Chapter 24 The Urinary System -PDF 1

Basic A & P II Dr. L. Bacha Chapter Outline (Martini & Nath 2010)

list the three major functions of the urinary system: by examining Fig. 24-1, list the organs of the urinary system:

describe the location of the kidneys relative to the vertebral column: they are retroperitoneal, which means they are situated between the parietal peritoneum

and muscles of the back

describe the fibrous capsule: does adipose tissue surround the kidneys? a typical adult kidney is about the size of a large bar of soap! describe the hilum:

SECTIONAL ANATOMY OF THE KIDNEYS

describe the following (and examine the kidney in Figs. 24-3 and 24-4)

renal sinus -

renal cortex - renal medulla -

Chapter 24 The Urinary System -PDF 2

renal papilla - renal lobe: consists of one renal pyramid and the overlying area of renal cortex

on Fig. 24-4, locate the following parts of the duct system that collects urine from the renal

papillae (a): ureter renal pelvis minor calyx major calyx

about how many nephrons are in each kidney?!

BLOOD SUPPLY AND INNERVATION OF THE KIDNEYS

what percent of the total cardiac output do the kidneys receive?

examine Fig. 24-5 and identify the first 2 blood vessels listed below:

renal artery (one branches to each kidney from the abdominal aorta)

various arteries which eventually branch into

cortical radiate arteries (extend up into the cortex)

afferent arterioles

glomerulus = a ball-like network of glomerular capillaries within the glomerular capsule

efferent arterioles

peritubular capillaries (network of capillaries that surround the PCTs, DCTs and part of collecting duct, which we will get to!) and vasa recta (capillary loops that surround the loop of Henle and collecting ducts)

various veins that eventually lead into renal veins inferior vena cava

THE NEPHRON

nephrons are the microscopic, functional units of the kidneys

about 1.25 million in each kidney!

each nephron consists of what two parts?

Chapter 24 The Urinary System -PDF 3

Renal Corpuscle - a renal corpuscle includes what two parts?

Glomerular Capsule (Bowman’s capsule): The glomerular capsule consists of three parts (see Fig. 24-8):

1. capsular epithelium = simple squamous epithelium; forms the outer wall of the capsule

2. visceral epithelium - formed by unique, modified epithelial cells called podocytes (we’ll get back to these!)

3. the capsular space - the space between the two epithelial layers into which fluid filtered from the glomerular capillaries enters

Glomerulus = a ball-like network of glomerular capillaries that are fenestrated capillaries

- the renal corpuscle is where blood is filtered during a process called glomerular filtration to form fluid called glomerular filtrate

Renal Tubule

◦ the renal tubule is the tubular portion of a nephron into which filtrate passes

◦ reabsorption and secretion occur here

◦ examine the parts of the renal tubule in Fig. 24-6

◦ parts of the renal tubule: proximal convoluted tubule (PCT)

loop of Henle (1) thick descending limb (2) thin segment (3) thick ascending limb

distal convoluted tubule [DCT] the DCTs of several nephrons empty into a collecting duct

Collecting duct - leads into a papillary duct that opens at the renal papilla

List and summarize the three distinct processes of urine formation (page 840; Fig. 24-9!):

What does “proximal” refer to

here? “distal”? “convoluted”?

The names of some of the parts

of the renal tubule make sense if

you know what the terms mean!

Chapter 24 The Urinary System -PDF 4

I. GLOMERULAR FILTRATION

during glomerular filtration, blood pressure forces water and small solutes across the filtration membrane into the capsular space (larger solutes, such as plasma proteins, and formed elements are excluded) glomerular filtration results in the formation of fluid called glomerular filtrate in the capsular

space

The Filtration Membrane is what substances must pass through from the blood of the glomerulus into the capsular space during glomerular filtration

The filtration membrane is formed by three components (see Fig. 24-8):

1. Fenestrations of the endothelial cells of the glomerular capillaries pores in the endothelial cells that prevent filtration of blood cells and platelets

2. Basement membrane of the glomerular capillaries

3. Filtration slits between the pedicels of the podocytes that form the visceral epithelium of the glomerular capsule

the visceral layer of the glomerular capsule consists of modified simple squamous cells called podocytes; these podocytes have numerous footlike processes called pedicels that wrap around the endothelial cells of the glomerular capillaries; the spaces between the pedicels are called filtration slits

Net Filtration Pressure glomerular filtration depends on three main pressures:

1. glomerular hydrostatic blood pressure (page 845) define glomerular hydrostatic pressure: how does it compare to capillary pressures elsewhere in the systemic circuit?

what is the average glomerular hydrostatic pressure?

2. capsular hydrostatic pressure capsular hydrostatic pressure is the pressure exerted against the filtration membrane

by the fluid that is already in the capsular space and renal tubule does this pressure oppose or enhance filtration? what is the average capsular hydrostatic pressure?

3. blood colloid osmotic pressure blood colloid osmotic pressure is the osmotic pressure in the blood of the glomerulus

resulting from the presence of what? does this pressure oppose or enhance filtration?

what is the average blood colloid osmotic pressure?

Chapter 24 The Urinary System -PDF 5

net filtration pressure is the total pressure that promotes filtration stay tuned…we will write an equation that shows how a normal NFP is calculated:

Glomerular Filtration Rate define glomerular filtration rate (GFR):

what is the average GFR?

read about the value of a creatinine clearance test:

Control of GFR (p. 846)

homeostasis of body fluids requires that the kidneys maintain a relatively constant GFR - what happens if glomerular filtration does not occur (or if the GFR is too low)?

filtration depends on what:

there are three mechanisms that stabilize GFR:

(1) Autoregulation of GFR - what does autoregulation maintain?

- maintenance of GFR is accomplished mainly by changing the diameter of the afferent

arterioles

e.g., decrease in systemic blood pressure afferent arterioles dilate blood flow increases through the glomerulus increases GFR to previous level

e.g., elevated systemic blood pressure afferent arterioles contract narrows the arteriole’s lumen blood flow decreases through the glomerulus reduces GFR to previous level

Chapter 24 The Urinary System -PDF 6

(2) Hormonal Regulation of GFR - when blood volume or hydrostatic blood pressure are low, the cells of the kidney secrete

a hormone called renin into the blood, which ultimately leads to the formation of angiotensin II

angiotensin II causes: 1. vasoconstriction of systemic arterioles, which increases systemic blood pressure 2. the adrenal cortex to release aldosterone

∙ aldosterone increases the reabsorption of sodium ions and water by the kidneys to increase blood volume and blood pressure back to normal

- atrial natriuretic peptide - is released when the walls of the atria are stretched; it increases GFR, so that the kidneys excrete more sodium and water into urine; this results in a decrease in blood volume and, therefore, hydrostatic blood pressure

(3) Autonomic Regulation of GFR

- sympathetic nerves cause vasoconstriction of afferent arterioles, decreasing the GFR and slowing the production of filtrate

e.g., with exercise or hemorrhage increase sympathetic stimulation of smooth muscle of afferent arterioles vasoconstriction of afferent arterioles decrease blood flow through glomerulus decreases GFR this lowering of renal blood flow, and therefore of GFR, has two consequences: it decreases urine output to help conserve blood volume and it allows greater blood flow to other parts of the body

II. TUBULAR REABSORPTION (this is my summary of all the detail in the book!!!)

as tubular fluid flows along the renal tubule and collecting duct, substances in the fluid pass from the lumen of the tubule through the epithelial cells lining the tubule into the blood in surrounding peritubular capillaries and vasa recta

different substances get reabsorbed through different parts of the renal tubule and by different mechanisms (active transport; diffusion, etc.)

e.g. of substances that get reabsorbed: A. electrolytes B. amino acids C. glucose (I will go over this in class)

Once the glomerular filtrate begins to flow from the capsular space down through the renal tubule, it is referred to as tubular fluid.

Chapter 24 The Urinary System -PDF 7

D. water - water reabsorption is so important it gets its own heading…

Regulation of Water Reabsorption (and therefore, Urine Volume and Concentration)

1. ADH (antidiuretic hormone)

- ADH is a hormone produced by neurosecretory cells in the hypothalamus of the brain; it is stored and released by the posterior pituitary gland (the neurohypophysis)

- target of ADH = epithelial cells of the distal convoluted tubules (DCT) and collecting duct

- effect of ADH = causes the cell membrane of the epithelial cells of the DCT and collecting duct to be more permeable to water; this allows an increase in reabsorption of water.

(A diuretic is a substance that causes an increase in urine production.) summary of water reabsorption and influence of ADH:

80% of water in the filtrate is reabsorbed from the PCT to thin segment of the loop of Henle. This occurs regardless of the volume and concentration of urine that is finally produced by the kidney, and is referred to as obligatory water reabsorption.

How much of the remaining 19% of the water that can be reabsorbed is variable and depends on ADH. If ADH is present, water is reabsorbed from the DCT and collecting duct, so that more water in the filtrate is reabsorbed and less water remains in the filtrate as part of urine. (The following steps correspond to the diagram on the next page.)

1. NaCl in the filtrate entering the PCT is actively transported out of the PCT. Water follows by osmosis.

2. The descending portion is freely permeable to water. Water moves out by osmosis, because the interstitial tissue of the medulla of the kidney is hypertonic to the fluid in the tubule.

3. NaCl is actively transported from the ascending portion of the loop of Henle into the interstitial tissue, but the ascending tubule is NOT permeable to water.

4. As tubular fluid passes through the DCT and the collecting duct, water reabsorption and,

therefore, urine volume and concentration now depend on antidiuretic hormone (ADH):

a. A drop in blood hydrostatic pressure or an increase in osmotic pressure of the blood stimulates an increase in ADH secretion into the blood cells of the collecting duct and DCT are more permeable to water more water moves out of the DCT and collecting duct by osmosis into the interstitial

fluid and into surrounding blood vessels this increase in water reabsorption causes the excretion of a smaller volume of more concentrated urine the body conserves water

Chapter 24 The Urinary System -PDF 8

b. A rise in blood hydrostatic pressure or a decrease in osmotic pressure of the blood causes a decrease in ADH secretion cells of the collecting duct and DCT are less permeable to water and water remains

in these tubules as it passes through them decrease in water reabsorption and excretion of larger volume of more dilute urine the body gets rid of excess water

NOTE: The ascending portion of the loop of Henle actively transports NaCl from the tubule into the interstitial fluid, but is impermeable to water. Therefore, large quantities of NaCl accumulate in the interstitial fluid in increasing concentration from the cortex through the medulla, creating a hypertonic environment.

What if you are dehydrated on a hot summer day… would more or less ADH be secreted? would more or less water be reabsorbed? would more or less urine be produced?

What if you drink too much water… would more or less ADH be secreted? would more or less water be reabsorbed? would more or less urine be produced?

Chapter 24 The Urinary System -PDF 9

2. Renin-Angiotensin-Aldosterone

A decrease in blood hydrostatic pressure stimulates cells of the kidney to secrete an enzyme called renin into the blood which leads to the formation of angiotensin II

angiotensin II:

a. is a potent vasoconstrictor that increases blood hydrostatic pressure

b. stimulates the cells of the adrenal cortex to increase aldosterone secretion aldosterone increases Na+ reabsorption by the DCT and collecting duct more water is reabsorbed due to osmosis blood hydrostatic pressure increases

c. increases the sensation of thirst, salt desire, and ADH secretion 3. Diuretics

Diuretics are agents that increase the rate of urine formation. Examples of diuretics are:

a. alcohol - inhibits secretion of ADH

b. sodium ion reabsorption inhibitors (loop diuretics; e.g., furosemide) - decrease the reabsorption of Na+ and CL- from the thick ascending portion of the

loop of Henle and, therefore, decrease the amount of water that moves out of the filtrate (decrease reabsorption of water)

c. caffeine - promotes renal vasodilation, which increases GFR - decreases reabsorption of NaCl → less water is reabsorbed

d. osmotic diuretics (e.g., mannitol) - increase the osmotic pressure of the filtrate and, therefore decrease the amount of

water that moves out of the filtrate (decrease reabsorption of water) 4. Temperature

An increase in body temperature causes:

a. cutaneous blood vessels to dilate

b. vessels to the abdominal organs constrict decrease in blood flow to the kidney decrease in GFR decrease in volume of urine produced

A decrease in body temperature causes:

a. cutaneous blood vessels to constrict

b. vessels to the abdominal organs dilate increase in blood flow to the kidney → increase in GFR → increase in volume of urine produced

Chapter 24 The Urinary System -PDF 10

III. TUBULAR SECRETION

tubular secretion is the process of adding substances from the blood into the tubular fluid within the renal tubule

some substances that get secreted are ammonia, K+, H+

tubular secretion is important for maintaining blood pH by excreting H ions or retaining them

The Composition of Normal Urine (p.857)

Characteristics of Normal of Urine from Table 24-5, list these characteristics and indicate the normal range for each: Urinalysis

Components of Urine water (95%) electrolytes organic waste products excreted in the urine (see the right column on page 840)

- urea - an organic waste from the breakdown of what? - creatinine - what is creatinine generated in? - uric acid - what is uric acid formed during?

Chapter 24 The Urinary System -PDF 11

URETERS (p.860) ∙ 2 ureters, one from each kidney ∙ what do the ureters extend between? ∙ about how many inches long is each ureter?

∙ they enter the urinary bladder medially from the posterior side ∙ urine moves by peristalsis, pressure and gravity to the urinary bladder

URINARY BLADDER ∙ describe the urinary bladder and indicate what it functions as: ∙ what is the trigone of the urinary bladder? ∙ describe the location of the internal urethral sphincter: - it is formed by what type of muscle tissue?

- what do the smooth muscle fibers of this sphincter provide? Histology of the Urinary Bladder ∙ the muscularis of the urinary bladder consists of layers of smooth muscle; collectively, these layers form what muscle?

- what is the effect of contraction of this muscle? URETHRA ∙ small tube leading from the neck of the urinary bladder to the exterior of body. ∙ what does the urethra transport?

∙female - 1.5 inches long; male - about 8 in. long. ∙ note the two urethral sphincters shown in Fig. 24-18 (c)

∙ the external urethral sphincter is formed by what type of muscle tissue? - so, is it voluntary or involuntary?

∙ the opening of the urethra to the outside is the external urethral orifice

Chapter 24 The Urinary System -PDF 12

The Micturition Reflex and Urination

urine reaches the urinary bladder by what? what is this process coordinated by?

micturition is commonly known as urination

when the urinary bladder is stretched (with about 200 ml of urine), stretch receptors send impulses to the sacral region of the spinal cord

the sacral region of the spinal cord sends parasympathetic impulses that cause: relaxation of the internal urethral sphincter contraction of the smooth muscle of the bladder wall

interneurons in the spinal cord relay impulses to the cerebral cortex, where the sensation of fluid pressure in the urinary bladder is perceived the cerebral cortex sends somatic motor impulses that cause:

relaxation of the external urethral sphincter

urination occurs ! urination is a reflex, but after about 2 years of age, it can be initiated or stopped voluntarily by

impulses from the cerebral cortex why do infants lack voluntary control over urination? define incontinence:

THE END!