Biology Lecture 17: Urinary System PDF

Biology Lecture 17 : Urinary System Urinary system : Kidney :  Pair of bean-shaped structures  Located just above the waist.  The adrenal glands sit on top of each kidney and function as a component of the endocrine system.  Nephrons are the structural and functional units of the kidneys.  A si...

Biology Lecture 17 : Urinary System Urinary system : Kidney :  Pair of bean-shaped structures  Located just above the waist.  The adrenal glands sit on top of each kidney and function as a component of the endocrine system.  Nephrons are the structural and functional units of the kidneys.  A single kidney may have more than a million nephrons! Urinary system : Kidney : Functions of kidney : 1. Filter and purify blood many times per day , produce filtrate (urine) 2. Maintain osmotic balance and blood pressure in the body, 3. Have endocrine function : a. Erythropoietin stimulates bone marrow to produce red blood cells b. Secrete Renin, which regulates blood pressure c. Calcitriol, the active form of vitamin D Urinary system : Kidney : Urinary system : Kidney : Urinary system : Renal circulation :  Blood enters the kidney through the renal artery,  Renal arteries split to give branching of afferent arterioles, and then the capillaries supplying the nephrons.  When blood passes through capillaries of the glomerulus of a nephron, blood pressure forces some of the water and dissolved substances in the blood to cross the capillary walls into Bowman’s capsule. Urinary system : Urine Formation :  The filtered substances pass to the renal tubule of the nephron.  In the renal tubule, some of the filtered substances are reabsorbed and returned to the bloodstream.  Other substances are secreted into the fluid.  The fluid passes to a collecting duct, which reabsorbs some of the water and returns it to the bloodstream.  The fluid that remains in the collecting duct is urine. Urinary system : Urine Formation : Urinary system : Urine Formation : Urinary system : Urine Formation :  From the collecting ducts of the kidneys, urine enters the ureters ; two muscular tubes (30 cm long) that move the urine by peristalsis to the bladder.  The bladder is a hollow, sac-like organ that stores urine.  When the bladder is about half full, it sends a nerve impulse to a sphincter to relax and let urine flow out of the bladder and into the urethra.  The urethra is a muscular tube that carries urine out of the body.  Urine leaves the body through another sphincter in the process of urination.  This sphincter and the process of urination are normally under conscious control. Urinary system : Urine Formation : Urinary system : Urethra :  All urine transport structures are identical in male and female except urethra.  In females, the urethra is relatively short length, about 4 cm, and is less of a barrier to fecal bacteria than the longer male urethra (approximately 20 cm).  A greater incidence of urinary tract infections (UTIs) in women related to length difference. Urinary system : Urethra : Urinary system : Kidney Structure : Internally, the kidney has three regions : 1) Outer cortex, a 2) Medulla in the middle, and the 3) Renal pelvis in the the hilum of the kidney. The hilum is the concave part where blood vessels and nerves enter and exit the kidney; and is exit point for the ureters. Urinary system : Nephron : Consists of three parts a) Renal corpuscle, b) A renal tubule, c) And the associated capillary network. Urinary system : Nephron : Urinary system : Kidney Structure :  Nephron tubules can be found throughout the renal cortex and renal pyramids,  Renal pyramids make up the majority of the renal medulla.  There are, on average, eight renal pyramids in each kidney.  Urine that is produced by the nephrons travels into the renal pelvis and then into the ureters. Urinary system : Kidney Structure : Renal Corpuscle and Renal Tubule : Renal Corpuscle :  Located in the renal cortex, made up of a network of capillaries known as the glomerulus and the capsule, a cup-shaped chamber that surrounds it, called the glomerular or Bowman's capsule. Urinary system : Kidney Structure : Renal Tubule :  The renal tubule is a long and convoluted structure , emerges from the glomerulus  Divided into three parts based on function. 1. The proximal convoluted tubule (PCT) , 2. The loop of Henle, Forms a loop (with descending and ascending limbs). 3. Distal convoluted tubule (DCT) , Connects and empties its contents into collecting ducts. The urine will ultimately move into the renal pelvis and then into the ureters. Urinary system : Kidney Structure : Renal Tubule : Urinary system : Kidney Structure : Capillary Network within the Nephron  The capillary network that originates from the renal arteries supplies the nephron with blood  Afferent arteriole is the branch that enters the glomerulus  Efferent arteriole is the branch that exits the glomerulus  Glomerular capillary bed is within the glomerulus  Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary network, which surrounds and interacts with parts of the renal tubule. Urinary system : Kidney Structure : Urinary system : Kidney Structure : Urinary system : Kidney Function : Kidney Function and Physiology  Kidneys act on blood in a three-step process.  First, the nephrons filter blood that runs through the capillary network in the glomerulus.  Almost all solutes, except for proteins, are filtered out into the glomerulus by a glomerular filtration.  Second, the filtrate is collected in the renal tubules.  Third , Most of the solutes get reabsorbed in the PCT by tubular reabsorption process. Urinary system : Kidney Function : Kidney Function and Physiology :  In the loop of Henle, the filtrate continues to exchange solutes and water with the peritubular capillary network.  Water is also reabsorbed during this step.  Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion, the opposite process to tubular reabsorption.  The collecting ducts collect filtrate coming from the nephrons ( urine) Urinary system : Kidney Function : Glomerular filtration :  Filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole.  The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure !!  The leaky connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily.  All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion.  Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys , is an important indicator of kidney function. Urinary system : Kidney Function : Tubular Reabsorption and Secretion  Tubular reabsorption mostly occurs in the PCT.  Almost all nutrients are reabsorbed (either by passive or active transport).  Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones.  Sodium (Na+) is the most abundant ion and most of it is reabsorbed by active transport Urinary system : Kidney Function : Tubular Reabsorption and Secretion  Na+ is actively transported out of the tubule, water follows it.  Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT.  In the loop of Henle, the permeability of the membrane changes.  The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Urinary system : Kidney Function : Aldosterone :  A hormone synthesized by the adrenal cortex.  Affects urine concentration by regulating sodium levels in the blood.  Almost all of the sodium in the blood is reclaimed by the renal tubules under the influence of aldosterone.  Because sodium is always reabsorbed by active transport and water follows sodium to maintain osmotic balance, aldosterone manages not only sodium levels but also the water levels in urine.  Aldosterone favors the production of a concentrated urine. Urinary system : Kidney Function : Anti‒diuretic Hormone :  Antidiuretic hormone or ADH, helps the body conserve water when body fluid volume, especially that of blood, is low.  It is formed by the hypothalamus and is stored and released from the posterior pituitary gland.  It acts on the collecting ducts and promotes reabsorption of water.  This action results in the formation of a concentrated urine.  ADH also acts as a vasoconstrictor and increases blood pressure during hemorrhaging. Urinary system : Kidney Function : Anti‒diuretic Hormone :