Urinary System and Excretion

Outline Overview of the Urinary System Functions of the Urinary System Urination Kidney Structure Nephrons Urine Formation Maintaining Water-Salt Balance Reabsorption of Water Reabsorption of Salt Kidney Problems

Urinary System Kidneys are primary organs of the urinary system. The concave side of a kidney has depression called the hilum where the renal artery enters and the renal vein and ureters exit the kidney. Urinary bladder gradually expands as urine enters. Urethra extends from the urinary bladder to an exterior opening.

The Urinary System

Functions of the Urinary System Function of the urinary system is to produce urine and conduct it outside the body. Excretion is the removal of metabolic wastes from the body. Defecation refers to the elimination of feces from the body.

Functions of the Urinary System As urine is being produced, the kidneys: Carry out excretion of metabolic wastes. Urea is the primary nitrogenous end product, but ammonium, creatinine, and uric acid are also excreted. Maintain salt-water balance of blood and thus blood volume and blood pressure. Maintain acid-base balance of the blood. Have a hormonal function.

Urination

Kidney Structure The kidney contains three major regions: Renal cortex. Renal medulla. Renal pelvis. Microscopically, the kidney is composed of over one-million nephrons that produce urine.

Kidney Gross Anatomy

Anatomy of a Nephron Each nephron has its own blood supply. From the renal artery, afferent arteriole leads to the glomerulus, then to the efferent arteriole and then to the peritubular capillary network which surrounds the nephron. From there blood goes into a venule that joins the renal vein.

Nephron Anatomy

Parts of a Nephron Each nephron is composed of several parts. Glomerular capsule. Proximal convoluted tubule. Loop of the nephron (loop of Henle). Distal convoluted tubule.

Urine Formation Urine formation is divided into three steps. Glomerular filtration. Tubular reabsorption. Tubular secretion.

Maintaining Water-Salt Balance Reabsorption of Water. Excretion of hypertonic urine is dependent upon reabsorption of water from the loop of the nephron and the collecting duct. Osmotic gradient exists within the tissues of the renal medulla. Antidiuretic hormone (ADH) released by the posterior lobe of the pituitary causes more water to be reabsorbed and less urine to form.

Reabsorption of Water

Maintaining Water-Salt Balance Reabsorption of Salt. Kidneys regulate blood’s salt balance by controlling excretion and reabsorption of various ions. Usually more than 99% of sodium filtered at the glomerulus is returned to the blood. Most is reabsorbed at the proximal convoluted tube, while a sizeable amount is reabsorbed from the convoluted tubule and collecting duct.

Reabsorption of Salt Aldosterone regulates sodium reabsorption. Renin is an enzyme that converts angiotensinogen. Atrial natriuretic hormone promotes sodium excretion. Diuretics increase urine flow. Alcohol. Caffeine.

Homeostasis The kidneys are primary organs of homeostasis because they maintain acid-base balance and water-salt balance of blood. Kidneys reabsorb bicarbonate ions and excrete hydrogen ions as needed to maintain normal blood pH.

Problems with Kidney Function Many types of illness cause progressive renal disease and ultimate renal failure. Diabetes. Hypertension. Inherited conditions. Treatment of renal failure can include hemodialysis and kidney replacement.

Review Overview of the Urinary System Functions of the Urinary System Urination Kidney Structure Nephrons Urine Formation Maintaining Water-Salt Balance Reabsorption of Water Reabsorption of Salt Kidney Problems