Transport and Excretion: Quick Revision

A brief note on Transport and Excretion. Read it before a test. Try each question at the end before you open its answer.

The heart and circulation

• The heart has four chambers. The atria receive blood, and the ventricles pump it out. The septum keeps the two sides apart.
• The right side holds blood with little oxygen and sends it to the lungs. The left side holds oxygen-rich blood and sends it to the body.
• Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta → body.
• Valves stop the blood from flowing back.
• In double circulation, the blood passes through the heart twice in one round. Fish have a two-chambered heart and a single circulation.

Vessels and blood pressure

• Normal blood pressure is about $120/80$ mm of Hg, measured with a sphygmomanometer. Pressure that stays high is hypertension.

Blood, lymph and plants

• Plasma carries food, carbon dioxide and wastes. Red cells carry oxygen with haemoglobin, and platelets start a clot.
• Lymph forms from fluid that leaks out of the capillaries. It carries absorbed fat and drains into the veins.
• Xylem carries water and minerals up. Root pressure helps at night, and the transpiration pull is the main force in the day.
• Phloem carries food, as sucrose, up or down. This translocation uses energy from ATP.

Excretion

• Kidneys → ureters → urinary bladder → urethra.
• In a nephron, the blood is filtered in the glomerulus inside Bowman's capsule. The tubule takes back glucose, amino acids, salts and much water.
• About $180$ L of filtrate a day gives only a litre or two of urine.
• Dialysis cleans the blood when the kidneys fail, but it has no reabsorption.
• Plants lose oxygen by diffusion, store wastes in vacuoles, drop old leaves, and store resins and gums.

A worked example

A heart beats $68$ times a minute and pushes out $75$ mL of blood with each beat. How long does $4.5$ L of blood take to go round once?

1. Blood in a minute. $68 \times 75\ \text{mL} = 5100\ \text{mL} = 5.1\ \text{L}$.
2. Time for one round. $\dfrac{4.5\ \text{L}}{5.1\ \text{L per minute}} = 0.88$ minute.
3. In seconds. $0.88 \times 60 = 53$ s.

So all the blood goes round in less than a minute, even at rest.

Practice with new numbers

A program to try

The program follows a red blood cell through one full round of the body, and counts its passes through the heart.

Mistakes to avoid

• Calling the pulmonary artery oxygen-rich. It carries blood with little oxygen to the lungs.
• Saying that the kidneys make urea. The liver makes it, and the kidneys remove it.
• Thinking that phloem moves food only downwards. It moves food wherever it is needed.
• Mixing up the sides of a heart diagram. The heart's right side is on your left.

Check yourself

1. Which vessels bring oxygen-rich blood to the left atrium?

2. Why is a double circulation useful for birds and mammals?

3. What is taken back into the blood from the tubule of a nephron?

4. At night, what pushes water up the xylem of a small plant?

5. Name two ways in which a plant gets rid of its wastes.