The Urinary System and Excretion

Urinary System The kidneys are the primary organs of excretion. Excretion is the removal of metabolic wastes from the body. Functions of the Urinary System The urinary system produces urine and conducts it to outside the body. As the kidneys produce urine, they carry out four functions: 1.excretion of metabolic wastes. 2. maintenance of water-salt balance. 3. maintenance of acid-base balance. 4. and secretion of hormones.

Excretion of MetabolicWastes The kidneys excrete metabolic wastes, notably nitrogenous wastes. Urea is the primary nitrogenous end product of metabolism in human beings, but humans also excrete some ammonium, creatinine, and uric acid. Maintenance of Water-Salt Balance A principal function of the kidneys is to maintain the appropriate water-salt balance of the blood. The kidneys also maintain the appropriate level of other ions (electrolytes), such as potassium ions (K), bicarbonate ions (HCO3), and calcium ions (Ca2), in the blood.

Maintenance of Acid-Base Balance The kidneys regulate the acid-base balance of the blood. the blood pH should be just about 7.4. The kidneys monitor and control blood pH, mainly by excreting hydrogen ions (H) and reabsorbing the bicarbonate ions (HCO3) as needed to keep blood pH at about 7.4. Secretion of Hormones 1.The kidneys release renin, a substance that leads to the secretion of the hormone aldosterone from the adrenal cortex which promotes the reabsorption of sodium ions (Na) by the kidneys. 2. The kidneys secrete the hormone erythropoietin, which stimulates red blood cell production. 3.The kidneys also help activate vitamin D from the skin.

Organs of the Urinary System The urinary system consists of the kidneys, ureters, Urinary bladder, and urethra.

Kidneys The kidneys are paired organs located near the small of the back in the lumbar region on either side of the vertebral column. Each kidney is usually held in place by connective tissue. Masses of adipose tissue adhere to each kidney. The kidneys are bean-shaped and reddish-brown in color. The fist-sized organs are covered by a tough capsule of fibrous connective tissue, called a renal capsule. The concave side of a kidney has a depression called the hilum where a renal artery enters and a renal vein and a ureter exit the kidney.

Ureters The ureters, which extend from the kidneys to the bladder, are small, muscular tubes. Each descends behind the parietal peritoneum, from the hilum of a kidney, to enter the bladder posteriorly at its inferior surface. The wall of a ureter has three layers: 1.The inner layer is a mucosa (mucous membrane), 2.the middle layer consists of smooth muscle, 3. the outer layer is a fibrous coat of connective Tissue.

Urinary Bladder The urinary bladder is located in the pelvic cavity. In males, it is directly anterior to the rectum; in females, it is anterior to the vagina and inferior to the uterus. Its function is to store urine until it is expelled from the body. The bladder has three openings—two for the ureters and one for the urethra, which drains the bladder. The bladder has other features that allow it to retain urine. After urine enters the bladder from a ureter, small folds of bladder mucosa act like a valve to prevent backward flow.

Urethra The urethra is a small tube that extends from the urinary bladder to an external opening. The urethra is a different length in females than in males.

Anatomy of the Kidney and Excretion A kidney has many branches of the renal artery and renal vein reach inside a kidney. Removing the blood vessels shows that a kidney has three regions The renal cortex is an outer The renal medulla. The renal pelvis is a central space, or cavity, that is continuous with the ureter.

Anatomy of a Nephron the kidney is composed of over one million nephrons, Each nephron has its own blood supply, including two capillary regions: From the renal artery,an afferent arteriole leads to the glomerulus, a knot of capillaries inside the glomerular capsule. Blood leaving the glomerulus enters the efferent arteriole. The efferent arteriole takes blood to the peritubular capillary network, which surrounds the rest of the nephron. From there, the blood goes into a venule that joins the renal vein.

Nephron Anatomy

Parts of a Nephron Each nephron is made up of several parts. First, the closed end of the nephron is pushed in on itself to form a cuplike structure called the glomerular capsule (Bowman’s capsule). -Next, there is a proximal convoluted tubule (PCT), -loop of the nephron (loop of Henle). Each loop consists of a descending limb and an ascending limb. -the distal convoluted tubule (DCT) -The distal convoluted tubules of several nephrons enter one collecting duct. Many collecting ducts carry urine to the renal pelvis.

Excretion urine formation, which is divided into these steps: 1.glomerular filtration, 2.tubular reabsorption, 3.and tubular secretion.

Glomerular Filtration Glomerular filtration occurs when whole blood enters the afferent arteriole and the glomerulus. Due to glomerular blood pressure, water and small molecules move from the glomerulus to the inside of the glomerular capsule. This is a filtration process because large molecules and formed elements are unable to pass through the capillary wall. In effect, then, blood in the glomerulus has two portions: 1.the filterable components (Water, Nitrogenous wastes, Nutrients, Nutrients) 2. the nonfilterable components Formed elements (blood cells and platelets) Plasma proteins

Tubular Reabsorption Reabsorbed FiltrateComponents Most water Nutrients Required salts (ions) Nonreabsorbed Filtrate Components Some water Much nitrogenous waste Excess salta (ions)

Tubular Secretion Tubular secretion is a second way by which substances are removed from blood and added to the tubular fluid. Hydrogen ions, potassium ions, creatinine, and drugs such as penicillin are some of the substances that are moved by active transport from the blood into the distal convoluted tubule.

Regulation of glomerular filtration rate and renal blood flow The glomerular filtration rate is the total rate of renal fil- Tration in both kidnesys and normally equals about 120 ml min -1.This is remarkably constant over a wide range of conditions. Physiological control of filtration is acom- plex and poorly understood subject, but glomerular fil- tration rate may be increased by factors which increase any of the following variables: - Glomerular capillary hyd rostatic pressure Glomerular capillary flow rate- Glomerular capillary surface area -

The maintenance of renal - blood flow is particularly Important for normal filtration, since this is the main Determinant of glomerular capillary pressure and flow. Renal blood flow is high (about 1.2 min -1 ) and shows Remarkably little variation over a wide range of arterial Prssures, providing us with one of the best examples of Anutoregulation of local blood flow. Therefore, if arterial pressure falls, dilatation of the Afferent arterioles reduces renal vascular resistance and So helps to limit the decrease in blood flow At very low Pressures.autoregulation breaks down and renal blood flew. declines more rapidly. Reflex stimulation of sympathetic nerves to the kidney under these conditions may further reduce both glomerular blood flow and hydrostatic pressure by constricting the afferent arterioles. Therefore,both glomerular filtration and urine production may be dramatically reduced in surgical shock with marked arterial hypotension.