Lecture 20.1 Kidney Anatomy and Function PDF

Lecture 20.1: Kidney anatomy and function and urine formation Introduction In this short review, we will discuss the three major functions of the kidney, which include excretion, elimination, and homeostatic regulation. We will then link these three functions with the anatomy of the urinar...

Lecture 20.1: Kidney anatomy and function and urine formation Introduction In this short review, we will discuss the three major functions of the kidney, which include excretion, elimination, and homeostatic regulation. We will then link these three functions with the anatomy of the urinary system, which includes the kidneys, ureters, bladder, and urethra. Lastly, we will briefly examine how blood is circulated in the kidney, how blood is filtered by the nephrons, and how reabsorption and secretion ensure solutes end up back in the blood and waste products end up concentrated in the urine. Blood flow through the kidney Normally, the kidneys are perfused with 20-25% of cardiac output. This is necessary not only because the kidneys perform excretion and homeostatic regulation but its cells must also be provided with adequate levels of oxygen and nutrients. The tubular cells of the nephron generally have higher metabolic rates than most cells. Blood flows through the renal arteries to enter the kidney. This blood enters the segmental arteries, the interlobar arteries, the arcuate arteries, and then the cortical radiate arteries before entering the afferent arterioles. Blood will then enter the glomerular capillaries to be filtered and leave the glomerulus via the efferent arterioles. This blood, which is waste- and nutrient-poor, remains oxygenated and will enter the peritubular capillaries to feed the cells of the nephrons. Although “nutrient-poor”, sufficient nutrients remain for the tubular cells, while additional nutrients are reabsorbed from other parts of the nephron. Blood that enters the venules is deoxygenated and leaves the kidneys via a similar route taken to come in and is returned to the heart. 1 Functions of the urinary system The urinary system has three important functions: excretion, homeostatic regulation, and elimination. Excretion and homeostatic regulation occur along the nephron. 1. Excretion is the process whereby solutes of the blood are filtered out of the blood and into the tubular fluid. This occurs across the glomeruli. Filtration ensures nitrogenous wastes, like urea, creatinine, ammonia, and uric acid, are removed from the blood, but it is indiscriminate, as glucose, amino acids, sodium, potassium, calcium, magnesium, bicarbonate, and hydrogen ions are also removed. Blood cells and large proteins (like albumin, globulins, and fibrinogen) are not filtered. 2. Homeostatic regulation is the process whereby important solutes are reabsorbed, while other solutes are secreted further along the nephron. The kidneys can make adjustments to the levels of sodium, potassium, hydrogen, and bicarbonate ions as necessary. Water commonly follows solute levels (notably sodium), so vascular volume can also be adjusted here. 3. Elimination is the process whereby urine is transported outside the body. 2 Additional details on renal physiology 1. Excretion occurs across the glomerular capillaries, whereby solutes from the blood enter the glomerular capsule; we call this filtrate. Filtration depends on the glomerular filtration rate (flow rate of filtered fluid through the kidney), which is largely dependent on arterial pressure but can be modified by adjusting the diameters of the afferent and efferent arterioles when arterial pressure changes. Arterial pressure and dilation of the afferent and efferent arterioles help define the glomerular hydrostatic pressure, which is the driving force that filters solutes and water from the blood into the glomerular capsule. Colloid pressure prevents excessive volumes of water from leaving the blood and entering the glomerular capsule; it is created by the plasma proteins in the blood (which do not enter the filtrate). 2. Homeostatic regulation occurs across the proximal convoluted tubule (PCT), the nephron loop, distal convoluted tubule (DCT), and collecting duct. The majority of reabsorption occurs across the PCT, less so along the nephron loop, and even less across the DCT. The PCT and nephron loop contribute greatly to hydrogen ion secretion. Further adjustments to serum electrolytes occur as the tubular cells along the DCT and collecting duct secrete and reabsorb certain ions; as these parts of the nephron are sensitive to hormones like antidiuretic hormone and aldosterone. For example, in the case of metabolic acidosis, the cells of the DCT can exchange H+ ions for K+ ions, effectively increasing serum pH fluids. Another important role of the kidneys is its endocrine function: it produces erythropoiesis (EPO) to stimulate red blood cell production in the red bone marrow, calcitriol to increase blood calcium levels, and renin to increase blood pressure by increasing sodium and water retention and triggering vasoconstriction. 3. Elimination occurs as urine flows from the renal pelvis into the ureters and is stored in the bladder. Urine remains stored in the bladder until sufficient pressure is created to encourage you to urinate. When this happens, urine is eliminated from the body via the urethra. 3 Glomerular filtration rate Glomerular filtration rate (GFR) is a measure of kidney function and describes the flow rate of fluid through the kidney (ml/min). A patient’s GFR is a function of the volume of serum filtered per minute and is adjusted for body surface area (ml/min/1.73m 2). A normal GFR measures between 100 and 130. Creatinine clearance is used to estimate GFR, as this measures the amount of creatinine cleared from the blood by the kidneys. However, it remains inaccurate because creatinine secretion increases when GFR decreases; this may overestimate GFR values. Inulin clearance remains the most accurate measurement of GFR because it is simply filtered by the kidneys; it is neither reabsorbed nor secreted by the tubular cells. Introduction to Kidney failure Renal failure occurs as the kidneys are unable to remove metabolic wastes and regulate fluid, electrolyte, and pH balances. Acute renal failure has an abrupt onset and is often reversible, while chronic renal failure is commonly the end result of irreparable damage. Acute renal failure is characterized by a rapid decline in kidney function resulting in homeostatic failure, and mortality rates more commonly higher in patients experiencing other morbidities. A common indicator of renal failure is azotemia, the accumulation of nitrogenous wastes in the blood (including BUN and creatinine), and a dec. in GFR. There are three types of acute renal failure: 1) prerenal, 2) intrinsic, and 3) postrenal. Prerenal and intrinsic acute renal failure account for up to 95% of cases. These will be discussed in lecture. A patient’s GFR (ml/min/1.73m2) will characterize their stage of chronic kidney disease: 1. >90: kidney damage is present but GFR appears normal. 2. 60-89: kidney damage is present with a mild decrease in GFR. 3. 30-59: kidney disease with a moderate decrease in GFR. 4. 15-29: kidney disease with severe decrease in GFR. 5.