Kidneys, Ureters, and the Urinary System pt1

• The kidney controls fluid and electrolyte balance by controlling urine volume and composition.
• To form urine, the nephron performs three processes: glomerular filtration, tubular reabsorption, and tubular secretion.
• The renal system is also called the renal system and acts as a purification plant, cleaning the blood of waste materials.
• Kidneys have three main jobs: filter blood, reabsorb and secrete ions, and produce urine.
• The urinary system controls the body's fluid and electrolyte balance and eliminates waste products.
• The body would die in a few days without the urinary system due to the loss of homeostasis.
• The urinary system consists of 2 kidneys, 2 ureters, a urinary bladder, and a urethra.

Kidneys and Ureters

• The Kidneys are bean-shaped organs located in the superior dorsal abdominal cavity that filter blood and make urine.
• The two ureters are tubes which carry urine from the kidneys to the urinary bladder.
• The urinary bladder is an expandable sac that holds urine.
• The urethra is the tube that transports urine from the bladder to the outside of the body.

Filtration

• Filtration moves fluid and chemicals from the blood into the nephron with the substance that enters into the nephron is called the filtrate.
• Blood cells, platelets, and large molecules (like protein) cannot get into a healthy kidney, and therefore stay in the blood vessel.
• Filter size determines what gets through the filter during filtration.
• Substances are moved through a filter by differences in pressure and the filter is the capillary wall.
• Pressure pushes substances through the holes in the filter, with more pressure resulting in more substances being pushed.
• Filtration rate changes with blood pressure.
• Sympathetic nervous system activation (fight or flight) and adrenaline release cause vasoconstriction, constricting afferent arterioles and slowing urine formation.
• The kidney regulates fluid volume and can work with the cardiovascular system to regulate blood pressure.
• Blood is filtered by millions of tiny filters in the cortex and what passes through the filter is called a filtrate.

Anatomy of the Kidney

• The kidney is covered by a fibrous layer of connective tissue called the renal capsule.
• The indentation that gives the kidney its bean shape is called the renal hilum.
• At the Hilum are renal arteries (which bring blood to the kidneys to be filtered), renal veins (which take the filtered blood away from the kidney to go back to the heart), and the ureter (which is attached at the hilum to transport urine from the kidney to the bladder).
• The internal kidney is divided into three parts: the renal cortex, the renal medulla, and the renal pelvis.
• ** The Renal Cortex is the outer layer that appears grainy and is where blood filtration occurs.
• ** The Renal Medulla is the middle layer that contains triangle-shaped, striped areas called renal pyramids, which are composed of collecting tubules for the urine that is formed.
• ** The Renal Pelvis is the inner layer which is a funnel divided into 2 or 3 large collecting cups called major calyces.
• The filtrate flows through tiny tubules in the medulla and collects in the renal pelvis.
• The renal pelvis (the enlarged proximal portion of the ureter) empties into the ureter, where the filtered urine is carried to the bladder.

Blood Vessels and Nephrons

• Kidneys work closely with the cardiovascular system.
• Blood supply to the kidney is crucial for proper function.
• A single renal artery enters each kidney at the hilum, branching into smaller arteries that eventually pass through the renal pyramids and collecting tubules.
• Afferent arterioles lead to a ball of capillaries called a glomerulus.
• Efferent arterioles leave from the glomerulus and then travel to a series of capillaries called pertibular capillaries and collecting tubules, which are part of the renal nephron.
• Blood flows out a series of veins that are the direct reverse of the arteries (excluding segmental veins), while the blood leaves the kidney via the renal vein.

Reabsorption and Secretion

• Water, salts, and amino acids that pass through the filter into the glomerular capsule can be reabsorbed into the peritubular capillaries, where this occurs in the proximal tubule, Loop of Henle, and distal tubule.
• Secretion is the movement of substances from the capillaries into the renal tubule, eventually leaving in the urine.
• During reabsorption, substances pass from the renal tubule into the peritubular capillaries and return to the bloodstream.
• Secreted substances, like urea, pass through the peritubular capillaries into the renal tubule (collecting duct) and are expelled from the body as urine.
• Glucose is completely reabsorbed, while metabolic waste products like urea and creatinine are almost completely secreted as urine.
• Tubular reabsorption and secretion control the contents and volume of urine.
• Reabsorbed substances move from the tubule back to the bloodstream via the peritubular capillaries while secreted substances (ammonia and some electrolytes) stay in the tubule and leave the body via urine.
• Tubule permeability affects tubular reabsorption and secretion and it indicates how easy it is for substances (like glucose) to pass through the wall.
• Diffusion or osmosis can occur in this process where each section of the tubule can reabsorb and secrete different substances.
• Many substances are reabsorbed in the proximal tubule.
• Water is reabsorbed via osmosis in the descending loop of Henle.
• Ions like sodium and potassium are reabsorbed in the ascending loop of Henle.
• Potassium and urea are filtered into the distal tubule and collecting duct.

The Nephron

• The functional unit of the kidney is the nephron, consisting of millions of microscopic funnels and tubules.
• The nephron can be divided into two distinct parts: the renal corpuscle and the renal tubule.
• ** The Renal Corpuscle is the first part of the nephron and acts as a filter
• ** The Renal Tubule is where reabsorption and secretion take place.

Renal Corpuscle

• Blood enters the renal corpuscle via the glomerulus, which is surrounded by the glomerular capsule, or Bowman’s capsule.
• Blood flows into the glomerulus, and everything EXCEPT blood cells and a few large molecules is pushed from the capillaries across the filter and into the glomerular capsule, making it a very efficient filter.
• The material filtered from the blood into the glomerular capsule is called glomerular filtrate.

Renal Tubule

• The rest of the nephron is a series of tubes known as renal tubules.
• Glomerular filtrate travels from the glomerular capsule into the first part of the renal tubule, the proximal tubule, eventually flowing into the nephron loop (or the Loop of Henle).
• The nephron loop consists of the descending loop of Henle and the ascending loop of Henle.
• Glomerular filtrate then travels from the nephron loop to the distal tubule.
• From the distal tubule, glomerular filtrate flows into one of several collecting ducts, which then lead to minor calyces, then to major calyces, the renal pelvis, and the ureter.
• The renal process can be divided into filtration between the glomerulus and Bowman's capsules, reabsorption of nutrients, salts, and water in the renal tubule, and secretion of unwanted substances are secreted in the renal tubule.
• At the point that the glomerular filtrate reaches urine.
• Blood vessels are positioned right beside the nephrons so that substances move between the tubules and the bloodstream in three ways:
• ** Blood approaches the nephron via the afferent arteriole.
• ** Blood flows from the afferent arteriole into the glomerulus.
• ** Blood flows from the glomerulus via the efferent arteriole into the peritubular capillaries and collecting ducts.

Kidneys in Homeostasis

• The kidney maintains electrolyte balance by selectively excreting or reabsorbing electrolytes as required, within the tubular system.
• The relationship between the acid of hydrogen ions (H+) and the base of bicarbonate ions (HCO3) balances the blood pH in what is called the acid/base relationship.
• Hormones released from endocrine glands assist the kidneys in maintaining optimal internal balance.
• Some hormones increase the filtration rate while others decrease it, affecting urine production, through homeostasis.

Urinary Bladder and Urination Reflex

• Filtrate flows out of the collecting duct into the minor calyces, then the major calyces.
• Once glomerular filtrate leaves the collecting ducts, its concentration cannot be changed, and is then considered urine.
• The urine collects in the renal pelvis and flows from the ureters to the urinary bladder, where it is stored.
• The urinary bladder is a small hollow organ lined with transitional epithelium to allow for expansion with the ability to stretch enhanced by pleats rugae.

Urination Reflex

• As urine accumulates, the bladder fills and stretches, triggering the urinary reflex and urge to urinate.
• When the bladder is full, signals are sent from the bladder to the spinal cord to the brain where the brain then sends signals down the spinal cord, causing contraction of the muscular walls of the bladder, and the bladder empties.
• Urine exits via the urethra, a thin muscular tube lined with different types of epithelium along its length.

Urinary Tract Infections (UTIs)

• UTIs are caused by the movement of fecal bacteria into the urinary tract.
• UTI symptoms include frequent, painful urination, bloody or cloudy urine with an unusual odor, and low abdominal/pelvic pain caused by bladder spasm.
• Treatment should begin promptly to avoid kidney damage from infection traveling from the bladder to the kidney.
• UTIs are more common in women because their urethra is shorter, so drinking plenty of water can help to prevent infections.

Aging Process

• As people age, there is a loss of muscle tone in the bladder which can lead to urinary incontinence.
• Between the ages of 2 to 80, individuals lose roughly 50% of kidney function.
• Medications are excreted by the kidneys; if not excreted properly, they can build up in the body and become toxic.