KIDNEY STRUCTURE AND PHYSIOLOGY LEARNING GUIDE, Resúmenes de Fisiología

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KIDNEY

STRUCTURE

AND

PHYSIOLOGY

L E A R N I N G G U I D E

ACKNOWLEDGEMENTS

EDITOR

Anthony A. Killeen, M.D., Ph.D., the editor of this entry in the Abbott Learning Guide series, is Professor and Vice-Chair for Clinical Affairs in the Department of Laboratory Medicine and Pathology at the University of Minnesota®. His research interests include quality performance in clinical laboratories, and clinical trials related to the epidemiology of cardiovascular disease.

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INTRODUCTION

KIDNEY STRUCTURE AND PHYSIOLOGY LEARNING GUIDE........ 5

SECTION 1

THE KIDNEYS.................................................. 6

SECTION 2

URINE FORMATION, TRANSPORT AND ELIMINATION............. 16

SECTION 3

RENAL REGULATION OF BLOOD COMPOSITION AND VOLUME... 25

SECTION 4

MINERAL HOMEOSTASIS...................................... 33

APPENDICES

APPENDIX A: GLOSSARY OF TERMS............................. 40

APPENDIX B: REFERENCES..................................... 42

APPENDIX C: CORRECT RESPONSES............................ 43

CONTENTS

4 | Learning Guide: Kidney Structure and Physiology

INTRODUCTION

KIDNEY STRUCTURE

AND PHYSIOLOGY

LEARNING GUIDE

Correct functioning of the urinary system is a critical component of homeostasis in the body. Urine formation and the excretion of wastes is one of the primary functions of the kidneys. The kidneys also help to maintain blood composition, pH and osmolarity, and regulate blood volume, blood pressure and red blood cell production through hormonal mechanisms.

The kidneys are also responsible for producing biologically active vitamin D hormone (calcitriol), which plays a critical role in the regulation of mineral homeostasis, and of calcium and phosphate in particular. The body’s delicate mineral balance is orchestrated by complex hormonal feedback mechanisms that involve the kidneys, the thyroid and parathyroid glands, bone, and the intestines, and are mediated by parathyroid hormone (PTH), calcitriol and other hormones.^1

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FAST FACT

The adrenal glands are a pair of flattened, pyramid-shaped glands that lie on top of the kidneys. They produce the hormones norepinephrine and epinephrine. These two hormones regulate the stress response, which increases heart rate and blood pressure. The adrenal glands also produce aldosterone, which will be discussed later in this module.^1

OVERVIEW OF THE KIDNEYS

The kidneys are a pair of kidney bean–shaped organs located just above the waist, between the peritoneum and the back of the abdomen. The two kidneys lie behind the liver and the intestines in the small of the back. They are partially protected by the 11th and 12th pair of ribs. As shown in Figure 1-1 , the left kidney is slightly higher than the right, since the liver occupies the space above the kidney on the right side.^2

Right kidney Left kidney

Adrenal gland

Right ureter Left ureter Urinary bladder Urethra

Figure 1-1: The organs of the urinary system.^2

Describe the functions of the kidney Identify the basic structures of the kidney Describe the function of the nephron Describe the structures that transport urine out of the body after it exits the kidneys

LEARNING

OBJECTIVES

After completing this section, you will be able to:

1 2 3 4

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FUNCTIONS

The kidneys are the hardest working organs of the urinary system. The other components of the urinary system serve mainly as passageways or for storage of urine.

The functions of the kidneys include^2 :

• Excreting wastes and foreign substances in the urine
• Regulating various properties of blood, including:
  • Ionic composition — by regulating the concentrations of several ions, such as sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl-) and phosphate (HPO 4 2-)
  • pH — by excreting hydrogen ions (H+) and conserving bicarbonate ions (HCO 3 - )
  • Osmolarity — by separately regulating the loss of water and solutes in the urine
  • Blood volume — by conserving or eliminating water in the urine, blood pressure is increased or decreased
  • Blood pressure — by secreting the enzyme renin , a component of the renin-angiotensin- aldosterone (RAA) system ; renin causes an increase in blood pressure
  • Blood glucose levels — by synthesizing and releasing new glucose molecules
• Producing hormones :
  • Calcitriol — the active form of vitamin D, which helps regulate calcium levels
  • Erythropoietin (EPO) — which stimulates the production of red blood cells

CLINICAL CONNECTION

The kidneys regulate blood pressure by producing the enzyme renin. An increase in the production of renin results in an increase in blood pressure.^2

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STRUCTURE OF THE KIDNEYS

FAST FACT

A typical adult kidney is 10–12 cm (4–5 in) long, 5–7 cm (2–3 in) wide and 3 cm (1 in) thick, roughly the size of a bar of bath soap, and weighs 135–150 g (4.5–5 oz).^2

If one was to slice a kidney in half (as illustrated by Figure 1-3 ), two distinct regions would be revealed:

• A superficial, smooth-textured reddish exterior portion called the renal cortex
• A deep, reddish-brown inner region called the renal medulla

Adrenal gland

Renal artery

Renal vein

Renal pelvis Fat in renal sinus

Ureter

RENAL CORTEX

(renal pyramid)

Minor calyx

Major calyx

RENAL MEDULLA

Figure 1-3: Internal anatomy of the kidneys.^2

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RENAL CORTEX

The renal cortex refers to the smooth-textured area extending from the exterior (renal capsule) to the bases of striated, cone-shaped structures called renal pyramids and into the spaces between them. The renal capsule is the membrane that covers the surface of the kidney.

RENAL MEDULL A The renal medulla consists of 8–18 renal pyramids. The base of each pyramid faces toward the renal cortex, and the apex of each pyramid, called the renal papilla, points to the renal hilum (the area of the kidney through which the ureter, blood vessels, lymphatic vessels, and nerves emerge).^2 The portions of the renal cortex that extend between renal pyramids are called renal columns. A renal lobe consists of 2 :

• A renal pyramid
• The overlying area of the renal cortex
• Half of each adjacent renal column

NEPHRON Together the renal cortex and renal medulla constitute the functional portion of the kidney, called the parenchyma. The parenchyma contains approximately 1 million microscopic structures called nephrons , which are the filtration units of the kidney.^2

FAST FACT

The nephrons are the functional units of the kidneys. Nephrons and collecting ducts perform three basic functions: filtration of the blood, reabsorption of water and solutes, and secretion of wastes from the blood. These processes help to maintain homeostasis within the body.^2

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LOOP OF HENLE

The loop of Henle connects the proximal and distal convoluted tubules. The descending part of this structure dips into the renal medulla. It next makes a hairpin turn and ascends to the renal cortex. Approximately 80%–85% of nephrons reside in the outer portion of the renal cortex. The other 15%–20% of nephrons exist deep in the cortex. Additional filtered ions and water are reabsorbed from the loop of Henle.^2

DISTAL CONVOLUTED TUBULES Resorption of remaining Na+, Cl-, Ca2+^ and water continues as fluid flows along the distal convoluted tubule. By the time the fluid reaches the end of the distal convoluted tubule, 90%–95% of filtered solutes and water have returned to the bloodstream. Remaining substances that have not been reabsorbed are excreted in the urine.^2

DUCTS, CALYCES AND THE RENAL PELVIS Distal convoluted tubules from several nephrons empty into a single collecting duct. The collecting ducts unite and later converge into large papillary ducts, which then drain into cuplike structures called calyces. Urine then moves into a single large cavity called the renal pelvis, and subsequently through the ureter into the urinary bladder.^2

CLINICAL CONNECTION

Acute kidney injury (AKI), an abrupt and sustained decline in renal function, leads to loss of the affected nephrons. In 80% of patients, necrosis (death) of tubular cells, but not glomeruli or surrounding tissues, occurs. Causes of AKI may include oxygen deficiency, sepsis or toxic kidney injury.^4

RENAL BLOOD SUPPLY

The kidneys are supplied with numerous blood vessels due to their critical function in waste removal and the regulation of blood volume and composition. Despite their small size (less than 0.5% of total body mass), the kidneys receive 20%–25% of the volume of blood pumped by the heart while at rest (resting cardiac output). In healthy adults, renal blood flows through both kidneys at a rate of approximately 1,200 mL per minute.^2

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LOWER URINARY TRACT

Urine drains from the kidneys through the ureters into the bladder, and subsequently out through the urethra during urination ( micturition ).^2

URETERS There are two ureters — one for each kidney. They transport urine from the renal pelvis to the urinary bladder. Along with gravity and hydrostatic pressure, muscular contractions push urine through the ureters. The ureters are long and thick-walled narrow tubes. They measure 25–30 cm (10–12 in) in length and vary in diameter from 1–10 mm between the renal pelvis and the urinary bladder. Like the kidneys, the ureters are positioned behind the peritoneum. They curve at their ends and pass through the wall of the bladder.^2

Although there is no anatomical valve between each ureter and the bladder, a physiological one exists. As the bladder fills with urine, it compresses the openings to the ureters, and prevents the backflow of urine. If this valve does not operate properly, microbes may travel up the ureters to cause a kidney infection.^2

URINARY BLADDER The bladder stores urine. Situated in the pelvic cavity, the urinary bladder is a hollow, distensible muscular organ. In males it is directly in front of the rectum, whereas in females it is in front of the vagina and below the uterus. Held in place by folds of the peritoneum, the bladder becomes slightly distended and spherical as it fills with urine. When empty, the bladder collapses. Urinary bladder capacity ranges from 700–800 mL, and is slightly smaller in females due to the position of the uterus.^2

FAST FACT

When the volume of the bladder exceeds 200–400 mL, pressure within the bladder increases. Stretch receptors in the bladder transmit nerve impulses into the spinal cord, triggering a spinal reflex that gives the sensation of a full bladder. This creates a conscious desire to urinate. By controlling the urethral sphincter and pelvic muscles, a person can initiate or delay urination for a limited period.^2

URETHRA

The urethra carries urine out of the body. It is a small tube that leads from the urinary bladder to the exterior of the body^2 :

• In females, the urethra has a length of 4 cm (1.5 in) and opens to the exterior through the external urethral orifice between the clitoris and the vaginal opening
• In males, the urethra is approximately 20 cm (8 in) long and extends from the urinary bladder to the exterior, but the urethra first passes through the prostate, then through the deep muscles of the perineum and lastly through the penis

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SECTION 2

URINE FORMATION,

TRANSPORT AND

ELIMINATION

COMPOSITION OF URINE 17 GLOMERULAR FILTRATION 20

TUBULAR REABSORPTION AND SECRETION 22

REVIEW QUESTIONS 24

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COMPOSITION OF URINE

Routine assessment of kidney function involves the evaluation of both the quantity and quality of urine, as well as the levels of waste in the blood. The analysis of the volume, physical, chemical and microscopic properties of urine is called a urinalysis. It is one test physicians use to assist them in their routine assessment of kidney function and general physical health.^2 A normal adult will eliminate 1–2 liters of urine per day. The characteristics of normal urine are listed in Table 2-.

Understand the normal composition of urine Describe the process of urine formation Describe how glomerular filtration rate (GFR) is related to kidney function Identify the differences between the reabsorption and secretion processes

LEARNING

OBJECTIVES

After completing this section, you will be able to:

1 2 3

4

Characteristic Description

Volume 1–2 liters in 24 hours Can vary with fluid intake

Color

Yellow or amber Varies with concentration and diet Certain medications and certain diseases may also affect color

Turbidity Transparent when freshly voided Becomes turbid (cloudy) upon standing

Odor

Mildly aromatic Becomes ammonia-like upon standing Can also vary with diet and certain diseases

pH 4.6–8.0 (average of 6.0) Varies with diet

Specific gravity (density) 1.001–1.035 Increases as concentration of solutes increases

Table 2-1: Characteristics of normal urine.^2

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CLINICAL CONNECTION

Several different tests are used to evaluate kidney disease. These tests include measurement of protein or albumin levels in urine and calculation of the rate at which creatinine is cleared in the urine. The urinary albumin-to-creatinine ratio (UACR) is one method of quantifying proteinuria.^5 Blood tests, as well as urinalysis, can be used to evaluate kidney function. Blood urea nitrogen (BUN) is a test that measures the levels of blood nitrogen from the urea that is generated by the breakdown of amino acids. BUN is elevated when renal disease or obstruction of the urinary tract occurs and glomerular filtration rate (GFR) is decreased or when the renal blood flow is reduced.^2 Estimates of GFR (eGFR) are considered the best overall measure of the level of kidney function. To estimate the GFR, prediction equations take into account serum creatinine concentration and some or all of the following variables: age, gender, race and body size.^5 An emerging method of estimating GFR is based on blood levels of cystatin C , a small protein freely filtered by the glomerular membrane and degraded in the proximal tubules.^6

Table 2-2 lists the components of urine, and the amount filtered, reabsorbed and excreted per day.

Substance

Filtereda (enters glomerular capsule per day)

Reabsorbed (returned to blood per day)

Urine (excreted per day)

Water 180 liters 178–179 liters 1–2 liters Proteins 2.0 g 1.9 g 0.1 g Sodium ions (Na+) 579 g 575 g 4 g Chloride ions (Cl-) 640 g 633.7 g 6.3 g Bicarbonate ions (HCO 3 - ) 275 g 274.97 g 0.03 g Glucose 162 g 162 g 0 g Urea 54 g 24 g 30 gb Potassium ions (K+) 29.6 g 29.6 g 2.0 gc Uric acid 8.5 g 7.7 g 0.8 g Creatinine 1.6 g 0 g 1.6 g a. Assuming GFR is 180 liters per day. b. In addition to being filtered and reabsorbed, urea is secreted. c. After virtually all filtered K+^ is reabsorbed in the convoluted tubules and loop of Henle, a variable amount of K+^ is secreted in the collecting duct.

Table 2-2: Substances filtered, reabsorbed and excreted by the healthy kidney.^2

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GLOMERULAR FILTRATION

As shown in Figure 2-1 , glomerular filtration is the initial step toward urine production. Water and most of the solutes in blood plasma move across the walls of the glomerular capillaries (the smallest of the blood vessels) into the glomerular capsule and into the renal tubule. Solutes that are in the fluid draining into the renal pelvis stay in the urine and are eventually excreted.^2

1. FILTRATION

Peritubular capillaries

Renalcorpuscle

BLOOD FLOW

Glomerulus capsule

Afferent arteriole Efferent arteriole

2. REABSORPTION (solutes, water) 3. SECRETION (additional wastes) 4. EXCRETION

Nephron

Renaltubule

Figure 2-1: Relation of the structure of a nephron to its three basic functions: glomerular filtration, tubular reabsorption and tubular secretion.^2

FILTRATION PROCESS

Filtration is the first basic function of the nephron. The endothelial cells of glomerular capillaries, plus a layer of epithelial cells that encircle the capillaries, form a leaky barrier known as the filtration membrane. It allows the filtration of most water and small solutes, but prevents the filtration of plasma proteins, blood cells and platelets. The filtration membrane is a sandwich-like assembly. A solute’s size and charge determine how easily it can pass through glomerular capillary membranes.^2

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