The Kidney: Anatomy and Physiology PDF

**The Kidney: Anatomy and Physiology** The kidneys are vital organs responsible for filtering blood, removing waste products, regulating electrolyte balance, and maintaining acid-base homeostasis. This unit will cover the anatomy and physiology of the kidneys, the urinary tract, and the mechanisms...

**The Kidney: Anatomy and Physiology** The kidneys are vital organs responsible for filtering blood, removing waste products, regulating electrolyte balance, and maintaining acid-base homeostasis. This unit will cover the anatomy and physiology of the kidneys, the urinary tract, and the mechanisms underlying urine formation. It will also address common clinical conditions related to kidney function and the urinary system. **1. Anatomy of the Kidney** The kidneys are paired, bean-shaped organs located on either side of the spine, just below the rib cage. They play a crucial role in filtering blood and producing urine. **Cortical and Juxtamedullary Nephrons:** - **Nephrons:** The functional units of the kidney, each kidney contains approximately 1 to 1.5 million nephrons. - **Cortical Nephrons:** Located primarily in the renal cortex, these nephrons have shorter loops of Henle and are responsible for the bulk of filtration. - **Juxtamedullary Nephrons:** These nephrons have longer loops of Henle that extend deep into the medulla. They are crucial for concentrating urine and conserving water. **Glomerulus and Bowman\'s Capsule:** - **Glomerulus:** A network of capillaries located at the beginning of the nephron. The glomerulus filters blood, allowing water, electrolytes, and small molecules to pass into the Bowman\'s capsule while retaining larger molecules like proteins and blood cells. - **Bowman\'s Capsule:** A cup-like sac that surrounds the glomerulus and collects the filtrate to be processed in the renal tubules. **2. Anatomy of the Bladder** The bladder is a hollow, muscular organ that stores urine before it is excreted from the body. **Bladder Structure:** - The bladder has a muscular wall made of smooth muscle fibres known as the detrusor muscle. The internal urethral sphincter, made of smooth muscle, controls the release of urine from the bladder. **Bladder Function:** - The bladder's primary function is to store urine until it reaches a volume that triggers the urge to urinate, at which point the bladder contracts and the sphincter muscles relax to allow urine to flow out through the urethra. **3. Common Clinical Conditions** **Anuria:** - Defined as the absence of urine production or a very minimal amount (less than 50 ml/day). It can indicate severe kidney dysfunction or obstruction in the urinary tract. **Polyuria:** - The production of abnormally large volumes of urine, often exceeding 3 litres per day. This can occur in conditions like diabetes mellitus or diabetes insipidus. **Oliguria:** - A significantly reduced urine output, typically less than 400 ml/day or less than 20 ml/hour. It may be a sign of acute kidney injury or severe dehydration. **Urinary Tract Infection (UTI):** - Infections of any part of the urinary system, including the kidneys, bladder, or urethra. Symptoms often include a strong urge to urinate, burning sensation during urination, and cloudy or strong-smelling urine. **Benign Prostatic Hyperplasia (BPH):** - Also known as prostate gland enlargement, BPH is common in older men and can cause urinary retention, nocturia, and a weak urine stream. **Urinary Retention:** - The inability to empty the bladder completely. It can be acute or chronic and may result from nerve dysfunction, obstruction, or weakened bladder muscles. **Overactive Bladder:** - A condition characterized by a frequent and sudden urge to urinate that may be difficult to control. It can lead to incontinence and significantly affect quality of life. **Paruresis (Shy Bladder Syndrome):** - A type of social anxiety where a person is unable to urinate in the presence of others, even when there is an urgent need to do so. **Bladder Sphincter Dyssynergia:** - A condition where there is a lack of coordination between the detrusor muscle and the external urethral sphincter, leading to difficulty in urination and incomplete bladder emptying. **Stress Incontinence:** - Involuntary leakage of urine during activities that increase intra-abdominal pressure, such as coughing, sneezing, or lifting heavy objects. **Haematuria:** - The presence of blood in the urine, which can be a sign of infection, kidney stones, or more serious conditions like bladder or kidney cancer. **Proteinuria:** - The presence of an abnormal amount of protein in the urine, often indicative of kidney disease. **4. Physiology of the Kidney** The kidneys perform several critical functions essential for maintaining homeostasis. **Excretion and Water-Electrolyte Balance:** - **Filtration:** Blood is filtered at the glomerulus, forming a filtrate that enters the nephron. This process occurs at a rate of approximately 125 ml/min, resulting in 180 litres of filtrate per day. - **Selective Reabsorption:** The nephron selectively reabsorbs water, electrolytes, and nutrients back into the blood, depending on the body\'s needs. - **Selective Secretion:** Additional waste products and excess ions are secreted into the tubular fluid, which becomes urine. **Acid-Base Balance:** - The kidneys regulate the body\'s pH by excreting hydrogen ions and reabsorbing bicarbonate. This process helps maintain the blood pH within a narrow range. **Endocrine Secretions:** - **Erythropoietin:** A hormone produced by the kidneys that stimulates the production of red blood cells in the bone marrow in response to low oxygen levels. - **Renin:** An enzyme released by the kidneys in response to low blood pressure or low sodium levels. Renin initiates the renin-angiotensin-aldosterone system (RAAS), which helps regulate blood pressure and fluid balance. - **Vitamin D Activation:** The kidneys convert vitamin D into its active form, calcitriol (vitamin D3), which is essential for calcium absorption and bone health. **5. Ultrafiltration, Selective Reabsorption, and Secretion** **Glomerular Filtration Rate (GFR):** - **Definition:** The rate at which filtrate is formed in the kidneys, typically about 125 ml/min in healthy adults. This translates to around 180 litres of filtrate produced per day. - **Urine Formation:** Of the 180 litres of filtrate, only about 1 to 2 litres are excreted as urine each day. The rest is reabsorbed back into the bloodstream. **Obligatory and Facultative Reabsorption:** - **Obligatory Reabsorption:** The reabsorption of water and solutes that occurs regardless of the body\'s needs, primarily in the proximal convoluted tubule. - **Facultative Reabsorption:** The adjustable reabsorption of water and solutes in response to the body\'s needs, regulated by hormones like antidiuretic hormone (ADH). **Anti-Diuretic Hormone (ADH):** - **V1 and V2 Receptors:** ADH acts on V1 receptors to cause vasoconstriction and on V2 receptors in the kidneys to promote water reabsorption, concentrating the urine. **Renin-Angiotensin-Aldosterone System (RAAS):** - **Mechanism:** Renin is released in response to low blood pressure or low sodium levels, converting angiotensinogen to angiotensin I. Angiotensin I is then converted to angiotensin II, a potent vasoconstrictor that stimulates aldosterone release, promoting sodium and water retention to increase blood pressure. ### 6. Role of the Kidneys in pH Regulation The kidneys play a vital role in maintaining acid-base balance by regulating the levels of bicarbonate (HCO₃⁻) in the blood, as well as by excreting hydrogen ions (H⁺). This regulation is crucial for maintaining the body\'s pH within the narrow range necessary for proper cellular function. #### Metabolic Acidosis: - **Cause:** Metabolic acidosis occurs when there is an excess of acid in the body or a loss of bicarbonate, leading to a decrease in blood pH. This can be caused by conditions such as renal failure, diabetic ketoacidosis, or severe diarrhea. - **Compensation:** In response to metabolic acidosis, the lungs increase the rate and depth of breathing (hyperventilation) to expel more CO2, which helps raise the pH. The kidneys contribute by increasing the reabsorption of bicarbonate and increasing the excretion of hydrogen ions in the urine, helping to restore pH balance. #### Metabolic Alkalosis: - **Cause:** Metabolic alkalosis occurs when there is an excess of bicarbonate in the blood or a loss of acid, leading to an increase in blood pH. This can be caused by conditions such as prolonged vomiting, excessive use of antacids, or diuretic use. - **Compensation:** In response to metabolic alkalosis, the lungs decrease the rate and depth of breathing (hypoventilation) to retain more CO2, which helps lower the pH. The kidneys respond by excreting excess bicarbonate in the urine and reducing the secretion of hydrogen ions, thereby helping to lower the pH to normal levels. **Multiple Choice Questions (MCQs)** 1. **What is the primary function of the juxtamedullary nephrons?** - a\) Filtration of blood - b\) Concentration of urine - c\) Secretion of erythropoietin - d\) Regulation of blood pressure 2. **Which hormone is primarily responsible for the regulation of water reabsorption in the kidneys?** - a\) Aldosterone - b\) Erythropoietin - c\) Renin - d\) Antidiuretic hormone (ADH) 3. **What condition is characterised by the presence of blood in the urine?** - a\) Proteinuria - b\) Haematuria - c\) Anuria - d\) Polyuria 4. **Which part of the nephron is primarily involved in selective reabsorption?** - a\) Bowman\'s capsule - b\) Proximal convoluted tubule - c\) Loop of Henle - d\) Collecting duct 5. **In which condition does the bladder fail to empty completely?** - a\) Polyuria - b\) Oliguria - c\) Urinary retention - d\) Stress incontinence **Clinical Cases** **Case 1: Acute Kidney Injury (AKI)** **Presentation:**\ A 65-year-old man presents with oliguria, swelling in the lower limbs, and fatigue. His recent blood tests show elevated creatinine and blood urea nitrogen (BUN) levels. **Discussion:** - **Question:** What could be the potential causes of acute kidney injury in this patient? Discuss the physiological mechanisms leading to oliguria in AKI. - **Answer:** Causes of AKI include reduced blood flow to the kidneys (prerenal), damage to the kidney tissue (intrarenal), or obstruction of urine flow (postrenal). Oliguria in AKI results from decreased glomerular filtration rate (GFR), increased reabsorption of water, or obstruction in the urinary tract. **Case 2: Urinary Tract Infection (UTI)** **Presentation:**\ A 30-year-old woman presents with dysuria, frequent urination, and lower abdominal pain. Urinalysis reveals the presence of bacteria and white blood cells. **Discussion:** - **Question:** Describe the pathophysiology of UTIs and why they are more common in women. What are the potential complications if left untreated? - **Answer:** UTIs occur when bacteria enter the urinary tract, usually through the urethra, and multiply. Women are more prone to UTIs due to their shorter urethra and proximity to the anus. Complications of untreated UTIs include pyelonephritis (kidney infection) and sepsis.