The Human Excretory The Human Excretory SystemSystem

Excretory SystemExcretory SystemThe The kidneys kidneys regulate the amount of water, salts regulate the amount of water, salts and other substances in the blood. and other substances in the blood. The kidneys are fist-sized, bean shaped The kidneys are fist-sized, bean shaped structures that remove nitrogenous wastes structures that remove nitrogenous wastes (urine) and excess salts from the blood. (urine) and excess salts from the blood. The The ureters ureters are tubes that carry urine from the are tubes that carry urine from the pelvis of the kidneys to the urinary bladder. pelvis of the kidneys to the urinary bladder. The The urinary bladder urinary bladder temporarily stores urine temporarily stores urine until it is released from the body. until it is released from the body. The The urethra urethra is the tube that carries urine from is the tube that carries urine from the urinary bladder to the outside of the body. the urinary bladder to the outside of the body. The outer end of the urethra is controlled by a The outer end of the urethra is controlled by a circular muscle called a sphincter.circular muscle called a sphincter.

Excretory SystemExcretory System

The KidneyThe KidneyEach kidney is composed of three Each kidney is composed of three sections: sections: – the outer (renal) cortex, the (renal) medulla the outer (renal) cortex, the (renal) medulla

(middle part) and the hollow inner (renal) (middle part) and the hollow inner (renal) pelvis. pelvis.

The The cortex cortex is where the blood is filtered. is where the blood is filtered.

The The medulla medulla contains the collecting ducts which contains the collecting ducts which carry filtrate (filtered substances) to the pelvis. carry filtrate (filtered substances) to the pelvis.

The The pelvis pelvis is a hollow cavity where urine is a hollow cavity where urine accumulates and drains into the ureter.accumulates and drains into the ureter.

The Parts of the Kidney

How blood is FilteredHow blood is Filtered

The filtering units of the kidneys are the The filtering units of the kidneys are the nephronsnephrons. . There are approximately one million nephrons in There are approximately one million nephrons in each kidney. each kidney. The nephrons are located within the cortex and The nephrons are located within the cortex and medulla of each kidney. medulla of each kidney. The tubes of the nephron are surrounded by The tubes of the nephron are surrounded by cells and a network of blood vessels spreads cells and a network of blood vessels spreads throughout the tissue. Therefore, material that throughout the tissue. Therefore, material that leaves the nephron enters the surrounding cells leaves the nephron enters the surrounding cells and returns to the bloodstream by a network of and returns to the bloodstream by a network of vessels. vessels.

How the kidney does its job!How the kidney does its job!Kidney ActionKidney ActionBlood enters Bowman’s capsule through a tiny Blood enters Bowman’s capsule through a tiny artery — (artery — (the renal arterythe renal artery). ). The artery branches to form a The artery branches to form a glomerulusglomerulus. . Blood pressure forces some blood plasma and Blood pressure forces some blood plasma and small particles into the surrounding capsule — small particles into the surrounding capsule — (this is called the (this is called the nephric filtratenephric filtrate). ). Large particles such as blood cells and proteins Large particles such as blood cells and proteins remain in capillaries. remain in capillaries. The nephric filtrate is pushed out of the capsule The nephric filtrate is pushed out of the capsule and into the and into the proximal tubule proximal tubule . . This is where reabsorption begins. This is where reabsorption begins.

Kidney action (cont)Kidney action (cont)

The filtrate reaches the end of the The filtrate reaches the end of the proximal tubule and the fluid is isotonic proximal tubule and the fluid is isotonic with the surrounding cells. with the surrounding cells.

Glucose and amino acids have been Glucose and amino acids have been removed from the filtrate. removed from the filtrate.

The filtrate then moves to the The filtrate then moves to the loop of loop of Henle Henle whose primary function is to whose primary function is to remove water from the filtrate by osmosis. remove water from the filtrate by osmosis.

Kidney action (cont)Kidney action (cont)

Only materials needed by the body are Only materials needed by the body are returned to bloodstream — for example, returned to bloodstream — for example, 99 % of water, all glucose and amino acids 99 % of water, all glucose and amino acids and many salts are reabsorbed. and many salts are reabsorbed. Osmosis , diffusion, and active transport Osmosis , diffusion, and active transport draw water , glucose , amino acids and draw water , glucose , amino acids and ions from filtrate into surrounding cells. ions from filtrate into surrounding cells. Small villi like projections help in the active Small villi like projections help in the active transport of glucose out of the filtrate and transport of glucose out of the filtrate and speed up the reabsorption process. speed up the reabsorption process. From here, these components return to From here, these components return to the bloodstream.the bloodstream.

Water (cont)Water (cont)

The filtrate moves to the The filtrate moves to the distal tubule distal tubule where tubular secretion occurs. where tubular secretion occurs. Active transport is used to pull hydrogen Active transport is used to pull hydrogen ions, creatinine, drugs such as penicillin ions, creatinine, drugs such as penicillin out of the blood and into the filtrate. out of the blood and into the filtrate. Fluid from a number of nephrons moves Fluid from a number of nephrons moves from the distal tubules to a common from the distal tubules to a common collecting duct collecting duct which carries what can which carries what can now be called now be called urine urine to the to the renal pelvisrenal pelvis..

Function of the Kidney

The principal function of the kidney is to filter blood in order to remove cellular waste products from the body.

At any given time, 20 % of blood is in the kidneys. Humans can function with one kidney.

If one ceases to work, the other increases in size to handle the workload.

The kidney has other functions but it is usually associated with the excretion of cellular waste such as :

1) urea (a nitrogenous waste produced in the liver from the breakdown of protein. It is the main component of urine) ;

2) uric acid (usually produced from breakdown of DNA or RNA) and

3) creatinine (waste product of muscle action).

All of these compounds have nitrogen as a major component.

The kidneys are more than excretory organs.

They are one of the major homeostatic organs of the body.

They control water pH, secrete erythropoietin (a hormone that stimulates red blood cell production) and activate vitamin D production in the skin.

That is why a doctor can tell so much from a urine sample.