Human Physiology lessons

Cells, tissues and organs

Cells of one type group together to form a tissue, such as muscle tissue. Different tissues join to make an organ, such as the heart.

Organs that share a job form an organ system. All your systems together make an organism: you.

Order the levels of organisation from smallest to largest, in three rounds.

The heart

The heart is a muscular pump with four chambers. The top two are the atria, which take blood in, and the bottom two are the ventricles, which pump it out.

The right side pumps blood to the lungs and the left side pumps it to the body. Blood passes through the heart twice on each full trip, which is called double circulation.

Diagrams show the heart as if you are facing the person. So the right side of the heart is on the left of the picture.

Example: a labelled heart diagram that follows blood through each chamber, valve and vessel.

Practice: Label the heart

Drag names onto the numbered chambers and vessels of the heart.

Cardiac output

At rest, each ventricle pumps out about 70\ \text{ml} of blood per beat. This is the stroke volume.

The volume pumped each minute is the cardiac output. It is the heart rate times the stroke volume.

Cardiac output: \text{cardiac output} = \text{heart rate} \times \text{stroke volume}

At 70 beats a minute that is 70 \times 70\ \text{ml} = 4900\ \text{ml}, about 4.9 litres a minute. During hard exercise both numbers rise, and output can reach four times that.

Type the cardiac output, heart rate or stroke volume, with random values.

Arteries, veins and capillaries

Arteries carry blood away from the heart at high pressure. Their walls are thick, muscular and stretchy.

Veins carry blood back to the heart at low pressure. They have thinner walls, a wide space inside and valves that stop blood flowing backwards.

Capillaries link arteries to veins. Their walls are one cell thick, so oxygen and food pass easily into the cells around them.

Example: cross-sections of an artery, a vein and a capillary, with wall, valves and pressure.

Practice: Sort the vessels

Sort features into artery, vein or capillary.

What blood is made of

Blood is a pale yellow liquid called plasma, full of cells. Plasma is mostly water and carries dissolved glucose, hormones, carbon dioxide and waste.

Red blood cells carry oxygen using a protein called haemoglobin. White blood cells fight infection, and platelets help blood clot at a cut.

An adult has about 5 litres of blood. Just one cubic millimetre holds about 5 million red blood cells.

Example: a blood sample settles into plasma, white cells and platelets, and red cells, with real counts.

Practice: Match the parts of blood

Match each part of blood to its job, then to how it looks.

Breathing

Breathing moves air in and out of your lungs. The main muscle is the diaphragm, a dome-shaped sheet under the lungs.

To breathe in, the diaphragm contracts and flattens while rib muscles lift the ribs up and out. The chest gets bigger, so the pressure inside drops and air flows in.

To breathe out, these muscles relax. The chest gets smaller, the pressure rises and air flows out.

Example: the chest breathing in and out, with diaphragm, ribs, lung volume and pressure.

Practice: Breathing in or out

Pick what each part does when breathing in or breathing out.

Gas exchange in the alveoli

Your airways end in tiny air sacs called alveoli. Each one is wrapped in a net of capillaries.

Oxygen diffuses from the air into the blood, where there is less of it. Carbon dioxide diffuses the other way, into the air you breathe out.

Alveoli are built for this, with walls just one cell thick. Your lungs hold about 480 million of them, with a total area of around 70 square metres.

Example: oxygen diffuses into red cells passing an alveolus while carbon dioxide leaves.

Neurons and reflexes

Your nervous system sends messages as electrical impulses along nerve cells called neurons. The brain and spinal cord form the central nervous system.

Sensory neurons carry impulses from receptors, like those in your skin. Motor neurons carry impulses out to effectors, the muscles and glands that respond.

A reflex is a fast, automatic response that skips conscious thought. A relay neuron in the spinal cord passes the impulse straight from the sensory neuron to a motor neuron.

Example: an impulse travels a reflex arc from a hot pan to the arm muscle.

Practice: Order the reflex arc

Put the steps of a reflex arc in order.

The brain

Your brain holds about 86 billion neurons. Different regions do different jobs.

The cerebrum is the large, wrinkled top part. It handles thinking, memory, language and the movements you choose to make.

The cerebellum at the back keeps your balance and coordinates movement. The brain stem joins the spinal cord and runs automatic jobs like breathing and heart rate.

Example: a side view of the brain that highlights each region and its jobs.

Digestion

Digestion breaks big food molecules into small ones that can pass into your blood. Food travels along one long tube, the gut, from mouth to anus.

Teeth and the stomach break food up, and muscles in the gut wall squeeze it along. Chemicals called enzymes break it down further.

The small molecules are absorbed in the small intestine. Its lining is folded into millions of tiny fingers called villi, giving an area of about 30 square metres.

Example: a meal travels the gut, stopping at each organ with what happens and how long it takes.

Digestive enzymes

Enzymes are proteins that speed up reactions without being used up. Each digestive enzyme breaks down one kind of food molecule.

Amylase breaks starch into sugars. Proteases break proteins into amino acids, and lipases break fats into fatty acids and glycerol.

Bile from the liver is not an enzyme. It splits fat into tiny droplets, which gives lipase more surface to work on.

Example: amylase, protease and lipase cut their food molecules into small pieces.

Practice: Match the enzymes

Match enzymes to the food they break down, the products and where they are made.

Defending the body

A pathogen is a microbe that causes disease, such as some bacteria and viruses. Your first defences are barriers that keep them out.

Skin blocks them and stomach acid kills many that you swallow. In your airways, sticky mucus traps them and tiny hairs called cilia sweep it away.

Pathogens that get in meet white blood cells. Phagocytes engulf and digest them, and lymphocytes make antibodies that lock onto them.

Example: a phagocyte engulfs a bacterium and antibodies clump the rest together.

Vaccination

Each antibody fits the antigens, the marker molecules, of one pathogen only. The first time, making enough of the right antibodies takes one to two weeks, so you may fall ill.

Memory cells then remember it. If it comes back, antibodies are made faster and in far larger amounts, so you usually stay well.

A vaccine holds a harmless form of a pathogen, a piece of one, or instructions to make a piece. It triggers that first response safely, so the fast response is ready for the real thing.

Example: antibody levels after a first and a second meeting with the same pathogen.

Practice: Sort the defences

Sort defences into barriers, phagocytes and lymphocytes.

Homeostasis

Homeostasis means keeping conditions inside your body steady. Your cells only work well within a narrow range of temperature, water and glucose.

It works by negative feedback. Receptors detect a change, a control centre in the brain decides, and effectors act to undo it.

Your brain keeps your core temperature near 37\ \degree\text{C}. If you get too hot you sweat, and if you get too cold you shiver.

Example: negative feedback brings body temperature back to 37 degrees after it rises and after it falls.

Blood glucose

Cells use glucose for energy, so its level in the blood must stay steady. In a healthy person it stays between about 4 and 8 millimoles per litre, written \text{mmol/L}.

After a meal, glucose rises and the pancreas releases insulin. Insulin makes cells take in glucose and the liver store the extra as glycogen.

When glucose falls, the pancreas releases glucagon instead. Glucagon makes the liver turn glycogen back into glucose.

Example: blood glucose through a morning, with insulin after meals and glucagon before lunch.

Practice: Control blood glucose

Read a glucose level, then pick the hormone and what the liver does.

Practice: Systems working together

Questions that link the systems during a run.