The Urinary System Removing waste, balancing blood pH, and maintaining water balance

The Importance of Removing Wastes The cells of the body obtain energy by converting complex organic compounds into simpler compounds. Many of these simpler compounds can be harmful and must be eliminated The lungs eliminate carbon dioxide The large intestine removes toxic wastes from the digestive system

Removing Waste The liver transforms ingested toxins, such as alcohol and heavy metals, and the hazardous products of protein metabolism into soluble compounds that can be eliminated by the kidneys The kidneys play a crucial role in removing waste, balancing blood pH, and maintaining water balance

Metabolic Waste When protein is used for energy, it is deaminated before it can enter the Krebs cycle The byproduct of deamination is ammonia Ammonia is extremely toxic— a buildup of as little as mg can kill humans In the liver, two molecules of ammonia combine with carbon dioxide, to form urea. Urea is times less toxic than ammonia.

The Kidney Renal arteries branch from the aorta and carry blood to the kidneys. Mass: about 0.5 kg May hold as much as 25% of the body’s blood at any given time.

The Urinary System Wastes are filtered from the blood and conducted to the urinary bladder by ureters. A urinary sphincter muscle located at the base of the bladder acts as a valve, permitting the storage of urine. Urine is voided through the urethra

on-of-the-Nephron on-of-the-Nephron

Structure of the Kidney The cross section of the kidney reveals three structures: – An outer layer of connective tissue: the cortex – An inner layer: the medulla, – A hollow chamber: the renal pelvis, joins the kidney with the ureter

Nephrons Approximately one million slender tubules, called nephrons, are the functional units of the kidneys

Parts of the Nephron Afferent arterioles: small branches that carry blood to the glomerulus Glomerulus: high-pressure capillary bed that is the site of filtration Efferent arterioles: small branches that carry blood away from the glomerulus to a capillary net Peritubular capillaries: network of small blood vessels that surround the nephron

Parts of a Nephron Bowman’s capsule: cuplike structure that surrounds the glomerulus Proximal tubule: section of the nephron joining the Bowman’s capsule with the loop of Henle Loop of Henle: carries filtrate from the proximal tubule to the distal tubule

Parts of the Nephron Distal tubule: conducts urine from the loop of Henle to the collecting duct Collecting duct: tube that carries urine from nephrons to the pelvis of a kidney

Nephron Small branches from the renal artery, the afferent arterioles, supply the nephrons with blood. The afferent arterioles branch into a capillary bed, called the glomerulus.

Nephrons Blood leaves the glomerulus by way of the efferent arterioles. Blood is carried from the efferent arterioles to a net of capillaries called peritubular capillaries that wrap around the kidney tubule.

The Nephron The glomerulus is surrounded by a funnel-like part of the nephron, called the Bowman’s capsule. The Bowman’s capsule, the afferent arteriole, and the efferent arteriole are located in the cortex of the kidney.

The Nephron Fluids to be processed into urine enter the Bowman’s capsule from the blood. The capsule tapers to a thin tubule, called the proximal tubule. Urine is carried from the proximal tubule to the loop of Henle, which descends into the medulla of the kidney.

The Nephron Urine moves through the distal tubule, the last segment of the nephron, and into the collecting ducts. As the name suggests, the collecting ducts collect urine from many nephrons that, in turn, merge in the pelvis of the kidney.

Nephrons

The Formation of Urine Urine formation depends on three functions: – Filtration – Reabsorption – Secretion

Filtration Each nephron of the kidney has an independent blood supply Blood moves through the afferent arteriole into the glomerulus which is a high-pressure filter. Normally, pressure in a capillary bed is about 25 mm Hg The pressure in the glomerulus is about 65 mm Hg

Filtration Dissolved solutes and water pass through the walls of the glomerulus into the Bowman’s capsule. Plasma protein, blood cells, and platelets are too large to move through the walls of the glomerulus. Smaller molecules such as Na +, H + glucose, amino acids, vitamins, minerals, urea, and uric acid pass through the cell membranes and enter the nephron.

Reabsorption On average, about 600 mL of fluid flows through the kidneys every minute. Approximately 20% of the fluid, or about 120 mL, is filtered into the nephrons. Only 1 mL of urine is formed for every 120 mL of fluids filtered into the nephron. The remaining 119 mL of fluids and solutes is reabsorbed.

Reabsorption in the Proximal Tubule 85% of the water in the filtrate is reabsorbed here Selective reabsorption of nutrients such as glucose and amino acids, vitamins and minerals occurs by active and passive transport. pH is controlled by secretion of hydrogen ions (H + ) and reabsorption of bicarbonate ions (HCO 3 - )

Reabsorption in the Descending Loop of Henle Descending limb of loop of Henle is permeable to water, resulting in loss of water from filtrate by osmosis. Salt (NaCl) becomes concentrated in filtrate as descending limb penetrates inner medulla

Reabsorption in the Ascending Loop of Henle The ascending limb of loop of Henle is permeable to salt, resulting in diffusion of salt out of ascending limb.

Reabsorption in the Distal Tubule Selective reabsorption of penicilin and other medications from blood into nephron occurs by active transport. Distal tubule helps regulate potassium (K + ) and salt (NaCl) concentration of body fluids.

Reabsorption in the Collecting Tubule As water is reabsorbed from the nephron, the remaining solutes become more concentrated. Molecules such as urea and uric acid will diffuse from the nephron back into the blood, although less is reabsorbed than was originally filtered.

Secretion Secretion is the movement of wastes from the blood into the nephron. Nitrogen-containing wastes, excess H + ions, and minerals such as K + ions are examples of substances secreted. Even drugs such as penicillin can be secreted. Tubular secretion occurs by active transport Molecules are shuttled from the blood into the nephron

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