The urinary system consists of two kidneys that filter blood, two ureters, a urinary bladder, and a urethra to convey waste substances to the outside.
The kidney is a reddish-brown, bean-shaped organ 12 centimeters long: it is enclosed in a tough fibrous capsule. The kidneys are positioned on both sides of the vertebral column between the 12th thoracic and the 3rd lumbar vertebrae. A medial depression leads to a hollow renal sinus into which blood vessels, nerves, lymphatic vessels, and ureter enter. Inside that is the renal pelvis which is divided into major and minor calyces. Their are two distinct regions within the kidney: the renal medulla, which houses tubes leading to the papillae, and the renal cortex, which contains the nephrons.
Function of Kidney
The kidney functions to help regulate volume, composition and pH of body fluids, and to remove metabolic wastes from the blood. It also helps control the rate of red blood formation. A kidney contains one million nephrons, each consists of a renal corpuscle and a renal tubule. The renal corpuscle is the filtering part of the nephron, it is made of capillaries called the glomerulus and a glomerular capsule that receives the filtrate.
Urine formation involves glomerular filtration, tubular reabsorption, and tubular secretion.
Urine formation begins when the fluid portion of the blood is filtered by the glomerulus and enters the glomerular capsule as glomerular filtration. The main force responsible for moving substances by filtration through the glomerular capillary wall is the hydrostatic pressure of the blood inside. due t plasma proteins osmotic pressure of the blood resists filtration , as does hydrostatic pressure inside the glomerular capsule.
Factors that affect the filtration rate are filtration pressure, glomerular plasma osmotic pressure , and hydrostatic pressure in the glomerular capsule. When the afferent arteriole constricts in response to sympathetic stimulation, filtration pressure, and thus filtration rate declines. When the afferent arteriole constricts, filtration pressure increases, which increases the filtration rate. When hydrostatic pressure inside the glomerular capsule is high, filtration rate declines.
On average, filtration rate is 125 milliliters per minute, 180 liters in 24 hours, most of which is reabsorbed further in the nephron. Glomerular filtration rate is relatively constant, sympathetic impulses may decrease the filtration rate.
Another control over filtration rate is the renin-angiotension system, which regulates sodium excretion. When he sodium chloride concentration in the tubular fluid decreases, the macula densa senses these changes and causes the juxtaglomerular cells to secrete renin. Secretion of renin triggers a series of reactions leading to the production of angiotensin II, which acts as a vasoconstrictor; this may affect filtration rate. Presence of angiotensin II also increases the secretion of aldosterone, which stimulates reabsorption of sodium. The heart can also increase filtration rate when blood volume is high.
Tubular Reabsorption: Changes in the fluid composition from the time glomerular filtrate is formed when urine arrives at the collecting duct are largely the result of tubular reabsorption of selected substances. Most of the reabsorption occurs in the proximal convoluted tubule, where cells possess microvilli with carrier proteins. Carrier proteins have limited transport capacity, so excessive amounts of a substance will be excreted into the urine. Glucose and amino acids are reabsorbed by active transport, water by osmosis, and proteins by pinocytosis.
Sodium and Water Reabsorption: Sodium ions are reabsorbed by active transport, and negatively charged ions follow passively. As sodium is reabsorbed, water follows by osmosis
.Regulation of Urine Concentration and Volume: Most of the sodium ions are reabsorbed before the urine is excreted, and sodium is concentrated in the renal medulla by the countercurrent mechanism. Normally the distal convoluted tubule and collecting duct are impermeable to water unless the hormone ADH is present.
Tubular Secretion: Tubular secretion transports substances from the plasma into the renal tubule. Active transport mechanisms move excess hydrogen ions into the renal tubule along with various organic compounds. Potassium ions are secreted actively and passively into the distal convoluted tubule and the collecting duct.
Urine Composition: Urine composition varies from time to time, reflecting the amount of water and solutes that the kidneys eliminate to maintain homeostasis. Urine is 95% water, and also contains urea, uric acid, a trace of amino acids, and electrolytes.
After forming in the nephrons, urine passes from the collecting ducts to the renal papillae, then to the minor and major calyces, and out the renal pelvis to the ureters, urinary bladder, and finally to the urethra, which conveys urine to the outside.
Ureters: The ureters are muscular tubes extending from the kidneys to the base of the urinary bladder. The wall of the ureter is composed of three layers: mucous coat, muscular coat, and outer fibrous coat. Muscular peristaltic waves convey urine to the urinary bladder where it passes through a flaplike valve in the mucous membrane of the urinary bladder.
Urinary Bladder: The urinary bladder is a hollow, distensible, muscular organ lying in the pelvic cavity. The internal floor of the bladder includes the trigone, which is composed of the openings of the two ureters and the urethra. The wall of the urinary bladder is made of four coats: Inner mucous coat, sub-mucous coat, muscular coat made up of detrusor muscle, and outer serous coat. The portion of detrusor muscle that surrounds the neck of the bladder forms an internal sphincter muscle.
Micturition: Urine leaves the bladder by the micturation reflex. The detrusor muscle contracts and the external urethral sphincter (in the urogenital diaphragm) must also relax. Stretching of the urinary bladder triggers the micturition reflex center located in the sacral portion of the spinal cord. Return parasympathetic impulses cause the detrusor muscle to contract in waves, and an urge to urinate is sensed. When these contractions become strong enough, the internal urethral sphincter is forced open. The external urethral sphincter is composed of skeletal muscle and is under conscious control.
Urethra:The urethra is a tube that conveys urine from the urinary bladder to the outside. It is a muscular tube with urethral glands that secrete mucus into the urethral canal.