11 3 1 define excretion n.
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11.3.1 Define Excretion. Excretion refers to the removal of the waste products of metabolic pathways from the body. This can occur in a number of ways but is mainly through the lungs, skin and kidney and urinary system.

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11 3 1 define excretion
11.3.1 Define Excretion

Excretion refers to the removal of the waste products of metabolic pathways from the body. This can occur in a number of ways but is mainly through the lungs, skin and

kidney and urinary system.


The substance that are divided into four groups.1- Toxins and other substances that are ingested and absorbed but are not fully metabolized by the body, for example betain pigment in beet, and also drugs.


2- Excess salt, absorbed from food in the gut.3- Excess water, produced by cell respiration and absorbed from the food in the gut.4- Nitrogenous wastes– mainly urea.


Substances in group 1 and 2 are excretory products, as they are metabolic products of cells. Groups 3 and 4 are not excretory products because they are not produced by the body cells.


The kidneys filter off about one fifth of the volume of plasma from the blood flowing through them. This filtrate contains all of the substances in plasma apart from large proteins molecules.


The kidneys then actively reabsorb the specific substances in the filtrate that the body needs. The results of this process in the unwanted substance pass out of the body in the urine.


Table 1 shows the flow rate of blood to the kidney and other organs, the rate of oxygen delivery and oxygen consumption. All of the values are given per 100 g of tissue or organs. The rates are for a person in a warm environment.


1- Compare the rate of blood flow to the kidney with flow to the other organs.2- Calculate the volume of oxygen delivered to the organs per liter of blood.

3- In the brain, 34 per cent of the oxygen that is delivered is consumed. Calculate the same percentage for the other organs.

4- Discuss the reasons for the difference between the kidney and the other organs in the volume of blood flowing to the organ, and the percentage of oxygen in the blood that is consumed.

5- Predict, with a reason, one change in blood flow that would occur if the person were moved to a cold environment.
11 3 2 draw and label a diagram of the kidney
11.3.2 Draw and label a diagram of the kidney.

Humans have two kidneys, situated low in the abdominal

cavity, near the back.


Each kidney has a renal artery leading to it and a renal vein and a ureter leading

away from it . The renal vein takes the ‘clean’ blood away from the kidney while the ureter leads the urine to the bladder.

When a kidney is examined, the cortex on the outside and the medulla on the inside can be seen. In the centre is the renal pelvis



Renal pelvis

Renal artery

Renal vein



The photograph is of a large pig kidney. There are three distinct regions based on the distribution of the different sections of the nephron. The human kidney contains approx 106nephrons.

Cortex: Lighter brown colour contains the Malpighian bodies which are the capsules that contains Bowman's capsule and a glomerulus at the expanded end of a nephron. There are also the proximal and distal convoluted tubules and the upper sections of collecting ducts.

Medulla:The darker, redder region composed of loops of henle and the lower sections of the collecting ducts. Notice that it seems to form triangular regions which are called the pyramids.

Pelvis: This Is a cavity which collects the urine that emerges from the open ends of the collecting ducts. The nephrons open on the margin of the pyramids and pelvis. The white tissue forms a funnel called the ureter which conducts the urine to the bladder

11 3 3 annotate a diagram of a glomerulus and associated nephron to show the function of each part
11.3.3 Annotate a diagram of a glomerulus and associated nephron to show the function of each part.

Nephron structure

  • Browman’s capsule- a cup- shaped structure with a highly porous inner wall, which collects the fluid filtered from the blood.
  • Proximal convoluted tubule- a highly twisted section of the nephron, with cells in the wall having many mitochondria and microvilli projecting into the lumen of the tube.
  • Loop of Henle– a tube shaped like a hairpin, consisting of a descending limb that carries the filtrate deep into the medulla of the kidney, and an ascending limb that brings it back out to the cortex.

-Distal convoluted tubule- another highly twisted section, but with fewer, shorter microvilli and fewer mitochondria.

- Collecting duct- a wider tube that carries the filtrate back through the cortex and medulla to the renal pelvis.


Associated with the nephrons are blood vessels. Blood flows through the following sequence.

  • Afferent arteriole- brings blood from the renal artery.
  • Glomerulus – a tight, knot like, high pressure capillary bed that is the site of blood filtration.
  • Efferent arteriole- narrow vessel that restrictis blood flow, helping to generate high pressure in the glomerulus.

Peritubular capillaries- a low- pressure capillary bed that runs around the convoluted tubules, absorbing fluid from them.

  • Vasa recta – unbranched capillaries that are similar in shape to the loops of Henle, with a descending limb that carries blood deep into the medulla and an ascending limb bringing it back to the cortex.
  • Venules – carry blood to the renal vein.
11.3.4 Explain the process of ultrafiltration, including blood pressure, fenestrated blood capillaries and basement membrane.