nternal urethral sphincter at the junction of the urinary bladder and the urethra.

The external layer consists of the parietal peritoneum, but it covers only the superior portion of the urinary bladder. The remainder of the urinary bladder surface is coated with dense irregular connective tissue.

Urethra

The urethra extends from the bladder to the surface of the body. It consists of an epithelium-lined lumen and a smooth muscle layer. Urine exiting the urethra must pass through the muscular external sphincter.

Male

The length of a male urethra is about 18-20 cm. It stretches from the internal urethral orifice in the neck of the urinary bladder to the external urethral orifice (EUO) at the tip of the glans penis. In flaccid state of the penis, the long axis of the urethra presents 2 curvatures and is consequently S-shaped. In erect state of the penis, the distal curvature goes away and consequently, it becomes ‘J-shaped’.

Female

The female urethra is all about 4 cm long. It starts at the internal urethral orifice in the neck of bladder and enters downward and forwards embedded in the anterior wall of the vagina via urogenital diaphragm. Urethra pierces the perineal membrane, and opens in the vestibule of vagina in front of the vaginal orifice. In the vestibule of vagina, the urethral orifice is situated in front of the vaginal orifice and about 2.5 cm behind the glans of clitoris.

Functions of The Urinary System

The normal metabolic activities of body cells produce a number of waste materials that tend to change the balance of water and dissolved substances in body fluids. The basic function of the urinary system is to maintain the volume and composition of body fluids within normal limits.

1. Maintenance of body fluid composition. One major function of the kidneys is to keep the volume and composition of blood plasma at homeostasis. This is accomplished by balancing of the concentration of water and electrolytes, in addition to blood pH, through the formation of urine.
2. Maintenance of blood pressure. Whenever the kidney senses a decrease in blood pressure, they secrete renin. Renin is an enzyme that triggers the renin-angiotensin mechanism, which increases blood pressure.
3. Secretion of erythropoietin. When the blood oxygen level falls below normal, the kidneys release more erythropoietin, which stimulates RBC formation by red bone marrow. The increase in RBC number helps increase the blood oxygen level.
4. Conversion of vitamin D. In response to parathyroid hormone (PTH), the kidney converts inactive vitamin D to its active form. Active vitamin D is important in maintaining blood Ca²⁺ levels.
5. Excretion of nitrogenous wastes. The kidneys do not remove all nitrogenous wastes but keep their concentrations in the blood within tolerable limits. The primary nitrogenous wastes produced by cellular metabolism are urea, uric acid, and creatinine.

• Urea is a waste product of amino acid metabolism. In order for amino acids to be used as an energy source in cellular respiration or converted into glucose or fat, the liver removes the amine (—NH₂) groups from them. The amine groups react to form ammonia, which is converted to the less toxic urea by the liver.
• Uric acid is a waste product of nucleic acid metabolism. An abnormally elevated concentration of uric acid in the blood and the deposition of uric acid crystals in joints are characteristic of a hereditary disorder called gout. Joints of the hands and feet are often the sites of uric acid deposition, which produces inflammation and severe pain.
• Creatinine is a waste product of muscle metabolism and, specifically, the breakdown of creatine phosphate.